Purpose of Chromosome replication before meiosis?

[Spirochete]

During meiosis the cell duplicates all its chromosomes, then divides twice to ultimately end up with 4 haploid cells. What is the logic behind duplicating the chromosomes first? Couldn't the organism get all the genetic recombination benefits of meiosis (independent assortment, crossing over, etc) by simply having homologous chromosomes separate without duplicating themselves first? Obviously the only one division would be required.

 

[Monique]

It allows for the process of recombination to occur: during prophase I homologous chromosomes pair and can form synapses.

 

[Spirochete]

Yes but think about an alternative version where the cell didn't duplicate its DNA first. You still have homologous chromosomes which can form tetrads and cross over. They can still separate randomly during anaphase. A single cytokinesis lwould still lead to 2 haploid cells with rearranged genetic information.

I actually did some more reading in a good cell biology book and it turns out my idea is a plausible alternative and in fact nobody knows why it doesn't happen this way.

 

[DaveC426913]

Yes but think about an alternative version where the cell didn't duplicate its DNA first. You still have homologous chromosomes which can form tetrads and cross over. They can still separate randomly during anaphase. A single cytokinesis lwould still lead to 2 haploid cells with rearranged genetic information.

I actually did some more reading in a good cell biology book and it turns out my idea is a plausible alternative and in fact nobody knows why it doesn't happen this way.

Because evolution isn't goal-driven?

 

[jim mcnamara]

Seems to me this is a teleological argument - where teleology is defined as "the cause and direction of changes in phenomena are determined by a previously existing plan or purpose, as opposed to mechanism"

In other words, exactly what Dave said. Evolution does not strive toward a purpose beforehand. Ain't no "pre-planned" purpose for meiosis.

 

[Spirochete]

Seems to me this is a teleological argument - where teleology is defined as "the cause and direction of changes in phenomena are determined by a previously existing plan or purpose, as opposed to mechanism"

In other words, exactly what Dave said. Evolution does not strive toward a purpose beforehand. Ain't no "pre-planned" purpose for meiosis.

Yes but often there might be a hidden logic to a process, or at least some kind of partial logic which is built into another arbitrary frame work. A random example of this would be 5' to 3' DNA replication. At first it seems like a random quirk of evolution. In fact there's a reason: the 5' end of a nucleotide carries the "high energy" phosphate bond. During DNA replication, 3' exonuclease activity by DNA polymerase occasionally chops nucleotides off the growing strand. If the growing strand contained the phosphate bond, editing by Pol III would remove the energy source for polymerization.

Of course this leads to a whole other set of "why" questions about the process which are better answered by your responses that evolution isn't goal driven. I would speculate that as processes became more complex during evolutionary history, they become more and more susceptible to so called "mistakes" of evolution which constrain their efficiency.

 

[BoomBoom]

I would speculate that as processes became more complex during evolutionary history, they become more and more susceptible to so called "mistakes" of evolution which constrain their efficiency.

I am not sure I would characterize the duplication of chromosomes in meiosis that leads to crossover and greater genetic diversity as a "mistake". The process seems to work just fine as it is and has for some time now.

It seems the "mechanism" that is in place to allow for crossover to occur requires 2 sets of homologous chromosomes. There are probably very good reasons for this process that you may be overlooking...perhaps to prevent accidental crossover from occurring during mitosis? (That could create problems.) Also, this process probably increases the variability of all the daughter cells since the possible number of genetic exchanges are doubled.

 

[PedroAndrade]

Not sure if I'm saying anything really wrong, but meiosis as a process could have "descended" from mitosis. It's more parsimonious for this process to emerge from a pre-existing one, rather than being a new thing altogether, and so keeps some of its trademarks (in this case, DNA replication).

 

[BoomBoom]

Not sure if I'm saying anything really wrong, but meiosis as a process could have "descended" from mitosis. It's more parsimonious for this process to emerge from a pre-existing one, rather than being a new thing altogether, and so keeps some of its trademarks (in this case, DNA replication).

I agree 100% with what you say here.

Once the pathway for meiosis diverged and was established within the euks, it became entrenched in the way we reproduce...as is the case with any successful pathway.

 

[Spirochete]

Not sure if I'm saying anything really wrong, but meiosis as a process could have "descended" from mitosis. It's more parsimonious for this process to emerge from a pre-existing one, rather than being a new thing altogether, and so keeps some of its trademarks (in this case, DNA replication).

That sounds logical.

 

[Renge Ishyo]

DNA duplication is a much more time consuming process (at the cellular level) compared to the relatively short process of meiosis. It seems reasonable to get that long process completely out of the way while the cell engages in normal functioning before proceeding with the many dynamic (and therefore energy consuming) process which occur during the brief meiosis state (the cell is literally bent out of shape while it is engaging in meiosis, and so it makes sense to minimize the time spent in such a stressful state as much as possible).

At a higher biological "macro" level we employ a similar "stock before you go" strategy for much the same reason. This can be observed by looking at male sexual behavior...after sex, the male is usually not ready to go again for a certain amount of time (this could be interpreted as the time necessary to stockpile another egg supply sufficient for another go at it). This gap between mating sessions can take days (or even weeks as you get older). Considering this, think about what is the more efficient way to reproduce, copying all our egg cells *before* engaging in the stressful act of sex to transmit them or to only copy egg cells *during* sex requiring the act itself to take place for days or even weeks? So it is with the cell, so it is with the organism at large.