BR Bio 9-6 #34 intrachromosomal recombination

[cbs21]

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can someone tell me what's going on here. when does the crossover bw non sister chromatids actually take place and where do all of the chromosomes come from? This makes no sense to me...

This is how they set it up. I don't understand how you have 4 different chromosomes and how this is intrachromosomal recombination...

--b-----a--
--b-----a--

X
--B----+--
--b-----a--


The bio passages in this chapter are taking me about ten minutes each and I'm getting a lot wrong!!

 

[Phantastic]

Homologous chromosomes recombine during Prophase I of Meiosis.

Homologous chromosomes each consist of a tetrad consisting of two sister chromatids each. So, there's 4 chromatids there, but yes, only 2 "chromosomes." Homologous chromosomes contain different sister chromatids and will therefore "shuffle their deck" to increase variation.

Just to help clairify:
In Mitosis, sister chromatids are lined up and pulled apart to each daughter cell. In Meiosis I, homologous chromosomes (consisting of two sister chromatids) are pulled apart to each daughter cell (and then each chromatid is pulled apart in Meiosis II, much like Mitosis).

 

[cbs21]

Homologous chromosomes recombine during Prophase I of Meiosis.

Homologous chromosomes each consist of a tetrad consisting of two sister chromatids each. So, there's 4 chromatids there, but yes, only 2 "chromosomes." Homologous chromosomes contain different sister chromatids and will therefore "shuffle their deck" to increase variation.

Just to help clairify:
In Mitosis, sister chromatids are lined up and pulled apart to each daughter cell. In Meiosis I, homologous chromosomes (consisting of two sister chromatids) are pulled apart to each daughter cell (and then each chromatid is pulled apart in Meiosis II, much like Mitosis).

--B----+--
--b-----a--

yes... but how can the sister chromatids come from a single chromosome (as in above)? don't the sister chromatids need to be identical since they are just copies?

i would understand if it was

--b-----a--
--b-----a--
X
--B----+--
--B----+--


but it's not...

 

[cbs21]

bump.. anyone??

 

[Rayhaan]

No they are not identical once they have crossed over. And only the chromatids involved in the link can cross over, your first diagram is right, the second is not.

To further clarify it says in BR bio (can't remember where) that the chromatids ARE NOT identical sister chromatids after this point, and therefore shouldn't technically be called sister as they are no longer the same.

 

[lorenzomicron]

PS this process of synapsis (see http://en.wikipedia.org/wiki/Synapsis) is one of the two key processes responsible for genetic diversity (the other is independent assortment).

 

[cbs21]

No they are not identical once they have crossed over. And only the chromatids involved in the link can cross over, your first diagram is right, the second is not.

To further clarify it says in BR bio (can't remember where) that the chromatids ARE NOT identical sister chromatids after this point, and therefore shouldn't technically be called sister as they are no longer the same.

but that first diagram is before crossover occurs, not after. i don't understand how that could be correct? this is straight from the answers.

b---a
B---+
X
b---a
b---a

----->

b---a
B---a

and

b---+
b---a

maybe the diagram is just confusing is me?

 

[Rayhaan]

but that first diagram is before crossover occurs, not after. i don't understand how that could be correct? this is straight from the answers.

b---a
B---+
X
b---a
b---a

----->

b---a
B---a

and

b---+
b---a

maybe the diagram is just confusing is me?

Yeah this is right, look at how the two homologous chromosomes are arranged. You see only the 2 inner chromatids of the tetrad can form the synapse and share. The ones on the outside of the tetrad can't. That's why during crossover you are technically producing 4 different chromatids and the originally "sister" chromatids are technically no longer sister as they are not the same. This is intended as it allows for more genetic diversity. If this diagram is from the answers then I might see where you are getting confused by my explanation. The three b--a don't actually occur that simply, there are probably different versions of those alleles. The important thing to remember is simply that the bivalent tetrad forms, but ONLY the ones actually involved in the cross over can share anything, the other 2 stay the same.

 

[Rayhaan]

I reread your question and I think I understand now. You are asking how (in your very first diagram) how the bottom chromosome has non-identical chromatids before crossing over even occurs right?

I do not have an answer for this. This may just be an overanalyzing of the question, and it is just aiming to test knowledge of rearrangement. You are correct in saying that it makes no sense that the chromatids are not identical before crossing over.

I don't want to further confuse this issue but we could extrapolate that there might be various reasons for this. There might have been simply some sort of chromosomal reorganization via transposable elements. I wouldn't worry much about it really, just know how the tetrad links up.