8885dc24920-9472/11/041:36 PM Page928mac76mac76:385 928 Chapter 24 Genes and Chromosomes TABLE 24-2 DNA Gene, and Chromosome Content in Some Genomes lotal DNA (bp) Number of chromosomes number of genes Bacterium(Escherichia coll) 4,639221 4.405 Yeast(Saccharomyces cerevisiae Nematode(Caenorhabditis elegans) 97.000000 19.000 Plant(Arabidopsis thaliana) 125,00000 25,500 Fruit fly(Drosophila melanogaster) 180,000.000 13,600 Plant(Oryza sativa; rice) 480.000000 24 57 Mouse(Mus musculus) 2500,00000 30,000-35,000 Human(Homo sapiens) 3.200000000 30000-35,000 Note: This information is constantly being refined. For the most current information, consult the websites for the individual genome project. The diploid chomosome number is ghen for all eukaryotes ecept yeast. Haploid chromosome number. Wild yeast strains generally have eight (octoploid)or more sets of these chromosomes INumber for females, with bo x chromosomes. Males have an x but no y thus 11 chromosomes in all. Mitochondrial dna codes for the mitochondrial trnas In higher eukaryotes, the typical gene has much and rRNAs and for a few mitochondrial proteins. More more intron sequence than sequences devoted to ex- than 95% of mitochondrial proteins are encoded by nu- ons. For example, in the gene coding for the single clear DNA Mitochondria and chloroplasts divide when polypeptide chain of the avian egg protein ovalbumin the cell divides. Their DNA is replicated before and dur-(Fig. 24-7), the introns are much longer than the ex- ng division, and the daughter dNa molecules pass into ons; altogether, seven introns make up 85% of the gene's he daughter organelles DNA. In the gene for the B subunit of hemoglobin, a sin- gle intron contains more than half of the genes dNA. Eukaryotic Genes and Chromosomes The gene for the muscle protein titin is the intron cham- Are Very Complex pion, with 178 introns. Genes for histones appear to have no introns. In most cases the function of introns is not Many bacterial species have only one chromosome per clear. In total, only about 1.5% of human dNa is"cod- cell and, in nearly all cases, each chromosome contains ing or exon DNA, carrying information for protein or only one copy of each gene. A very few genes, such as RNA products. However, when the much larger introns those for rRNAS, are repeated several times. Genes and are included in the count, as much as 30% of the hu- regulatory sequences account for almost all the dna in man genome consists of genes prokaryotes. Moreover, almost every gene is precisely The relative paucity of genes in the human genome colinear with the amino acid sequence (or RNA se- leaves a lot of DNa unaccounted for. Figure 24-8 quence) for which it codes(Fig. 24-2) provides a summary of sequence types. Much of the The organization of genes in eukaryotic DNA is nongene DNA is in the form of repeated sequences of tructurally and functionally much more complex. The several kinds. Perhaps most surprising, about half the study of eukaryotic chromosome structure, and more human genome is made up of moderately repeated se- recently the sequencing of entire eukaryotic genomes, quences that are derived from transposable elements- has yielded many surprises. Many, if not most, eukary- segments of DNA, ranging from a few hundred to sev- otic genes have a distinctive and puzzling structural eral thousand base pairs long, that can move from one feature: their nucleotide sequences contain one or more location to another in the genome. Transposable ele intervening segments of DNa that do not code for the ments(transposons) are a kind of molecular parasite, amino acid sequence of the polypeptide product. These efficiently making a home within the host genome. Many nontranslated inserts interrupt the otherwise colinear have genes encoding proteins that catalyze the trans- relationship between the nucleotide sequence of the position process, described in more detail in Chapters gene and the amino acid sequence of the polypeptide it 25 and 26. Some transposons in the human genome are encodes. Such nontranslated DNA segments in genes active, moving at a low frequency, but most are inactive re called intervening sequences or introns, and the relics, evolutionarily altered by mutations. Although coding segments are called exons. Few prokaryotic these elements generally do not encode proteins or genes contain introns. RNAs that are used in human cells, they have played aMitochondrial DNA codes for the mitochondrial tRNAs and rRNAs and for a few mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nu￾clear DNA. Mitochondria and chloroplasts divide when the cell divides. Their DNA is replicated before and dur￾ing division, and the daughter DNA molecules pass into the daughter organelles. Eukaryotic Genes and Chromosomes Are Very Complex Many bacterial species have only one chromosome per cell and, in nearly all cases, each chromosome contains only one copy of each gene. A very few genes, such as those for rRNAs, are repeated several times. Genes and regulatory sequences account for almost all the DNA in prokaryotes. Moreover, almost every gene is precisely colinear with the amino acid sequence (or RNA se￾quence) for which it codes (Fig. 24–2). The organization of genes in eukaryotic DNA is structurally and functionally much more complex. The study of eukaryotic chromosome structure, and more recently the sequencing of entire eukaryotic genomes, has yielded many surprises. Many, if not most, eukary￾otic genes have a distinctive and puzzling structural feature: their nucleotide sequences contain one or more intervening segments of DNA that do not code for the amino acid sequence of the polypeptide product. These nontranslated inserts interrupt the otherwise colinear relationship between the nucleotide sequence of the gene and the amino acid sequence of the polypeptide it encodes. Such nontranslated DNA segments in genes are called intervening sequences or introns, and the coding segments are called exons. Few prokaryotic genes contain introns. In higher eukaryotes, the typical gene has much more intron sequence than sequences devoted to ex￾ons. For example, in the gene coding for the single polypeptide chain of the avian egg protein ovalbumin (Fig. 24–7), the introns are much longer than the ex￾ons; altogether, seven introns make up 85% of the gene’s DNA. In the gene for the
subunit of hemoglobin, a sin￾gle intron contains more than half of the gene’s DNA. The gene for the muscle protein titin is the intron cham￾pion, with 178 introns. Genes for histones appear to have no introns. In most cases the function of introns is not clear. In total, only about 1.5% of human DNA is “cod￾ing” or exon DNA, carrying information for protein or RNA products. However, when the much larger introns are included in the count, as much as 30% of the hu￾man genome consists of genes. The relative paucity of genes in the human genome leaves a lot of DNA unaccounted for. Figure 24–8 provides a summary of sequence types. Much of the nongene DNA is in the form of repeated sequences of several kinds. Perhaps most surprising, about half the human genome is made up of moderately repeated se￾quences that are derived from transposable elements— segments of DNA, ranging from a few hundred to sev￾eral thousand base pairs long, that can move from one location to another in the genome. Transposable ele￾ments (transposons) are a kind of molecular parasite, efficiently making a home within the host genome. Many have genes encoding proteins that catalyze the trans￾position process, described in more detail in Chapters 25 and 26. Some transposons in the human genome are active, moving at a low frequency, but most are inactive relics, evolutionarily altered by mutations. Although these elements generally do not encode proteins or RNAs that are used in human cells, they have played a 928 Chapter 24 Genes and Chromosomes TABLE 24–2 DNA, Gene, and Chromosome Content in Some Genomes Total DNA (bp) Number of Approximate chromosomes* number of genes Bacterium (Escherichia coli) 4,639,221 1 4,405 Yeast (Saccharomyces cerevisiae) 12,068,000 16† 6,200 Nematode (Caenorhabditis elegans) 97,000,000 12‡ 19,000 Plant (Arabidopsis thaliana) 125,000,000 10 25,500 Fruit fly (Drosophila melanogaster) 180,000,000 18 13,600 Plant (Oryza sativa; rice) 480,000,000 24 57,000 Mouse (Mus musculus) 2,500,000,000 40 30,000–35,000 Human (Homo sapiens) 3,200,000,000 46 30,000–35,000 Note: This information is constantly being refined. For the most current information, consult the websites for the individual genome projects. * The diploid chromosome number is given for all eukaryotes except yeast. † Haploid chromosome number. Wild yeast strains generally have eight (octoploid) or more sets of these chromosomes. ‡ Number for females, with two X chromosomes. Males have an X but no Y, thus 11 chromosomes in all. 8885d_c24_920-947 2/11/04 1:36 PM Page 928 mac76 mac76:385_reb: