Question

What is the trend in eukaryotes when considering gene density and genome size?

Answer 1

A gene is a segment of DNA which expresses to produce RNA or a polypeptide that would perform certain function. Now gene density means a measure of number of genes per million base pair (Mb or Mega base). A human genome has a gene density of 12-15 genes per Mb.

Now the eukaryotes contain a larger genome than prokaryotes and are complex too. But surprisingly, larger genome size does not relate with higher complexity and this paradoxical statement could be resolved by learning that all the genes in the genome are not functional in nature and genes are present that do no not code for a meaningful protein at all. Due the ratio of higher non coding regions, the size of genome might be large but less complex in nature.

As said above, the introns or non coding regions are found in between the coding regions or exons. A splicing mechanism takes place to remove to non coding introns which leads to meaningful coding regions that will transcribe into a mRNA followed by protein synthesis.

Another factor contributing for large gene density of eukaryotes are the presence of pseudo genes which are multiple copies of the same gene encoding a protein. Another factor is the highly repetitive genes which could occupy loads of space the eukatyotic genome. This is not the case w.r.t prokaryotes are the small and simple.

Coming to the gene density, let us look at an example. The E.coli genome consists of 4.6*10^6 base pairs and contains about 4288 genes which means almost 90% of the gene is used for protein coding. Whereas c.elegans, a simple eukaryote consists of 97*10^6 base pairs but only 19000 genes are coded for proteins which means that only 25% of the gene containing exons are coded.

This is the trend in eukaryotes. However, genome is higher order animals are more complex and consists of greater number of non coding regions.