Genome evolution

Genome evolution is the process by which a genome changes in structure (sequence) or size over time. The study of genome evolution involves multiple fields such as structural analysis of the genome, the study of genomic parasites, gene and ancient genome duplications, polyploidy, and comparative genomics. Genome evolution is a constantly changing and evolving field due to the steadily growing number of sequenced genomes, both prokaryotic and eukaryotic, available to the scientific community and the public at large.

Since the first sequenced genomes became available in the late 1970s, scientists have been using comparative genomics to study the differences and similarities between various genomes. Genome sequencing has progressed over time to include more and more complex genomes including the eventual sequencing of the entire human genome in 2001. By comparing genomes of both close relatives and distant ancestors the stark differences and similarities between species began to emerge as well as the mechanisms by which genomes are able to evolve over time.

Prokaryotic genomes have two main mechanisms of evolution: mutation and horizontal gene transfer. A third mechanism, sexual reproduction, prominent in eukaryotes, is not found in bacteria although prokaryotes can acquire novel genetic material through the process of bacterial conjugation in which both plasmids and whole chromosomes can be passed between organisms. An often cited example of this process is the transfer of antibiotic resistance utilizing plasmid DNA. Another mechanism of genome evolution is provided by transduction whereby bacteriophages introduce new DNA into a bacterial genome.

Genome evolution in bacteria is well understood because of the thousands of completely sequenced bacterial genomes available. Genetic changes may lead to both increases or decreases of genomic complexity due to adaptive genome streamlining and purifying selection. In general, free-living bacteria have evolved larger genomes with more genes so they can adapt more easily to changing environmental conditions. By contrast, most parasitic bacteria have reduced genomes as their hosts supply many if not most nutrients, so that their genome does not need to encode for enzymes that produce these nutrients themselves.

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