Genome evolution
[citation needed] Prokaryotic genomes have two main mechanisms of evolution: mutation and horizontal gene transfer.
Prokaryotes can acquire novel genetic material through the process of bacterial conjugation in which both plasmids and whole chromosomes can be passed between organisms.
The main mechanism of sexual interaction is natural genetic transformation which involves the transfer of DNA from one prokaryotic cell to another though the intervening medium.
[6] In general, free-living bacteria have evolved larger genomes with more genes so they can adapt more easily to changing environmental conditions.
The non-coding portions of the gene, known as introns, which are largely not present in prokaryotes, are removed by RNA splicing before translation of the protein can occur.
In other words, the genome size is much larger than would be expected given the total number of protein coding genes.
A famous example for such gene decay is the genome of Mycobacterium leprae, the causative agent of leprosy.
It is beneficial to an organism to rid itself of non-essential genes because it makes replicating its DNA much faster and requires less energy.
With the addition of more and more repeats to these regions the plants increase the possibility of developing new virulence factors through mutation and other forms of genetic recombination.
[21] In 1997, Wolfe & Shields gave evidence for an ancient duplication of the Saccharomyces cerevisiae (Yeast) genome.
Wolfe & Shields hypothesized that this was actually the result of an entire genome duplication in the yeast's distant evolutionary history.
Based on these observations, they determined that Saccharomyces cerevisiae underwent a whole genome duplication soon after its evolutionary split from Kluyveromyces, a genus of ascomycetous yeasts.
The "cut-and-paste" mechanism works by excising DNA from one place in the genome and inserting itself into another location in the code.
Such changes can lead to a frameshift mutation, causing the entire code to be read in a different order from the original, often resulting in a protein becoming non-functional.
This can result in a shift of reading frame, causing the gene to no longer code for the expected protein, introduce a premature stop codon or a mutation in the promoter region.
[32] Often cited examples of pseudogenes within the human genome include the once functional olfactory gene families.
No attempt to grow symbiotic P. necessarius outside their hosts has yet been successful, strongly suggesting that the relationship is obligate for both partners.
The genomes of 5 species have revealed that both the sequences but also the expression pattern of many genes has quickly changed over a relatively short period of time (100,000 to several million years).
Given that gene expression is driven by short regulatory sequences, this demonstrates that relatively few mutations are required to drive speciation.
The cichlid genomes also showed increased evolutionary rates in microRNAs which are involved in gene expression.
It has been hypothesized that as the first organisms evolved in a high-heat and pressure environment they needed the stability of these GC bonds in their genetic code.
De novo origin of (protein-coding) genes only requires two features, namely the generation of an open reading frame, and the creation of a transcription factor binding site.
For instance, Levine and colleagues reported the origin of five new genes in the D. melanogaster genome from noncoding DNA.
[49] For instance, Wu et al. (2011) reported 60 putative de novo human-specific genes all of which are short consisting of a single exon (except one).
RNA is composed of purine and pyrimidine nucleotides, both of which are necessary for reliable information transfer, and thus Darwinian natural selection and evolution.
Nam et al.[52] demonstrated the direct condensation of nucleobases with ribose to give ribonucleosides in aqueous microdroplets, a key step leading to formation of the RNA genome.
Also, a plausible prebiotic process for synthesizing pyrimidine and purine ribonucleotides leading to genome formation using wet-dry cycles was presented by Becker et al.[53]
