基因Ⅸ Chapter 5.pdfVIP

Genome Sequences and Gene Numbers CHAPTER OUTLINE .. Introduction • Most of the genes that are unique to vertebrates are con- . cerned with the immune or nervous systems . .. Bacterial Gene Numbers Range Over an Order How Many Genes Are Essential? of Magnitude • Not all genes are essential. In yeast and fly, deletions of Total Gene Number Is Known for Several Eukaryotes 50% of the genes have detectable effects. • There are 6000 genes in yeast; 18,500 in a worm; 13,600 in • When two or more genes are redundant, a mutation in any a fly; 25,000 in the small plant Arabidopsis; and probably one of them may not have detectable effects. 20,000 to 25,000 in mouse and man. • We do not fully understand the survival in the genome of .. How Many Different Types of Genes Are There? genes that are apparently dispensable . .. The Human Genome Has Fewer Genes Than Expected Genes Are Expressed at Widely Differing Levels • Only 1% of the human genome consists of coding regions. • In any given cell, most genes are expressed at a low level. • The exons comprise -5% of each gene, so genes (exons plus