While every living organism has a genome of some kind (be it RNA, or DNA), there is considerable variation in the sizes of these blueprints. One major factor that impacts genome size is whether the organism is prokaryotic or eukaryotic. In prokaryotes, the genome contains little to no non-coding sequence, such that genes are tightly clustered in groups or operons sequentially along the chromosome. Conversely, the genes in eukaryotes are punctuated by long stretches of non-coding sequence. Overall, this contributes to the phenomena that prokaryotic genomes tend to be smaller (i.e. contain fewer bases) on average than those of eukaryotes.
Unsurprisingly, given this observation, the smallest known genomes are mostly prokaryotes. Candidatus Carsonella rudii, for example, is a highly simplified proteobacterium which has a genome size of just 160 thousand base-pairs. Having lost many genes that it needed to synthesize life-essential proteins, it has evolved to be an obligate intracellular symbiont. At the opposite end of the spectrum, the eukaryote Japanese flowering plant Paris japonica is one of the largest known genomes, at around 150 billion base-pairs. Although the number of genes this encodes isn’t known, the genome shows vast amounts of duplication and non-coding sequence.
Within the genome of an average prokaryote there are roughly 3,000 genes. The average eukaryote has somewhere in the region of 20,000. But the genome size, especially in eukaryotes, is wildly variable - in large part due to the amount of non-coding sequence.
The Creation of New Genes
In order to evolve new genes, organisms have a few main options. The one thing most of these have in common is that they modify sequences that already exist.
Duplication plays an important role in creating new genes, and there are a few types of duplication that can result in these novel sequences. In gene duplication, a section of DNA containing a gene is duplicated. This second copy does not face the selection pressure which constrains the first, and so it can diverge. In time, this can lead to the evolution of novel genes, with new roles.
Another type of duplication - DNA shuffling - can result in just a section of a gene being duplicated and joining another gene. This can result in the creation of novel genes, with novel products.
Sometimes new genes simply evolve from accumulated mutations over time. This is known as intragenic mutation, and is most noticeable when comparing across species or divergent populations.
Finally, it is also possible to obtain new genes from external sources, in a process known as horizontal gene transfer. This means genetic material can be incorporated from other individuals, sometimes of the same species, but also potentially from another species entirely. This is a frequent source of novel genes in prokaryotes and archaea. It is less common in eukaryotes, but has been shown to occur, and eukaryotes can even pick up genetic information from sources as distant as bacteria or fungi.
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