I've been working on this one for a while. I view it as the first step in making sexual reproduction a more realistic possibility. By itself it has no real worth (as evidenced by the fact that the number of chromosomes an organism has is in no way reflective of how advanced it is).
The only problem I haven't solved yet is how to create centromeres between chromosomes. (edit: okay, telomeres and oris I haven't decided are necessary or not yet.) I'd like to have the process be:
- somewhat spontaneous
- more likely to, but not exclusively so, occur between identical strands than two entirely different ones
- be somewhat epigenetic.
The above list is how actual centromeres seem to work. In any system we come up with, we have to be able to seperate chromosomes paired together (meiosis) and recombine different chromosomes with the same centromere signature.
Real centromeres seem to be somewhat linked with alpha-satellite DNA (long, repeating patterns of Junk DNA).
This is what I was thinking:
Two chromosomes may spontaneously develop a centromere if they have a large area of identical, repeating DNA patterns (an alpha-satellite). The more repititous and shorter this pattern, the more likely it is to occur.
Something like
add sub 4 > add sub 4 > add sub 4 > add sub 4 > add sub 4 > add sub 4 >
in two seperate chromosomes may spontaneosuly connect with each other and form a centromere.
add add add add add add add add add add add add add add add add add add add add
will almost certainly develop a centromere, and very quickly.
I haven't worked out what kind of function will determine the liklihood yet.
Centromeres will be defined as a number. Like numbers stick together, unlike numbers don't.
After a centromere has formed, it will stay in that portion of the DNA forever more ( or until it spontaneously dissapears or something, I'm not sure yet). I'll have to find a way to record it in the DNA but keep it from mutating (that centromere is a physical attachment to the DNA strand, not DNA itself). If the alpha-satellite DNA disappears, it will continue to have a centromere where it used to be. (This is the epigenetic aspect).
During reproduction, if a chromosome has more than one centromere, then it will break into different chromsome strands (where the breakage occurs is random), with each centromere forming it's own chromosome. Then, if a chromosome has a centrome, the chromosome it was attached to has a more or less guarenteed chance of arriving in the opposite cell that it does. Chromosomes without centromeres have a 50/50 of going into either cell.
That is, if a cell has:
1-1 2-2 3 3, where - is a centromere and numbers are each a seperate chromosome strand, and it reproduces, then the two daughters can look something like:
1-1 2-2 and 1-1 2-2 3 3 3 3
Later, I can add a crossing-over, and other chromosomal 'mutations' between chromosomes. Crossing over will help keep chromosomes largely similar, and help decrease the liklihood of centromeres forming on a chromosome that already has a centromere.