Genomic characteristics of Drosophila melanogaster.

sequence among cloned fragments, making it difficult to assemble the clones in the correct order). Drosophila has more than 13,000 predicted genes. There are 14,113 RNA transcripts produced from these genes, with some genes encoding multiple transcripts through alternative splicing. Drosophila genes average four exons per gene, although this number is probably an underestimate. The average RNA molecule encoded by a gene is 3058 nucleotides in length.

Human genome The human genome is 3.4 billion base pairs in length (IFigure 19.27a). Only about 25% of the DNA is transcribed into RNA, and less than 2% actually encodes proteins (IFigure 19.27b). Active genes are often separated by vast deserts of noncoding DNA, much of which consists of repeated sequences derived from trans-posable elements.

The average gene in the human genome is approximately 27,000 bp in length, with about 9 exons. (Table 19.5). (One exceptional gene has 234 exons.) The introns of human genes are much longer, and there are more of them than in other genomes (I Figure 19.27c) The human genome does not encode substantially more protein domains (see Table 19.3), but the domains are combined in more ways to produce a relatively diverse proteome. Gene functions encoded by the human genome are presented in Figure 19.27b. A single gene often encodes multiple proteins through alternative splicing; each gene may encode, on the average, two or three different mRNAs, meaning that the human genome, with approximately 32,000 genes, might encode as many as 96,000 proteins.

Gene density varies among human chromosomes; chromosomes 17, 19, and 22 have the highest density and

(a) Homo sapiens (human)

(a) Homo sapiens (human)

24 pairs of linear chromosomes Genome size: 3.4 billion bp Number of genes: ~32,000 G + C content: 41%

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