1 of genome yielded 6o000 genes based on full genome

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Unformatted text preview: tion) Human! Yeast! Fruit fly! Maize! E. coli! (1) (2) Given all of this repetitive DNA, how many genes are in the human genome? Before considering the non- coding repetitive fraction, we came up with 118,500 genes If repetitive sequences constitute ~40% of the genome, perhaps there are 70,000 genes Looking at all of the different mRNAs produced, estimates ranged from 35,000 to 140,000 genes Extrapolations from sequencing human chromosome 22 (679 genes, spans 1.1% of genome) yielded 6o,000 genes Based on full genome sequencing and comparative analysis, the human gene catalog contains 20,488 protein- coding genes Now things begin to make sense: Resolving the C- value paradox There are several classes of repetitive sequences in a genome, and many are transposable genetic elements A transposable element is a sequence that can move to new genomic locations, often leaving a copy in the original location (pp. 576-582; Box.15.1) Parasitic elements. Transposable elements typically encode for enzymes that will replicate themselves. They can also work passively, through recombinance and crossing over and increasing in number through chance. Why is there so much repetitive DNA in a genome? What controls the amount? Transposable elements are viewed as selfish (or parasitic) genes that can replicate and transmit themselves at the expense of the host genome (they provide a good example of selection acting at different levels) Repetitive DNA can also increase by a passive process: Unequal crossing over Variation in genome size can also result from differences among organisms in their rates of deleting DNA Genome Size and Contents - Major points Cellular organisms vary over 1,000,000- fold in overall genome size, but only about 200- fold...
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