I am always confused about this...so when one cell which has 23 pairs of chromosomes undergoes the two meiotic divisions it produces four cells containing 1 chromatid of every chromosome pair so when this fertilizes the opposite gamete (of course it has also one chromatid of each chromosome pair) when they are fertilized they make one 23 chromosomes because 23 chromatid from one and 23 from other and they will form 23 chromosomes not 23 pairs of it, so my question is how they form 23 pair of it or to make it homologue..please clarify with some pics??...tnx

  • $\begingroup$ you've almost got me confused as well! 23 chromatids in each gamete: 23 + 23 = 23 pairs, no? the gamete chromosomes do not fuse or recombine in fertilization or anything like this. $\endgroup$
    – shigeta
    Mar 9 '14 at 13:36
  • $\begingroup$ I was confused by this for the longest time too...it's confusing how a both a single chromatid and a pair of chromatids can both be referred to as a chromosome. $\endgroup$
    – evamvid
    Mar 10 '14 at 1:07

After fertilisation the zygote has two copies of each of the chromosomes (not worrying about sex chromosomes). When each of these is replicated the cells will have chromosome pairs, which will then divide in mitosis.

The classical chromosome image of pairs of X-shaped chromosomes relates to the situation at mitotic metaphase when each of the chromosomes has replicated and consists of two sister chromatids. As anaphase proceeds the splitting of the centromeres means that each of the sister chromatids becomes a chromosome, and each cell ends up with a pair of each chromosome type, just like a zygote.


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