《生物学》课程参考书籍文献（英文版，Chapter 11 - Chapter 19）Part IV Reproduction and Heredity、Part V Molecular Genetics

Part IV Reproduction and Heredity ① Why Do Some Genes Maintain More Than One Common Allele in a Population? ∽ These bacterial cells are dividing. As the population grows, plant was supposed to look like: small plant with green gene variants arise by mutation. Do the new variants persist,or ds. But if all pea p nts are they eliminated by natural selection? ble to sort out th A Muller's classical model thus makes a very straightfor- les that ward prediction: in nature, most populations of asexual organisms should be genetically uniform most of the time. However, this is not at all what is observed. Natural popu- uding asexua nes like bacteria appear to have ise. Selection favors one form at a particular plac fferer at a differer varying sel crown onment, an en- each to b tion tween them G call balance by by natural selection, an d either wins or loses. tualism) or favors one (what a biologist calls commensal- There are no ties. One version of the gene becomes univer- sal in the population, and the other geneinated. popula ism). In essence, cooperation would be counterbalancing the effects of competition 205

205 Why Do Some Genes Maintain More Than One Common Allele in a Population? When Mendel did his crosses of pea plants, he knew what a pea plant was supposed to look like: a small plant with green leaves, purple flowers, and smooth seeds. But if all pea plants were like that, he would never have been able to sort out the rules of heredity—in a cross of green peas with green peas, there would have been no visible differences to reveal the 3:1 pattern of gene segregation. The variant alleles that Mendel employed in his studies—yellow leaves, white flow￾ers, wrinkled seeds—were rare “accidents” maintained in seed collections for their novelty. In nature, such unusual kinds of peas had never been encountered by Mendel. By the time Mendel’s work was rediscovered in 1900, Darwin had provided a ready explanation of why alterna￾tive alleles seemed to be rare in natural populations. Nat￾ural selection was simply scouring the population, cleansing it in each generation of less fit alternatives. While recombi￾nation can complicate the process in interesting ways among sexual organisms like peas, asexual organisms like bacteria were predicted to be very sensitive to the effects of selection. Left to do its work, natural selection should crown as winner in bacterial population the best allele of each gene, producing a uniform population. Why do populations contain variants at all? In 1932 the famous geneticist Herman Muller formulated what has come to be called the “classical model,” explaining gene variation in natural populations of asexual organisms as a temporary, transient condition, new variations arising by random muta￾tion only to be established or eliminated by selection. Except for the brief periods when populations are undergoing this periodic cleansing, they should remain genetically uniform. The removal of variants was proposed to be a very straightforward process. During the periodic cleansing pe￾riods envisioned by Muller, his classical model operates under a “competitive exclusion” principle first proposed by Gause: whenever a new variant appears, it is weighed in the balance by natural selection, and either wins or loses. There are no ties. One version of the gene becomes univer￾sal in the population, and the other is eliminated. Muller’s classical model thus makes a very straightfor￾ward prediction: in nature, most populations of asexual organisms should be genetically uniform most of the time. However, this is not at all what is observed. Natural popu￾lations of most species, including asexual ones like bacteria, appear to have lots of common variants—they are said to be “polymorphic.” So where are all of these variants coming from? Varia￾tion in the environment, either spatial or temporal, can be used to explain how some polymorphisms arise. Selection favors one form at a particular place and time, a different form at a different place or time. In a nutshell, varying selection can encourage polymorphism. Is that all there is to it? Is it really impossible for more than one variant to become common in a population, if the population lives in a constant uniform environment, an en￾vironment that does not vary from one place to another or from one time to another? Theory says so. Biologists that study microbial communities have begun to report that bacteria are not aware of Muller’s theory. Bacterial cultures started from a single cell living in simple unstructured environments rapidly become polymorphic. There is a way to reconcile theory and experiment. Per￾haps the variant individuals in the population are interact￾ing with one another. Muller’s theory assumes that every individual undergoes an independent trial by selection. But what if that’s not so? What if different kinds of individuals help each other out? Stable coexistence of variants in a population might be possible if interactions between them contribute to the welfare of both (what a biologist calls mu￾tualism) or favors one (what a biologist calls commensal￾ism). In essence, cooperation would be counterbalancing the effects of competition. Part .04 µm IV Reproduction and Heredity These bacterial cells are dividing. As the population grows, gene variants arise by mutation. Do the new variants persist, or are they eliminated by natural selection? Real People Doing Real Science

Acetate media E 0.06 ●cV101 strain 0.04 90.02 (a) (b) Maintaining stable polymorphism (a)Three new strains emerge in culture and are maintained. (b)Two strains are grown on media containing acetate. The strain CV103 was found to excrete acetate, while the strain CV101 was found to thrive in media with acetate as the sole source of carbon. Population growth is measured by an increase in the turbidity of the liquid medium; turbidity is measured as an increase in light absorbance at a wavelength of 420 nm(A420 nm) The Experiment The results To investigate this intriguing possibility, Julian Adams Three distinct variants were detected in the 773-generation and co-workers at the University of Michigan set out to E. coli, each being maintained at stable levels in the contin- see if polymorphism for metabolic abilities would de- uously growing culture. Clearly polymorphism can appear velop spontaneously in bacteria growing in a uniform within an initially uniform bacterial population growing in environment a simple homogeneous environment. For a bacterial subject they chose Escherichia coli When mixed together and allowed to compete, one (E coli), a widely studied bacterium whose growth under strain does not drive the other two to extinction, as theory laboratory conditions is well understood. Cultures of had predicted. Instead, the three new strains, CV101 Escherichia coli can be maintained in chemostat culture CV103, and CV116, all persist(see graph a above). for many hundreds of generations. A chemostat is a large The three strains were then analyzed to see how they ontainer holding liquid culture medium. A little bit of differed. CV103 exhibited the highest rate of glucose up- the liquid is continuously removed, and an equal amount take and produced the most acetate(an end product of glu- of fresh culture medium added to replace what leaves. cose aerobic fermentation). Is this difference important? The growth of the E coli culture is limited by the amount To see, the CV103 strain was co-cultured with CV101 of glucose remaining in the culture medium to feed the They maintained stable growth levels, which indicated that the contribution of the third strain. CV116 was not re Researchers inoculated a glucose-limited chemostat cul- quired to maintain their growth. ture media with the E. coli strain JA122, and maintained the What is the difference between cv1o1 and cv1033 continuous culture for 773 generations. A sample was CV101 could grow in culture filtrate of CV103 but in the taken from the chemostat after 773 generations and an reverse situation, CV103 could not grow. This indicates lyzed for the presence of new strains of E. coli. Any varia- that CV103 secretes a substance upon which CV101 can tion among the cells in the sample would indicate that grow. Is CV101 utilizing the acetate produced by CV103 polymorphism had arisen as its carbon source? To detect metabolic variation within the sample of To test this possibility, CV101 and CV103 were grown growing cells, Adams's team analyzed the rate of glucose together in media with acetate as the only carbon source uptake and the concentration of acetate, among other The results from this experiment are shown in graph b variables. By examining such biochemical parameters, above and indicate that Cv101 thrives on an acetate carbon the researchers could determine if the different strains source, while Cv103 does not and requires an additional were filling different metabolic "niches"that is, using carbon source such as glucose the metabolic environment in different ways. Metabolic These results indicate that two of the strains are main- niches were characterized by looking at the normal prod- tained in polymorphism at stable levels because they have ucts of aerobic fermentation, acetate and glycerol, which evolved different adaptations that allow them to coexist by appear in the growth medium as a by-product of E. coli filling different niches. One strain(Cv101)is maintained metabolism in the population because it is able to use a metabolic by To further classify the strains, batch cultures containing product released by another strain(CV103) wo strains were established to analyze interactions be To explore this experiment further, tween the two groups go to the virtual lab at mhhe. com/raven/vlab4. mhtml

The Experiment To investigate this intriguing possibility, Julian Adams and co-workers at the University of Michigan set out to see if polymorphism for metabolic abilities would de￾velop spontaneously in bacteria growing in a uniform environment. For a bacterial subject they chose Escherichia coli (E. coli), a widely studied bacterium whose growth under laboratory conditions is well understood. Cultures of Escherichia coli can be maintained in chemostat culture for many hundreds of generations. A chemostat is a large container holding liquid culture medium. A little bit of the liquid is continuously removed, and an equal amount of fresh culture medium added to replace what leaves. The growth of the E. coli culture is limited by the amount of glucose remaining in the culture medium to feed the growing cells. Researchers inoculated a glucose-limited chemostat cul￾ture media with the E. coli strain JA122, and maintained the continuous culture for 773 generations. A sample was taken from the chemostat after 773 generations and ana￾lyzed for the presence of new strains of E. coli. Any varia￾tion among the cells in the sample would indicate that polymorphism had arisen. To detect metabolic variation within the sample of growing cells, Adams’s team analyzed the rate of glucose uptake and the concentration of acetate, among other variables. By examining such biochemical parameters, the researchers could determine if the different strains were filling different metabolic “niches”—that is, using the metabolic environment in different ways. Metabolic niches were characterized by looking at the normal prod￾ucts of aerobic fermentation, acetate and glycerol, which appear in the growth medium as a by-product of E. coli metabolism. To further classify the strains, batch cultures containing two strains were established to analyze interactions be￾tween the two groups. The Results Three distinct variants were detected in the 773-generation E. coli, each being maintained at stable levels in the contin￾uously growing culture. Clearly polymorphism can appear within an initially uniform bacterial population growing in a simple homogeneous environment. When mixed together and allowed to compete, one strain does not drive the other two to extinction, as theory had predicted. Instead, the three new strains, CV101, CV103, and CV116, all persist (see graph a above). The three strains were then analyzed to see how they differed. CV103 exhibited the highest rate of glucose up￾take and produced the most acetate (an end product of glu￾cose aerobic fermentation). Is this difference important? To see, the CV103 strain was co-cultured with CV101. They maintained stable growth levels, which indicated that the contribution of the third strain, CV116, was not re￾quired to maintain their growth. What is the difference between CV101 and CV103? CV101 could grow in culture filtrate of CV103 but in the reverse situation, CV103 could not grow. This indicates that CV103 secretes a substance upon which CV101 can grow. Is CV101 utilizing the acetate produced by CV103 as its carbon source? To test this possibility, CV101 and CV103 were grown together in media with acetate as the only carbon source. The results from this experiment are shown in graph b above and indicate that CV101 thrives on an acetate carbon source, while CV103 does not and requires an additional carbon source such as glucose. These results indicate that two of the strains are main￾tained in polymorphism at stable levels because they have evolved different adaptations that allow them to coexist by filling different niches. One strain (CV101) is maintained in the population because it is able to use a metabolic by￾product released by another strain (CV103). Generations 0.4 0.6 0.8 Frequency in population Population growth (A420 1.0 nm) 10 20 30 0.2 0.0 Time (hours) 0.02 0.04 0.06 10 20 30 40 0.00 CV101 strain CV103 strain CV116 strain (a) (b) CV101 strain Acetate media CV103 strain Maintaining stable polymorphism. (a) Three new strains emerge in culture and are maintained. (b) Two strains are grown on media containing acetate. The strain CV103 was found to excrete acetate, while the strain CV101 was found to thrive in media with acetate as the sole source of carbon. Population growth is measured by an increase in the turbidity of the liquid medium; turbidity is measured as an increase in light absorbance at a wavelength of 420 nm (A420 nm). To explore this experiment further, go to the Virtual Lab at www.mhhe.com/raven6/vlab4.mhtml

How Cells divide Concept outline 11.1 Bacteria divide far more simply than do Cell Division in Prokaryotes. Bacterial cells divide by pitting in two 11.2 Chromosomes are highly ordered structures. Discovery of Chromosomes. All eukaryotic cells contain omosomes but different numbers of chromosomes The Structure of Eukaryotic Chromos play an important role in packaging DNA in chromosome 1.3 Mitosis is a key phase of the cell cycle. Phases of the Cell Cycle. The cell cycle cor growth phases, a nuclear division phase, and a division Interphase: Preparing for Mitosis. cell grows, replicates its DNA, and prepares for cell FIGURE 11.1 Mitosis. In prophase, the chromosomes condense and Cell division in bacteria. It,s hard to imagine fecal coliform microtubules attach sister chromosomes to opposite poles acteria as beautiful. but here is escherichia coli inhabitant of the of the cell In metaphase, chromosomes align along the large intestine and the biotechnology lab, spectacularly caught in center of the cell In anaphase, the chromosomes separate the act of fission in telophase the spindle dissipates and the nuclear envelope reforms Cytokinesis. In cytokinesis, the cytoplasm separates into Ai ces of organisms--bacteria, alligators, the weeds two roughly equal halves. n a lawn--grow and reproduce. From the smallest of 11. 4 The cell cycle is carefully controlled. creatures to the largest, all species produce offspring like themselves and pass on the hereditary information that General Strategy of Cell Cycle Control. At three points akes them what they are. In this chapter, we begin our in the cell cycle, feedback from the cell determines whether consideration of heredity with an examination of how cells reproduce(figure 11.1). The mechanism of cell reproduc Molecular Mechanisms of Cell Cycle Control. Special tion and its biological consequences have changed signifi Cancer and the Control of Cell Proliferation. Cancer cantly during the evolution of life on earth results from damage to genes encoding proteins that regulate the cell division cycle 207

207 11 How Cells Divide Concept Outline 11.1 Bacteria divide far more simply than do eukaryotes. Cell Division in Prokaryotes. Bacterial cells divide by splitting in two. 11.2 Chromosomes are highly ordered structures. Discovery of Chromosomes. All eukaryotic cells contain chromosomes, but different organisms possess differing numbers of chromosomes. The Structure of Eukaryotic Chromosomes. Proteins play an important role in packaging DNA in chromosomes. 11.3 Mitosis is a key phase of the cell cycle. Phases of the Cell Cycle. The cell cycle consists of three growth phases, a nuclear division phase, and a cytoplasmic division stage. Interphase: Preparing for Mitosis. In interphase, the cell grows, replicates its DNA, and prepares for cell division. Mitosis. In prophase, the chromosomes condense and microtubules attach sister chromosomes to opposite poles of the cell. In metaphase, chromosomes align along the center of the cell. In anaphase, the chromosomes separate; in telophase the spindle dissipates and the nuclear envelope reforms. Cytokinesis. In cytokinesis, the cytoplasm separates into two roughly equal halves. 11.4 The cell cycle is carefully controlled. General Strategy of Cell Cycle Control. At three points in the cell cycle, feedback from the cell determines whether the cycle will continue. Molecular Mechanisms of Cell Cycle Control. Special proteins regulate the “checkpoints” of the cell cycle. Cancer and the Control of Cell Proliferation. Cancer results from damage to genes encoding proteins that regulate the cell division cycle. All species of organisms—bacteria, alligators, the weeds in a lawn—grow and reproduce. From the smallest of creatures to the largest, all species produce offspring like themselves and pass on the hereditary information that makes them what they are. In this chapter, we begin our consideration of heredity with an examination of how cells reproduce (figure 11.1). The mechanism of cell reproduc￾tion and its biological consequences have changed signifi￾cantly during the evolution of life on earth. FIGURE 11.1 Cell division in bacteria. It’s hard to imagine fecal coliform bacteria as beautiful, but here is Escherichia coli, inhabitant of the large intestine and the biotechnology lab, spectacularly caught in the act of fission

11.1 Bacteria divide far more simply than do eukaryotes Cell Division in Prokaryotes In bacteria, which are prokaryotes and lack a nucleus, cell division consists of a simple procedure called binary fission (literally, "splitting in half), in which the cell divides into two equal or nearly equal halves(figure 11. 2). The genetic information, or genome, replicates early in the life of the cell. It exists as a single, circular, double-stranded DNA mole chis dna circle into the bacterial cell is a re- markable feat of packaging--fully stretched out, the dNA of a bacterium like Escherichia coli is about 500 times longer The dna circle is attached at one point to the cytoplas- mic surface of the bacterial cells plasma membrane. at a pecific site on the dna molecule called the replication ori- gin, a battery of more than 22 different proteins begins the process of copying the DNA(figure 11). When these en-FIGURE 11.2 “ daughter” genomes are attached side- by-side to the plasma two daughter cells幺巨 zymes have proceeded all the way around the circle of Fission(40,000x). Bacteria divide by a process of simple cell DNA, the cell possesses two copies of the genome. These fission. Note the newly formed plas membrane The growth of a bacterial cell to about twice its initial ize induces the onset of cell division a wealth of recent ev- idence suggests that the two daughter chromosomes are ac- ively partitioned during this process. As this process pro- cells are much larger than bacteria, and their genomes con- ceeds, the cell lays down new plasma membrane and cell tain much more DNA. Eukaryotic DNA is contained in a wall materials in the zone between the attachment sites of number of linear chromosomes, whose organization is much the two daughter genomes. A new plasma membrane grows more complex than that of the single, circular DNA mole- between the genomes; eventually, it reaches all the way into cules in bacteria. In chromosomes, DNA forms a complex the center of the cell, dividing it in two. Because the mem- with packaging proteins called histones and is wound into brane forms between the two genomes, each new cell is as- tightly condensed coils sured of retaining one of the genomes. Finally, a new cel wall forms around the new membrane Bacteria divide by binary The evolution of the eukaryotes introduced several addi middle of the cell. An activ hat one genome will end d up in ca Fission begins in the tioning process ensures tional factors into the process of cell division. Eukaryotic each daughter cell ⑧8 Replication FIGURE 11.3 How bacterial DNA repl The replication of the circular DNA molecule(blue)that constitutes the genome of a bacterium begins at a single site, called the rep origin. The replication enzymes move out in both directions from that site and make copies(red) of each strand in the dna duplex the enzymes meet on the far side of the molecule, replication is complete 208 Part IV Reproduction and Heredity

cells are much larger than bacteria, and their genomes con￾tain much more DNA. Eukaryotic DNA is contained in a number of linear chromosomes, whose organization is much more complex than that of the single, circular DNA mole￾cules in bacteria. In chromosomes, DNA forms a complex with packaging proteins called histones and is wound into tightly condensed coils. Bacteria divide by binary fission. Fission begins in the middle of the cell. An active partitioning process ensures that one genome will end up in each daughter cell. 208 Part IV Reproduction and Heredity Cell Division in Prokaryotes In bacteria, which are prokaryotes and lack a nucleus, cell division consists of a simple procedure called binary fission (literally, “splitting in half”), in which the cell divides into two equal or nearly equal halves (figure 11.2). The genetic information, or genome, replicates early in the life of the cell. It exists as a single, circular, double-stranded DNA mole￾cule. Fitting this DNA circle into the bacterial cell is a re￾markable feat of packaging—fully stretched out, the DNA of a bacterium like Escherichia coli is about 500 times longer than the cell itself. The DNA circle is attached at one point to the cytoplas￾mic surface of the bacterial cell’s plasma membrane. At a specific site on the DNA molecule called the replication ori￾gin, a battery of more than 22 different proteins begins the process of copying the DNA (figure 11.3). When these en￾zymes have proceeded all the way around the circle of DNA, the cell possesses two copies of the genome. These “daughter” genomes are attached side-by-side to the plasma membrane. The growth of a bacterial cell to about twice its initial size induces the onset of cell division. A wealth of recent ev￾idence suggests that the two daughter chromosomes are ac￾tively partitioned during this process. As this process pro￾ceeds, the cell lays down new plasma membrane and cell wall materials in the zone between the attachment sites of the two daughter genomes. A new plasma membrane grows between the genomes; eventually, it reaches all the way into the center of the cell, dividing it in two. Because the mem￾brane forms between the two genomes, each new cell is as￾sured of retaining one of the genomes. Finally, a new cell wall forms around the new membrane. The evolution of the eukaryotes introduced several addi￾tional factors into the process of cell division. Eukaryotic 11.1 Bacteria divide far more simply than do eukaryotes. FIGURE 11.2 Fission (40,000). Bacteria divide by a process of simple cell fission. Note the newly formed plasma membrane between the two daughter cells. Replication origin FIGURE 11.3 How bacterial DNA replicates. The replication of the circular DNA molecule (blue) that constitutes the genome of a bacterium begins at a single site, called the replication origin. The replication enzymes move out in both directions from that site and make copies (red) of each strand in the DNA duplex. When the enzymes meet on the far side of the molecule, replication is complete.

11.2 Chromosomes are highly ordered structures Discovery of Chromosomes Chromosomes were first observed by the german embryol ogist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the cells through what would now be a rather primitive light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Flem ing called their division mitosis, based on the greek word anng“t Chromosome number Since their initial discovery, chromosomes have been found in the cells of all eukaryotes examined. Their number may ary enormously from one species to another. a few kinds of organisms--such as the Australian ant My armeria, the pla Haplopappus gracilis, a relative of the sunflower that grows in FIGURE 11.4 North American deserts; and the fungus penicilliumn--have Human chromosomes. This photograph(950x) shows human only 1 pair of chromosomes, while some ferns have more chromosomes as they appear immediately before nuclear division than 500 pairs(table 11.1). Most eukaryotes have between Each DNA molecule has already replicated, forming identical 10 and 50 chromosomes in their body cells opies held together by a constriction called the centromere. Human cells each have 46 chromosomes. consist ing of 23 nearly identical pairs(figure 11.4). Each of these 46 chromosomes contains hundreds or thou- trisomy is fatal, and even in those few cases, sands of genes that play important roles in determin- problems result. Individuals with an extra copy of the ing how a person's body develops and functions. For very small chromosome 21, for example, develop this reason, possession of all the chromosomes is es more slowly than normal and are mentally retarded, a sential to survival. humans miss even one condition called Down syndrome mosome, a condition called monosomy, do not sur- vive embryonic development in most cases. Nor does All eukaryotic cells store their hereditary information in c. e human embryo develop proper. on called tri- chromosomes, but different kinds of organisms utilize very different numbers of chromosomes to store this somy. For all but a few of the smallest chromosomes, information Table 11.1 Chromosome Number in Selected Eukaryotes Total Number of Total Number of Total Number of Chromosomes Group Chromosomes Group Chromosomes FUNGI PLANTS VERTEBRATES Neurospora(haploid) Haplopappus graci veast Garden pea 14 Mor INSECTS Corn Bread wheat Human Drosophila 6826 48 Horsetail Ho Adder s tongue fern 1262 Chicken Chapter 11 How Cells Divide 209

Discovery of Chromosomes Chromosomes were first observed by the German embryol￾ogist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the cells through what would now be a rather primitive light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Flem￾ing called their division mitosis, based on the Greek word mitos, meaning “thread.” Chromosome Number Since their initial discovery, chromosomes have been found in the cells of all eukaryotes examined. Their number may vary enormously from one species to another. A few kinds of organisms—such as the Australian ant Myrmecia, the plant Haplopappus gracilis, a relative of the sunflower that grows in North American deserts; and the fungus Penicillium—have only 1 pair of chromosomes, while some ferns have more than 500 pairs (table 11.1). Most eukaryotes have between 10 and 50 chromosomes in their body cells. Human cells each have 46 chromosomes, consist￾ing of 23 nearly identical pairs (figure 11.4). Each of these 46 chromosomes contains hundreds or thou￾sands of genes that play important roles in determin￾ing how a person’s body develops and functions. For this reason, possession of all the chromosomes is es￾sential to survival. Humans missing even one chro￾mosome, a condition called monosomy, do not sur￾vive embryonic development in most cases. Nor does the human embryo develop properly with an extra copy of any one chromosome, a condition called tri￾somy. For all but a few of the smallest chromosomes, trisomy is fatal, and even in those few cases, serious problems result. Individuals with an extra copy of the very small chromosome 21, for example, develop more slowly than normal and are mentally retarded, a condition called Down syndrome. All eukaryotic cells store their hereditary information in chromosomes, but different kinds of organisms utilize very different numbers of chromosomes to store this information. Chapter 11 How Cells Divide 209 11.2 Chromosomes are highly ordered structures. FIGURE 11.4 Human chromosomes. This photograph (950×) shows human chromosomes as they appear immediately before nuclear division. Each DNA molecule has already replicated, forming identical copies held together by a constriction called the centromere. Table 11.1 Chromosome Number in Selected Eukaryotes Total Number of Total Number of Total Number of Group Chromosomes Group Chromosomes Group Chromosomes FUNGI Neurospora (haploid) 7 Saccharomyces (a yeast) 16 INSECTS Mosquito 6 Drosophila 8 Honeybee 32 Silkworm 56 PLANTS Haplopappus gracilis 2 Garden pea 14 Corn 20 Bread wheat 42 Sugarcane 80 Horsetail 216 Adder’s tongue fern 1262 VERTEBRATES Opossum 22 Frog 26 Mouse 40 Human 46 Chimpanzee 48 Horse 64 Chicken 78 Dog 78

The Structure of Eukaryotic more,if the strand of DNA from a single chromosome were laid out in a straight line, it would be about 5 cen chromosomes timeters(2 inches)long. Fitting such a strand into a nu- In the century since discovery of chromosomes, we have cleus is like cramming a string the length of a football field learned a great deal about their structure and composition into a baseball-and thats only 1 of 46 chromosomes! In the cell, however, the DNA is coiled, allowing it to fit into a much smaller space than would otherwise be possible Composition of Chromatin Chromosomes are composed of chromatin, a complex of DNA and protein; most are about 40% DNA and 60% Chromosome coilin protein. A significant amount of RNA is also associated How can this long DNA fiber coil so tightly? If we gently with chromosomes because chromosomes are the sites of disrupt a eukaryotic nucleus and examine the DNA with RNA synthesis. The DNA of a chromosome is one very electron microscope, we find that it resembles a string of long, double-stranded fiber that extends unbroken through beads(figure 11.5). Every 200 nucleotides, the dNa du the entire length of the chromosome. a typical human plex is coiled around a core of eight histone proteins, form- chromosome contains about 140 million(1.4x 10%)nu- omplex known as a nucleosome. Unlike most cleotides in its DNA. The amount of information one proteins, which have an overall negative charge, histones chromosome contains would fill about 280 printed books of are positively charged, due to an abundance of the basic 1000 pages each, if each nucleotide corresponded to a amino acids arginine and lysine. They are thus strongly at "word"and each page had about 500 words on it. Further- tracted to the negatively charged phosphate groups of the within chromosome supercoil Levels of eukaryotic Chromatin hromosomal Nucleotides assemble into strano rands require further Chromatin fiber packaging to fit into the cell nucleus. The DNA duplex is tightly bound to and wound around proteins called bistones The DNA-wrapped histones are called CentralFNucleosome nucleosomes. The coalesce into chromatin fibers, ultimately coiling around into super coils that make up the form of DNA DNA double helix(duplex) 210 Part IV Reproduction and Heredity

The Structure of Eukaryotic Chromosomes In the century since discovery of chromosomes, we have learned a great deal about their structure and composition. Composition of Chromatin Chromosomes are composed of chromatin, a complex of DNA and protein; most are about 40% DNA and 60% protein. A significant amount of RNA is also associated with chromosomes because chromosomes are the sites of RNA synthesis. The DNA of a chromosome is one very long, double-stranded fiber that extends unbroken through the entire length of the chromosome. A typical human chromosome contains about 140 million (1.4 × 108) nu￾cleotides in its DNA. The amount of information one chromosome contains would fill about 280 printed books of 1000 pages each, if each nucleotide corresponded to a “word” and each page had about 500 words on it. Further￾more, if the strand of DNA from a single chromosome were laid out in a straight line, it would be about 5 cen￾timeters (2 inches) long. Fitting such a strand into a nu￾cleus is like cramming a string the length of a football field into a baseball—and that’s only 1 of 46 chromosomes! In the cell, however, the DNA is coiled, allowing it to fit into a much smaller space than would otherwise be possible. Chromosome Coiling How can this long DNA fiber coil so tightly? If we gently disrupt a eukaryotic nucleus and examine the DNA with an electron microscope, we find that it resembles a string of beads (figure 11.5). Every 200 nucleotides, the DNA du￾plex is coiled around a core of eight histone proteins, form￾ing a complex known as a nucleosome. Unlike most proteins, which have an overall negative charge, histones are positively charged, due to an abundance of the basic amino acids arginine and lysine. They are thus strongly at￾tracted to the negatively charged phosphate groups of the 210 Part IV Reproduction and Heredity Supercoil within chromosome Chromosomes Coiling within supercoil Chromatin Chromatin fiber Nucleosome DNA Central histone DNA DNA double helix (duplex) FIGURE 11.5 Levels of eukaryotic chromosomal organization. Nucleotides assemble into long double strands of DNA molecules. These strands require further packaging to fit into the cell nucleus. The DNA duplex is tightly bound to and wound around proteins called histones. The DNA-wrapped histones are called nucleosomes. The nucleosomes then coalesce into chromatin fibers, ultimately coiling around into supercoils that make up the form of DNA recognized as a chromosome

DNA. The histone cores thus act as"magnetic forms"that promote and guide the coiling of the DNA. Further coiling somes wraps up into ⅨKκ higher order coils called ighly condensed portions of the chromatin are called heterochromatin. Some of these portions remain perma K ! il nently condensed, so that their DNA is never expressed The remainder of the chromosome, called euchromatin, is condensed only during cell division, when compact packa ing facilitates the movement of the chromosomes. At all other times, euchromatin is present in an open configura tion, and its genes can be expressed. The way chromatin is packaged when the cell is not dividing is not well under 3 35 stood beyond the level of nucleosomes and is a topic of in- FIGURE 11.6 A human karyotype. The individual chromosomes that make up Chromosome Karyotypes the 23 pairs differ widely in size and in centromere position. In this preparation, the chromosomes have been specifically stained Chromosomes may differ widely in appearance. They vary to indicate further differences in their composition and to in size, staining properties, the location of the centromere(a tinguish them clearly from one another constriction found on all chromosomes), the relative length of the two arms on either side of the centromere. and the positions of constricted regions along the arms. The partic ular array of chromosomes that an individual possesses is called its karyotype(figure 11.6). Karyotypes show marked Centromere differences among species and sometimes even among indi- viduals of the same species chromatids To examine a human karyotype, investigators collect a cell sample from blood, amniotic fluid, or other tissue and Homologous add chemicals that induce the cells in the sample to di chromosomes vide. Later, they add other chemicals to stop cell division at a stage when the chromosomes are most condensed and thus most easily distinguished from one another. The cells are then broken open and their contents, including the chromosomes, spread out and stained. To facilitate the examination of the karyotype, the chromosomes are FIGURE 11.7 usually photographed, and the outlines of the chromo- The difference between homologous chromosomes and sister somes are cut out of the photograph and arranged in chromatids. Homologous chromosomes are a pair of the same chromosome-say, chromosome number 16. Sister chromatids order(see figure 11.6) are the two replicas of a single chromosome held together by the centromeres after DNA replication How Many Chromosomes Are in a Cell? With the exception of the gametes(eggs or sperm)and a few specialized tissues, every cell in a human body is luman body cell contains a total of 46 replicated chromo- diploid(2n). This means that the cell contains two nearly somes, each composed of two sister chromatids joined by identical copies of each of the 23 types of chromosomes, one centromere. The cell thus contains 46 centromeres and for a total of 46 chromosomes. The haploid(1n)gametes 92 chromatids(2 sister chromatids for each of 2 homo- contain only one copy of each of the 23 chromosome types, logues for each of 23 chromosomes). The cell is said to while certain tissues have unusual numbers of chromo ontain 46 chromosomes rather than 92 because by con somes-many liver cells, for example, have two nuclei, vention, the number of chromosomes is obtained by count while mature red blood cells have no nuclei at all. The two copies of each chromosome in body cells are called homol- gous chromosomes, or homologues(Greek bomm Eukaryotic genomes are larger and more complex than agreement"). Before cell division, each homologue repli those of bacteria. Eukaryotic DNA is packaged tightly cates,producing two identical sister chromatids joined at into chromosomes, enabling it to fit inside cells the centromere, a condensed area found on all eukaryotic Haploid cells contain one set of chromosomes, while hromosomes(figure 11.7). Hence, as cell division begins, a diploid cells contain two sets Chapter 11 How Cells Divide 211

DNA. The histone cores thus act as “magnetic forms” that promote and guide the coiling of the DNA. Further coiling occurs when the string of nucleosomes wraps up into higher order coils called supercoils. Highly condensed portions of the chromatin are called heterochromatin. Some of these portions remain perma￾nently condensed, so that their DNA is never expressed. The remainder of the chromosome, called euchromatin, is condensed only during cell division, when compact packag￾ing facilitates the movement of the chromosomes. At all other times, euchromatin is present in an open configura￾tion, and its genes can be expressed. The way chromatin is packaged when the cell is not dividing is not well under￾stood beyond the level of nucleosomes and is a topic of in￾tensive research. Chromosome Karyotypes Chromosomes may differ widely in appearance. They vary in size, staining properties, the location of the centromere (a constriction found on all chromosomes), the relative length of the two arms on either side of the centromere, and the positions of constricted regions along the arms. The partic￾ular array of chromosomes that an individual possesses is called its karyotype (figure 11.6). Karyotypes show marked differences among species and sometimes even among indi￾viduals of the same species. To examine a human karyotype, investigators collect a cell sample from blood, amniotic fluid, or other tissue and add chemicals that induce the cells in the sample to di￾vide. Later, they add other chemicals to stop cell division at a stage when the chromosomes are most condensed and thus most easily distinguished from one another. The cells are then broken open and their contents, including the chromosomes, spread out and stained. To facilitate the examination of the karyotype, the chromosomes are usually photographed, and the outlines of the chromo￾somes are cut out of the photograph and arranged in order (see figure 11.6). How Many Chromosomes Are in a Cell? With the exception of the gametes (eggs or sperm) and a few specialized tissues, every cell in a human body is diploid (2n). This means that the cell contains two nearly identical copies of each of the 23 types of chromosomes, for a total of 46 chromosomes. The haploid (1n) gametes contain only one copy of each of the 23 chromosome types, while certain tissues have unusual numbers of chromo￾somes—many liver cells, for example, have two nuclei, while mature red blood cells have no nuclei at all. The two copies of each chromosome in body cells are called homol￾ogous chromosomes, or homologues (Greek homologia, “agreement”). Before cell division, each homologue repli￾cates, producing two identical sister chromatids joined at the centromere, a condensed area found on all eukaryotic chromosomes (figure 11.7). Hence, as cell division begins, a human body cell contains a total of 46 replicated chromo￾somes, each composed of two sister chromatids joined by one centromere. The cell thus contains 46 centromeres and 92 chromatids (2 sister chromatids for each of 2 homo￾logues for each of 23 chromosomes). The cell is said to contain 46 chromosomes rather than 92 because, by con￾vention, the number of chromosomes is obtained by count￾ing centromeres. Eukaryotic genomes are larger and more complex than those of bacteria. Eukaryotic DNA is packaged tightly into chromosomes, enabling it to fit inside cells. Haploid cells contain one set of chromosomes, while diploid cells contain two sets. Chapter 11 How Cells Divide 211 FIGURE 11.6 A human karyotype. The individual chromosomes that make up the 23 pairs differ widely in size and in centromere position. In this preparation, the chromosomes have been specifically stained to indicate further differences in their composition and to distinguish them clearly from one another. Sister chromatids Homologous chromosomes Centromere FIGURE 11.7 The difference between homologous chromosomes and sister chromatids. Homologous chromosomes are a pair of the same chromosome—say, chromosome number 16. Sister chromatids are the two replicas of a single chromosome held together by the centromeres after DNA replication

11.3 Mitosis is a key phase of the cell cycle Phases of the Cell Cycle The increased size and more complex organization of eu- Metaphase Anaphase karyotic genomes over those of bacteria required radical Telophase hanges in the process by which the two replicas of the are partitioned into the daughter cells during cell n. This division process is diagrammed as a cell consisting of five phases(figure 11. 8) The five phases D Interphase(G,,S,G2 phase GI is the primary growth phase of the cell. For many or- 口 Mitosis(M) ganisms, this encompasses the major portion of the cells ife span. S is the phase in which the cell synthesizes a eplica of the genome. G is the second growth phase, in which preparations are made for genomic separation During this phase, mitochondria and other organelles replicate, chromosomes condense, and microtubules begin to assemble at a spindle. Gl, S, and G2 together constitute interphase, the portion of the cell cycle be tween cell divisions FIGURE 11.8 M is the phase of the cell cycle in which the microtubu- The cell cycle. Each wedge represents one hour of the 22-hour lar apparatus assembles, binds to the chromosomes, and cell cycle in human cells growing in culture. GI represents the moves the sister chromatids apart. Called mitosis, this primary growth phase of the cell cycle, S the phase during which a process is the essential step in the separation of the two eplica of the genome is synthesized, and Gy the second growth daughter genomes. We will discuss mitosis as it occurs in animals and plants, where the process does not vary much (it is somewhat different among fungi and some protists) 24 hours. but some cells. like certain cells in the human Although mitosis is a continuous process, it is traditionally liver, have cell cycles lasting more than a year. During subdivided into four stages: prophase, metaphase, anaphase and telophase he cycle, growth occurs throughout the GI and g2 C is the phase of the cell cycle when the cytoplasm di phases(referred to as"gap"phases, as they separate S vides, creating two daughter cells. This phase is called from M), as well as during the s phase. The M phase cytokinesis. In animal cells, the microtubule spindle takes only about an hour, a small fraction of the entire helps position a contracting ring of actin that constricts cycle like a drawstring to pinch the cell in two. In cells with a Most of the variation in the length of the cell cycle cell wall, such as plant cells, a plate forms between the di from one organism or tissue to the next occurs in the phase Cells often pause in Gi before DNA replication and enter a resting state called Go phase; they may re- Duration of the Cell Cycle main in this phase for days to years before resuming cell division. At any given time, most of the cells in an ani The time it takes to complete a cell cycle varies greatly mal's body are in Go phase. Some, such as muscle and nerve cells, remain there permanently; others, such as among organisms. Cells in growing embryos can com- liver cells, can resume Gi phase in response to factors re- plete their cell cycle in under 20 minutes; the shortest known animal nuclear division cycles occur in fruit fly leased during injury embryos(8 minutes). Cells such as these simply divide their nuclei as quickly as they can replicate their DNA, Most eukaryotic cells repeat a process of growth and ell th. half of the The cycle can vary half by m, and essentially none by gi or G2. Because ma- length from a few minutes to several years ture cells require time to grow, most of their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 212 Part IV Reproduction and Heredity

Phases of the Cell Cycle The increased size and more complex organization of eu￾karyotic genomes over those of bacteria required radical changes in the process by which the two replicas of the genome are partitioned into the daughter cells during cell division. This division process is diagrammed as a cell cycle, consisting of five phases (figure 11.8). The Five Phases G1 is the primary growth phase of the cell. For many or￾ganisms, this encompasses the major portion of the cell’s life span. S is the phase in which the cell synthesizes a replica of the genome. G2 is the second growth phase, in which preparations are made for genomic separation. During this phase, mitochondria and other organelles replicate, chromosomes condense, and microtubules begin to assemble at a spindle. G1, S, and G2 together constitute interphase, the portion of the cell cycle be￾tween cell divisions. M is the phase of the cell cycle in which the microtubu￾lar apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Called mitosis, this process is the essential step in the separation of the two daughter genomes. We will discuss mitosis as it occurs in animals and plants, where the process does not vary much (it is somewhat different among fungi and some protists). Although mitosis is a continuous process, it is traditionally subdivided into four stages: prophase, metaphase, anaphase, and telophase. C is the phase of the cell cycle when the cytoplasm di￾vides, creating two daughter cells. This phase is called cytokinesis. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant cells, a plate forms between the di￾viding cells. Duration of the Cell Cycle The time it takes to complete a cell cycle varies greatly among organisms. Cells in growing embryos can com￾plete their cell cycle in under 20 minutes; the shortest known animal nuclear division cycles occur in fruit fly embryos (8 minutes). Cells such as these simply divide their nuclei as quickly as they can replicate their DNA, without cell growth. Half of the cycle is taken up by S, half by M, and essentially none by G1 or G2. Because ma￾ture cells require time to grow, most of their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 24 hours, but some cells, like certain cells in the human liver, have cell cycles lasting more than a year. During the cycle, growth occurs throughout the G1 and G2 phases (referred to as “gap” phases, as they separate S from M), as well as during the S phase. The M phase takes only about an hour, a small fraction of the entire cycle. Most of the variation in the length of the cell cycle from one organism or tissue to the next occurs in the G1 phase. Cells often pause in G1 before DNA replication and enter a resting state called G0 phase; they may re￾main in this phase for days to years before resuming cell division. At any given time, most of the cells in an ani￾mal’s body are in G0 phase. Some, such as muscle and nerve cells, remain there permanently; others, such as liver cells, can resume G1 phase in response to factors re￾leased during injury. Most eukaryotic cells repeat a process of growth and division referred to as the cell cycle. The cycle can vary in length from a few minutes to several years. 212 Part IV Reproduction and Heredity 11.3 Mitosis is a key phase of the cell cycle. G2 S G1 C Metaphase Prophase Anaphase Telophase M Interphase (G1, S, G2 phases) Mitosis (M) Cytokinesis (C) FIGURE 11.8 The cell cycle. Each wedge represents one hour of the 22-hour cell cycle in human cells growing in culture. G1 represents the primary growth phase of the cell cycle, S the phase during which a replica of the genome is synthesized, and G2 the second growth phase

Interphase: Preparing for Mitosis Chromatid The events that occur during interphase, made up of the g S, and g phases, are very important for the successful con pletion of mitosis. During Gl, cells undergo the major por tion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids, which remain at- tached to each other at the centromere. The centromere is Kinetochore a point of constriction on the chromosome, containing specific DNA sequence to which is bound a disk of protein called a kinetochore. This disk functions as an attachment site for fibers that assist in cell division(figure 11.9). Each Centromere chromosome's centromere is located at a characteristic site The cell grows throughout interphase. The GI and gz segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cells DNA replicates only during the S phase of the cell cycle After the chromosomes have replicated in S phase, they chromosome remain fully extended and uncoiled. This makes them invis- ible under the light microscope. In G] phase, they begin the FIGURE 11.9 long process of condensation, coiling ever more tightly Kinetochores. In a metaphase chromosome, kinetochore involved in the rapid final conden- microtubules are anchored to proteins at the centromere. sation of the chromosomes that occurs early in mitosis. also during G phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite Interphase is that portion of the cell cycle in which the poles of the cell. In animal cells, a pair of microtubule- chromosomes are invisible under the light microscope organizing centers called centrioles replicate. All eukary- because they are not yet condensed. It includes the Gl otic cells undertake an extensive synthesis of tubulin, the S, and G2 phases. In the G2 phase, the cell mobilizes its rotein of which microtubules are formed resources for cell division A Vocabulary of chromatin The complex of DNA and kinetochore A disk of protein bound to proteins of which eukaryotic chromosomes the centromere and attached to micro- Cell Division are composed tubules during mitosis, linking each chro- chromosome The structure within cells matid to the spindle apparatus. that contains the genes. In eukaryotes, it microtubule A hollow cylinder, about 25 consists of a single linear DNA molecule as- nanometers in diameter, composed of sub binary fission Asexual reproduction of a sociated with proteins. The DNA is repli- units of the protein tubulin. Microtubules cell by division into two equal or nearly cated during S phase, and the replicas sepa- lengthen by the addition of tubulin subunits equal parts. Bacteria divide by binary rated during M phase to their end(s) and shorten by the removal cytokinesis Division of the cytoplasm of a of subunits centromere A constricted region of a cell after nuclear division. mitosis Nuclear division in which repl length, composed of highly repeated DNA some that is extended except during cell di- genetically identical daughter nuclei. When equences(satellite DNA). During mitosis, vision and from which rna is transcribed. accompanied by cytokinesis, it produces matids and is the site to which the kineto- heterochromatin The portion of a chro- two identical daughter cells the centromere joins the two sister chro chores are attached mosome that remains permanently con- nucleosome The basic packaging unit of densed and therefore is not transcribed eukaryotic chromosomes, in which the chromatid One of the two copies of a into RNA. Most centromere regions are DNA molecule is wound around a cluster of replicated chromosome, joined by a single heterochromatic histone proteins. Chromatin is composed of centromere to the other strand homologues Homologous chromosomes: long strings of nucleosomes that resemble of ch somes that carry equivalent genes Chapter 11 How Cells Divide 213

Interphase: Preparing for Mitosis The events that occur during interphase, made up of the G1, S, and G2 phases, are very important for the successful com￾pletion of mitosis. During G1, cells undergo the major por￾tion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids, which remain at￾tached to each other at the centromere. The centromere is a point of constriction on the chromosome, containing a specific DNA sequence to which is bound a disk of protein called a kinetochore. This disk functions as an attachment site for fibers that assist in cell division (figure 11.9). Each chromosome’s centromere is located at a characteristic site. The cell grows throughout interphase. The G1 and G2 segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cell’s DNA replicates only during the S phase of the cell cycle. After the chromosomes have replicated in S phase, they remain fully extended and uncoiled. This makes them invis￾ible under the light microscope. In G2 phase, they begin the long process of condensation, coiling ever more tightly. Special motor proteins are involved in the rapid final conden￾sation of the chromosomes that occurs early in mitosis. Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. In animal cells, a pair of microtubule￾organizing centers called centrioles replicate. All eukary￾otic cells undertake an extensive synthesis of tubulin, the protein of which microtubules are formed. Interphase is that portion of the cell cycle in which the chromosomes are invisible under the light microscope because they are not yet condensed. It includes the G1, S, and G2 phases. In the G2 phase, the cell mobilizes its resources for cell division. Chapter 11 How Cells Divide 213 Metaphase chromosome Kinetochore Kinetochore microtubules Centromere region of chromosome Chromatid FIGURE 11.9 Kinetochores. In a metaphase chromosome, kinetochore microtubules are anchored to proteins at the centromere. A Vocabulary of Cell Division chromatin The complex of DNA and proteins of which eukaryotic chromosomes are composed. chromosome The structure within cells that contains the genes. In eukaryotes, it consists of a single linear DNA molecule as￾sociated with proteins. The DNA is repli￾cated during S phase, and the replicas sepa￾rated during M phase. cytokinesis Division of the cytoplasm of a cell after nuclear division. euchromatin The portion of a chromo￾some that is extended except during cell di￾vision, and from which RNA is transcribed. heterochromatin The portion of a chro￾mosome that remains permanently con￾densed and, therefore, is not transcribed into RNA. Most centromere regions are heterochromatic. homologues Homologous chromosomes; in diploid cells, one of a pair of chromo￾somes that carry equivalent genes. kinetochore A disk of protein bound to the centromere and attached to micro￾tubules during mitosis, linking each chro￾matid to the spindle apparatus. microtubule A hollow cylinder, about 25 nanometers in diameter, composed of sub￾units of the protein tubulin. Microtubules lengthen by the addition of tubulin subunits to their end(s) and shorten by the removal of subunits. mitosis Nuclear division in which repli￾cated chromosomes separate to form two genetically identical daughter nuclei. When accompanied by cytokinesis, it produces two identical daughter cells. nucleosome The basic packaging unit of eukaryotic chromosomes, in which the DNA molecule is wound around a cluster of histone proteins. Chromatin is composed of long strings of nucleosomes that resemble beads on a string. binary fission Asexual reproduction of a cell by division into two equal or nearly equal parts. Bacteria divide by binary fission. centromere A constricted region of a chromosome about 220 nucleotides in length, composed of highly repeated DNA sequences (satellite DNA). During mitosis, the centromere joins the two sister chro￾matids and is the site to which the kineto￾chores are attached. chromatid One of the two copies of a replicated chromosome, joined by a single centromere to the other strand

Mitosis Chromosome Prophase: Formation of the Mitotic Apparatus When the chromosome condensation initiated in G2 phase reaches the point at which individual condensed chromo- omes first become visible with the light microscope rst stage of mitosis, prophase, has begun. The condensa tion process continues throughout prophase; consequently some chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bear Metaphase ing the rRNA genes is condensed Assembling the Spindle Apparatus. The assembly of the microtubular apparatus that will later separate the sister chromatids also continues during prophase. In ani- mal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules called the spindle apparatus between them. In plant cells,a similar bridge of microtubular fibers forms between op posite poles of the cell, although centrioles are absent in plant cells During the formation of the spindle apparatus, the nu clear envelope breaks down and the endoplasmic reticulum IGURE 11.10 reabsorbs its components. At this point, then, the micro- Metaphase In metaphase, the chromosomes array themselves in tubular spindle fibers extend completely across the cell, a circle around the spindle midpoint. from one pole to the other. Their orientation determines he plane in which the cell will subsequently divide hrough the center of the cell at right angles to the spindle apparatus. trous. The attachment of the two sides of a centromere In animal cell mitosis, the centrioles extend a radial to the same pole, for example, leads to a failure of the array of microtubules toward the plasma membrane when ter chromatids to separate, so that they end up in the they reach the poles of the cell. This arrangement of mi- same daughter cell crotubules is called an aster. Although the aster's func- tion is not fully understood, it probably braces the centri oles against the membrane and stiffens the point of Metaphase: Alignment of the Centromeres microtubular attachment during the retraction of the The second stage of mitosis, metaphase, is the phase spindle. Plant cells, which have rigid cell walls, do not where the chromosomes align in the center of the cell form asters When viewed with a light microscope, the chromosomes appear to array themselves in a circle along the inner cir Linking Sister Chromatids to Opposite Poles. Each cumference of the cell, as the equator girdles the earth(fig- chromosome possesses two kinetochores, one attached to ure 11.10). An imaginary plane perpendicular to the axis of the centromere region of each sister chromatid(see fig- the spindle that passes through this circle is called the ure 11.9). As prophase continues, a second group of mi- metaphase plate. The metaphase plate is not an actual struc- crotubules appears to grow from the poles of the cell to- ture, but rather an indication of the future axis of cell divi ward the centromeres. These microtubules connect the ion. Positioned by the microtubules attached to the kine- kinetochores on each pair of sister chromatids to the two tochores of their centromeres, all of the chromosomes line poles of the spindle. Because microtubules extending up on the metaphase plate(figure 11. 11). At this point, from the two poles attach to opposite sides of the cen- which marks the end of metaphase, their centromeres are tromere,they attach one sister chromatid to one pole and neatly arrayed in a circle, equidistant from the two poles of the other sister chromatid to the other pole. This the cell, with microtubules extending back towards the arrangement is absolutely critical to the process of mito- posite poles of the cell in an arrangement called a spi sis; any mistakes in microtubule positioning can be disas because of its shape 214 Part IV Reproduction and Heredity

Mitosis Prophase: Formation of the Mitotic Apparatus When the chromosome condensation initiated in G2 phase reaches the point at which individual condensed chromo￾somes first become visible with the light microscope, the first stage of mitosis, prophase, has begun. The condensa￾tion process continues throughout prophase; consequently, some chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bear￾ing the rRNA genes is condensed. Assembling the Spindle Apparatus. The assembly of the microtubular apparatus that will later separate the sister chromatids also continues during prophase. In ani￾mal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers. By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules called the spindle apparatus between them. In plant cells, a similar bridge of microtubular fibers forms between op￾posite poles of the cell, although centrioles are absent in plant cells. During the formation of the spindle apparatus, the nu￾clear envelope breaks down and the endoplasmic reticulum reabsorbs its components. At this point, then, the micro￾tubular spindle fibers extend completely across the cell, from one pole to the other. Their orientation determines the plane in which the cell will subsequently divide, through the center of the cell at right angles to the spindle apparatus. In animal cell mitosis, the centrioles extend a radial array of microtubules toward the plasma membrane when they reach the poles of the cell. This arrangement of mi￾crotubules is called an aster. Although the aster’s func￾tion is not fully understood, it probably braces the centri￾oles against the membrane and stiffens the point of microtubular attachment during the retraction of the spindle. Plant cells, which have rigid cell walls, do not form asters. Linking Sister Chromatids to Opposite Poles. Each chromosome possesses two kinetochores, one attached to the centromere region of each sister chromatid (see fig￾ure 11.9). As prophase continues, a second group of mi￾crotubules appears to grow from the poles of the cell to￾ward the centromeres. These microtubules connect the kinetochores on each pair of sister chromatids to the two poles of the spindle. Because microtubules extending from the two poles attach to opposite sides of the cen￾tromere, they attach one sister chromatid to one pole and the other sister chromatid to the other pole. This arrangement is absolutely critical to the process of mito￾sis; any mistakes in microtubule positioning can be disas￾trous. The attachment of the two sides of a centromere to the same pole, for example, leads to a failure of the sis￾ter chromatids to separate, so that they end up in the same daughter cell. Metaphase: Alignment of the Centromeres The second stage of mitosis, metaphase, is the phase where the chromosomes align in the center of the cell. When viewed with a light microscope, the chromosomes appear to array themselves in a circle along the inner cir￾cumference of the cell, as the equator girdles the earth (fig￾ure 11.10). An imaginary plane perpendicular to the axis of the spindle that passes through this circle is called the metaphase plate. The metaphase plate is not an actual struc￾ture, but rather an indication of the future axis of cell divi￾sion. Positioned by the microtubules attached to the kine￾tochores of their centromeres, all of the chromosomes line up on the metaphase plate (figure 11.11). At this point, which marks the end of metaphase, their centromeres are neatly arrayed in a circle, equidistant from the two poles of the cell, with microtubules extending back towards the op￾posite poles of the cell in an arrangement called a spindle because of its shape. 214 Part IV Reproduction and Heredity Chromosome Centrioles Metaphase plate Aster Spindle fibers FIGURE 11.10 Metaphase. In metaphase, the chromosomes array themselves in a circle around the spindle midpoint