7 replies to this topic

[mr02077]

Hi. I am a mathematician who just likes reading on molecular biology.

My question is:
why does meiosis needs chromosomes replication in S phase? Why would not it have been developed through evolution, for example, just to do recombination between homologous chromosomes right away and split the recombined chromosomes to get two haploid cells?

Meosis is uniform in the way it functions for all multicellulals, where it is used, so it must be that the current process details are advantegous to other combinations.

One benefit I would see is production of more gametes, which deals with two-fold cost of sex better. But again, the cell could be replicating genes for 2 or more times, in meiosis, to be even more efficient, but only this unique model with single replication is ubiqutous.
What are the exact benefits of it?

I hope you understand my question well. Thank you for helping

Edited by mr02077, 14 September 2018 - 07:28 AM.

[pito]

Hi. I am a mathematician who just likes reading on molecular biology.

My question is:
why does meiosis needs chromosomes replication in S phase? Why would not it have been developed through evolution, for example, just to do recombination between homologous chromosomes right away and split the recombined chromosomes to get two haploid cells?

Meosis is uniform in the way it functions for all multicellulals, where it is used, so it must be that the current process details are advantegous to other combinations.

One benefit I would see is production of more gametes, which deals with two-fold cost of sex better. But again, the cell could be replicating genes for 2 or more times, in meiosis, to be even more efficient, but only this unique model with single replication is ubiqutous.
What are the exact benefits of it?

I hope you understand my question well. Thank you for helping

Not sure what you mean, but you say that it is rather strange to have a duplication (followed by the meiosis) rather than not have duplication and just have the homologues split?

[mr02077]

The duplication is a part of the meiosis, but you are right, the question is: why would not it do homologous recombination in a diploid cell and do the split to two haploid gametes?

Edited by mr02077, 15 September 2018 - 02:09 AM.

[pito]

The duplication is a part of the meiosis, but you are right, the question is: why would not it do homologous recombination in a diploid cell and do the split to two haploid gametes?

It is not really "part of" the meiosis. It happens before it, another phase, but yeah,you say it is part of it.

 

Why not just homologues recombination in a diploid cell and then split?

very simple: you have less "shuffling" of genes if you do it like that.

The fact that you have 2 divisions will generate more differences! 

You mix the genetic material more like this.

[Trof]

I think the cell division might just be so linked to replication, that it is required for meiosis as well.The centromere organization and all the spindle aparatus required to make a cell division precise would be difficult to form on a haploid chromosome, where is naturaly not formed.

[mr02077]

Apologies for replying so late, the reason was my long travelling.

@Trof, I think you brought quite a valid point. I myself did not pay attention to the detail that existense of centromeres is@exactly the difference between the scenario that I described and how it actually develops in the nature.

@Pito, it is just quantitatively bigger number of combinations, mathematically it is not - whatever combination you get at the end of meiosis, you could get by recombination between homologous chromosomes right away.

If it was up to as many combinations as possible it could be 2, 3 or more replications. But everywhere in the nature meiosis works exactly the same - one replication, 2 divisions, 4 resulting genotype combinations. So it has its causative reasons.

[pito]

Apologies for replying so late, the reason was my long travelling.

@Trof, I think you brought quite a valid point. I myself did not pay attention to the detail that existense of centromeres is@exactly the difference between the scenario that I described and how it actually develops in the nature.

@Pito, it is just quantitatively bigger number of combinations, mathematically it is not - whatever combination you get at the end of meiosis, you could get by recombination between homologous chromosomes right away.

If it was up to as many combinations as possible it could be 2, 3 or more replications. But everywhere in the nature meiosis works exactly the same - one replication, 2 divisions, 4 resulting genotype combinations. So it has its causative reasons.

 

Not sure I get you or you got me.

 

If you have 2 divisions you have 2 chances of getting a recombination of genes.

With 1 , you have only 1 divisions, so les chance of having different combinations.

You have a more diverse "offspring" when you use 2 divisions compared to 1.

Not sure how you can say that this is not true mathematically.
 

1 division= 2 gametes

2 divions = 4 gametes.

 

4 > 2 ... so not sure what you mean with it is the same mathematically.

 

 

Also: homologues recombination is not the same als the splitting of the homologues.

 

 

And: why does it have meiosis and 2 divisions and not 2 times meiosis or X times and X divisions to have even more recombination (and gametes?) => every system is limited , it is a trait off between more "recombination" (more options) and more chance in mutations.

The more you mess around (more divisions and so on) the more chances you have for mutations.

 

So nature just somehow evolved to a system that gave the best option.

 

However (!) not all organisms work with meiosis. Certain fungi for example do not use this system and have a system more like you are explaining.

And it turns out their rate of recombination is higher! They have a lot more of (random) recombinations, so even more differences in their genes for the offspring.... And it turns out that for those organisms it is a HUGE benefit because they have so many different daughter cells that there will always be a few adapted to certain conditions.

The disadvantage is however also a huge loss of cells that got stuck with a bad set of genes.

 

Imagine this in humans => with our rate of an average of 2-3 babies per family we wouldnt survive for long... however for those fungi it doesn't matter because they make millions of "babies" so they can afford to have this type of recombination.

 

 

One can question whether the start of meiosis (sexual reproduction) is part of the evolution towards humans....

[pito]

I think the cell division might just be so linked to replication, that it is required for meiosis as well.The centromere organization and all the spindle aparatus required to make a cell division precise would be difficult to form on a haploid chromosome, where is naturaly not formed.

 

But this part of a bigger discussion.

 

Does one follow from the other one?

Consequence or prerequisite?