Hi compmage,
A diploid cell (ie us) has 2 PAIRS of chromosomes per cell. Early in meiosis each chromosome doubles, giving 2 chromatids, joined at the centromere (the familiar X shape). Homologous chromosomes (two chromosomes of each, remember)pair up & crossing over/recombination takes place. The doubled up chromosomes (note there are 4 homologous chromatids at this point, at the middle of the cell) are then randomly assorted, with 23 doubled up homologous chromosome pairs being pulled apart (in each case), & cell division occurs. This means there are 23 doubled up chromosomes (46 chromatids) in each daughter cell. The sister chromatids then split (becoming chromosomes in their own right), each homologous sister being pulled to opposite ends of the cell, which again divides. Note that there is only ONE chromosome set now per cell, the cells have divided twice, giving four haploid (1 chromosome set) cells, ie sperm or egg.
Fertilisation makes a diploid embryo out of two haploid cells.
When chromosomes replicate & form chromatids you have two strands, each of which forms a new strand based on nucleotide compatibility.
EG 1 chromosome.
ATAGGTTCCA
TATCCAAGGT
becomes two strands
ATAGGTTCCA & TATCCAAGGT
The other "missing strand" is copied using base pair complementing, from nucleotides in the cytoplasm. ie.
ATAGGTTCCA & TATCCAAGGT
TATCCAAGGT & ATAGGTTCCA
Giving 2 chromatids (note this happens for each homologous chromosome, giving four chromotids/somes). The chromatids are then passed on as a discrete unit, one ending up in each haploid cell. Thats two chromosomes becoming 4, being separated into 4 cells, by 2 cell divisions. In fertilisation, two chromosomes (from each parent) have
different sequences, which is where I think you're going wrong. These two (sperm/egg) chromosomes do not have to interact. This is why we have two copies of each gene (one from sperm, on from the egg (sex chromosomes excepted)), but they aren't always the same allele (version of same gene).
Put differently, you have two copies of chromosome 20, both are different. The splitting/copying of the chromosomes occurs in the parents, not during fertilisation.
Hope this helps.
Mark
[This message has been edited by mark24, 06-20-2002]