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Chromosomes and stuffs

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bucket:
Yeah I'm finding it hard to get my head around some of these things for some reason.

Firstly, is it true that chromosomes normally exist as a single chromatid, and only develop the other chromatid to form an X shape during DNA replication during the S-phase of interphase??
Second, in crossing over, if the swapping of genes occurs between a homologous pair of chromosomes, at the same gene loci, how is that a cause for variation? If they are homologous chromosomes doesn't this mean that they have the SAME genes at that same gene loci, and therefore would just be exchanging the SAME gene to one another? Lol I'm so confused!!

username:
Are you asking the first question because of the TSFX lecture thing and how she said something about a 'non dividing cell's chromatid being called a general chromosome or something? Cuz she lost me there.

bucket:
haha you were there too? the first presenter wasn't really good :S. Anyway yeah, she brought it to our attention but didn't explain why and stuff.

username:
I KNOW! I think I'm just going to ignore that detail though, because it's started to confuse me with everything else.

I didn't really like the second presenter either, the whole "Okay so now we're going to skip from page.. 10 to... 61" didn't cut it for me.

Were you there today for the Chemistry one? I found that hella good. But the seats in the med building are so terrible. I felt like someone blew off my lower torso.

bturville:

--- Quote from: bucket on October 02, 2008, 10:17:37 pm ---Firstly, is it true that chromosomes normally exist as a single chromatid, and only develop the other chromatid to form an X shape during DNA replication during the S-phase of interphase?

--- End quote ---
yep. you should be able to see this in most diagrams of mitosis.


--- Quote from: bucket on October 02, 2008, 10:17:37 pm ---Second, in crossing over, if the swapping of genes occurs between a homologous pair of chromosomes, at the same gene loci, how is that a cause for variation? If they are homologous chromosomes doesn't this mean that they have the SAME genes at that same gene loci, and therefore would just be exchanging the SAME gene to one another? Lol I'm so confused!!

--- End quote ---
the gene loci are the same yes, but the ALLELES at those loci are often different on the homologous chromosomes, leading to variation.

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