Genomes and Their Evolution

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Genome Size

Genomes of most bacteria and archaea range from 1 to 6 million base pairs (Mb); genomes of eukaryotes are usually larger

Most plants and animals have genomes greater than 100 Mb; humans have 3,200 Mb

Within each domain there is no systematic relationship between genome size and phenotype

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Table 21-1

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Number of Genes

Free-living bacteria and archaea have 1,500 to 7,500 genes

Unicellular fungi have from about 5,000 genes and multicellular eukaryotes from 40,000 genes

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Number of genes is not correlated to genome size

For example, it is estimated that the nematode C. elegans has 100 Mb and 20,000 genes, while humans have 3,200 Mb and 20,488 genes

Vertebrate genomes can produce more than one polypeptide per gene because of alternative splicing of RNA transcripts

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Gene Density and Noncoding DNA

Humans and other mammals have the lowest gene density, or number of genes, in a given length of DNA

Multicellular eukaryotes have many introns within genes and noncoding DNA between genes

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Concept 21.4: Multicellular eukaryotes have much noncoding DNA and many multigene families

The bulk of most eukaryotic genomes consists of noncoding DNA sequences, often described in the past as “junk DNA”

Much evidence indicates that noncoding DNA plays important roles in the cell

For example, genomes of humans, rats, and mice show high sequence conservation for about 500 noncoding regions

Sequencing of the human genome reveals that 98.5% does not code for proteins, rRNAs, or tRNAs

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About 24% of the human genome codes for introns and gene-related regulatory sequences

Intergenic DNA is noncoding DNA found between genes

Pseudogenes are former genes that have accumulated mutations and are nonfunctional

Repetitive DNA is present in multiple copies in the genome

About three-fourths of repetitive DNA is made up of transposable elements and sequences related to them

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Fig. 21-7

Exons (regions of genes coding for protein

or giving rise to rRNA or tRNA) (1.5%)

Repetitive

DNA that

includes

transposable

elements

and related

sequences

(44%)