Posted: May 23, 2016 1:40 pm
jamest wrote:I've already mentioned it. I was under the impression that each gene or combination thereof corresponded to particular biological structures and/or functions within an organism. Is that correct? If so, then I wonder why/how small and relatively simple organisms could have many more genes than larger and much more complex organisms. That's all I'm curious about here.
That's not entirely correct. Large sections of the genome are spam which doesn't contribute to the development of the organism. It's built up over the time being copied over and over again, with occasional copying errors increasing the quantity of spam. As long as it doesn't impede the function of the genetic material that does contribute to development it will continue being replicated.
Also, viruses reproduce by hijacking a host cell and making it replicate their genetic material. Sometimes viral genetic material is incorporated into the host DNA on a permanent basis and, like the spam, it goes on being replicated. These are called proviruses.
The addition of spam and proviruses into a genome is useful in telling the extent to which different genomes are related, since they enter at a given point on the evolutionary tree and are present in some branches but not others.