8885d_c24_920-9472/11/041:36 PM Page924mac76mac76:385 924 Chapter 24 Genes and Chromosomes description of the coiling of DNA molecules. Finally, we catalytic function. DNA also contains other segments or discuss the protein-DNA interactions that organize sequences that have a purely regulatory function. Reg hromosomes into compact structures ulatory sequences provide signals that may denote the beginning or the end of genes, or influence the tran scription of genes, or function as initiation points for 24.1 Chromosomal Elements replication or recombination(Chapter 28). Some genes can be expressed in different ways to generate multiple Cellular DNA contains genes and intergenic both of which may serve functions vital to the IS, gene products from one segment of DNA. The special le transcriptional and translational mechanisms that allow more complex genomes, such as those of C this are described in Chapters 26 through 28 cells, demand increased levels of chromosomal organi- We can make direct estimations of the minimum zation, and this is reflected in the chromosomes struc verall size of genes that encode proteins. As described tural features. We begin by considering the ditterent in detail in Chapter 27, each amino acid of a polypep- types of DNA sequences and structural elements within tide chain is coded for by a sequence of three consec- utive nucleotides in a single strand of DNA(Fig. 24-2) with these"codons"arranged in a sequence that corre- Genes Are Segments of DNA That Code sponds to the sequence of amino acids in the polypep- for Polypeptide Chains and RNAs tide that the gene encodes. a polypeptide chain of 350 amino acid residues(an average-size chain)corre- Our understanding of genes has evolved tremendot over the last century. Classically, a gene was defined as a portion of a chromosome that determines or affects a single character or phenotype(visible property), such DNA mRNA Polypeptide as eye color. George Beadle and Edward Tatum proposed a molecular definition of a gene in 1940. After exposing spores of the fungus Neurospora crassa to x rays and TIlIA other agents known to damage dNa and cause alterations in DNA sequence(mutations ), they detected mutant fungal strains that lacked one or another specific en- zyme, sometimes resulting in the failure of an entire TIIA metabolic pathway. Beadle and Tatum concluded that a gene is a segment of genetic material that determines or codes for one enzyme: the one gene-one enzym hypothesis. Later this concept was broadened to one TIIA gene-one polypeptide, because many genes code for proteins that are not enzymes or for one polypeptide of UACACUUUUG TIIA U a multisubunit protein. The modern biochemical definition of a gene is even more precise. a gene is all the dna that encodes the primary sequence of some final gene product, which can be either a polypeptide or an RNa with a structural or TIIA TIIA TIIA GCcGUUUCU CIlI TIIA termi Template strand FIGURE 24-2 Colinearity of the coding nucleotide sequences of DNA and mRNA and the amino acid sequence of a polypeptide chain. a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons)complementary to those of the DNA. In some bacte. orge W. Bead award L. Tatum al and many eukaryotic genes, coding sequences are interrupted at 1903-198 1909-1975 tervals by regions of noncoding sequences(called introns)description of the coiling of DNA molecules. Finally, we discuss the protein-DNA interactions that organize chromosomes into compact structures. 24.1 Chromosomal Elements Cellular DNA contains genes and intergenic regions, both of which may serve functions vital to the cell. The more complex genomes, such as those of eukaryotic cells, demand increased levels of chromosomal organi￾zation, and this is reflected in the chromosome’s struc￾tural features. We begin by considering the different types of DNA sequences and structural elements within chromosomes. Genes Are Segments of DNA That Code for Polypeptide Chains and RNAs Our understanding of genes has evolved tremendously over the last century. Classically, a gene was defined as a portion of a chromosome that determines or affects a single character or phenotype (visible property), such as eye color. George Beadle and Edward Tatum proposed a molecular definition of a gene in 1940. After exposing spores of the fungus Neurospora crassa to x rays and other agents known to damage DNA and cause alterations in DNA sequence (mutations), they detected mutant fungal strains that lacked one or another specific en￾zyme, sometimes resulting in the failure of an entire metabolic pathway. Beadle and Tatum concluded that a gene is a segment of genetic material that determines or codes for one enzyme: the one gene–one enzyme hypothesis. Later this concept was broadened to one gene–one polypeptide, because many genes code for proteins that are not enzymes or for one polypeptide of a multisubunit protein. The modern biochemical definition of a gene is even more precise. A gene is all the DNA that encodes the primary sequence of some final gene product, which can be either a polypeptide or an RNA with a structural or catalytic function. DNA also contains other segments or sequences that have a purely regulatory function. Reg￾ulatory sequences provide signals that may denote the beginning or the end of genes, or influence the tran￾scription of genes, or function as initiation points for replication or recombination (Chapter 28). Some genes can be expressed in different ways to generate multiple gene products from one segment of DNA. The special transcriptional and translational mechanisms that allow this are described in Chapters 26 through 28. We can make direct estimations of the minimum overall size of genes that encode proteins. As described in detail in Chapter 27, each amino acid of a polypep￾tide chain is coded for by a sequence of three consec￾utive nucleotides in a single strand of DNA (Fig. 24–2), with these “codons” arranged in a sequence that corre￾sponds to the sequence of amino acids in the polypep￾tide that the gene encodes. A polypeptide chain of 350 amino acid residues (an average-size chain) corre- 924 Chapter 24 Genes and Chromosomes George W. Beadle, 1903–1989 Edward L. Tatum, 1909–1975 U C U A G A C G U G C A G G A C C T U A C A T G A C U T G A U U U A A A G C C C G G G U U C A A 5 3 3 5 DNA mRNA T C T C G T G G A T A C A C T T T T G C C G T T 3 5 Arg Gly Tyr Thr Phe Ala Val Ser Carboxyl terminus Amino terminus Polypeptide Template strand FIGURE 24–2 Colinearity of the coding nucleotide sequences of DNA and mRNA and the amino acid sequence of a polypeptide chain. The triplets of nucleotide units in DNA determine the amino acids in a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons) complementary to those of the DNA. In some bacte￾rial and many eukaryotic genes, coding sequences are interrupted at intervals by regions of noncoding sequences (called introns). 8885d_c24_920-947 2/11/04 1:36 PM Page 924 mac76 mac76:385_reb: