Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: medhavika on 13/03/2004 07:38:43
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Prior to meiosis, chromosomes replicate and form sister chromatids. A set of maternal chromosome and a set of paternal chromosome in a fetlizized egg, all consisting of two sister chromatids, join together and undergo crossing over in prophase one. My question is how does each chromosome know which chromosome to form a chiasma with? Is crossing over a completely random process where any two chromosomes join and exchange segments or does anything facilitate the process? If it is random, can two paternal chromosomes undergo crossing over?
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Cohesins actually hold the homologous pairs together during meiosis. This actually occurs during the S phase while the chromosomes re replicating. I can link you to an experiment abstract where they studied this. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9778527&dopt=Abstract) Chiamata do not form unless there is crossing over. It is the formation of the chiasma during a crossover event that is responsible for interference with double crossovers.
I'm sure of the exact mechanism used by cohesins to ensure that only homologous pairs...it might be detailed in that experiment I linked to above, I only read the abstract.
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Oh, and I would say that if 2 different paternal chromosomes did link during meiosis, you'd some have messed up gametes coming out that if there was a crossover. Serious mutation scenario.
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Jason, please note that the reference you've cited actually refers to mitotic division involving division of the centromere and separation of the chromatids.
During the first meiotic division, when chiasmata form, homologous pairs of chromosomes (e.g. the two chromosome number 21's - one from the father, 1 from the mother) assemble at the cell equator. Under normal circumstances they only pair up with their corresponding partner. This can go awry when the individual carries a Robertsonian translocation. This occurs when the ends of 2 chromosomes fuse together. This causes non-disjunction and leads to extra copies (trisomy) of the affected chromosomes in the gametes and hence can cause syndromes like Down's.
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Surely if two paternal or maternal chromasomes formed a chiasmata an crossed over (IF) the effect would be null because the genes being swapped over would be identical?
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Not quite - remember that (under normal circumstances) you have 2 pairs of each chromosome - one inherited from your father and one from your mother.
These are called homologous chromosomes because they contain the same genes, in the same order as each other, although the message in the genes may be slightly different between the two. For instance the eye colour gene on the chromosome from your mother may code for blue eyes, whilst the eye colour gene on the chromosome you got from your father encodes brown eyes.
Crossing over only occurs between these homologous chromosomes. They exhange corresponding portions of the chromosome with each other. Since there is only one paternal and one maternal chromosome of each type, there is no way that 2 paternal chromosomes could get together and swap material along the lines that you suggest.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx