Opener Chapter 24 – Genome Evolution

Comparative Genomes Powerful tool for exploring evolutionary divergence among organisms Footprints on the evolutionary path between different species Helped to differentiate between different kingdoms Comparison between human and pufferfish genome – first time for two vertebrates Shared a common ancestor 450 million years ago Some genes common were conserved and many are unique to both 97% of human genome is repetitive DNA but less than 1/6 of Fugu sequence has repetitive DNA

Comparison between human and mouse genomes Humans and mice diverged 75 million years ago Human genome shares 99% of its homology with mice (300 genes are unique to each) Comparison between human and chimpanzee genomes Humans and chimpanzees diverged 35 million years ago and show high homology DNA mutations: Non-synonymous changes alter amino acid codes but synonymous do not alter them Single nucleotide substitutions, insertions and deletions of DNA bases found

Comparative Genomes Differences in generation time accounts for different rates of genomic evolution Organisms with smaller generation time have faster evolution Rate of mutation in germ line of mice and humans is same for each generation Plant genes responsible for photosynthesis has not been found in animals Plants, animals and fungi have 70% homology which is responsible for replication, repair, transcription and translation

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How Polyploids guides studies of genome evolution? Paleopolyploids- study of ancient polyploids Sequence comparisons and phylogenetic tools establish the time and pattern of polyploidy events Presence or absence of duplicated gene pairs provide information about when both genome duplication and gene loss occurred All copies of duplicated gene pairs might not exist thousands or millions years after polyploidization

Fig With passage of time, duplicated gene pairs might/will get lost

Synthetic polyploids provide another source for understanding genome evolution Synthesized by crossing closely related plants similar to their ancestral species and then inducing chromosome doubling via chemicals Produces sterility in the plant

Fig Genome downsizing has occurred in M.truncatula

Fig Elimination or rapid loss of genes or whole chromosomes over a longer period of time or in short span of few generations

How Polyploids guides studies of genome evolution? - Polyploidization leads to a change in gene expression Leads to methylation of cytosine Leads to short-term gene silencing - Polyploidization can lead to jumping of transposon Transposon can move to new position in genome giving rise to new phenotypes In plants – generation of biodiversity and adaptations

Evolution within Genomes – by duplication Aneuploidy is duplication or loss of an individual chromosome rather than entire genome DNA segments may be duplicated which can lead to loss of function, gain of a novel function or have the total function divide into two parts. Paralogues and Orthologues

Fig Two midsized ape chromosomes fused to form a human chromosome – rearrangement of chromosomes

Fig Conservation of synteny – Information of conservation of genes/synteny allows researchers to locate a gene in different species

Fig Gene inactivation results in pseudogenes 70% of olfactory receptor (OR) genes are inactive pseudogenes Half of OR genes in chimpanzee and gorilla function effectively and 95% of OR genes of New World monkey and all mouse OR genes work very well Humans started relying on other senses reducing selection pressure against loss of OR gene function by random mutation Comparison of human and chimpanzee genomes show that Or genes are becoming pseudogenes although the number is higher in humans than in chimpanzees

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