Genetics as it applies to evolution, molecular biology, and medical aspects.
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If DNA residing in the cell is loosly packed DNA called chromatin, and if in the Interphase the chromatin replicates to prepare for mitosis, then it must be that when the chromatin replicated there were only 23 modules of DNA in the cell for it to replicate. Is that correct? That way once the chromatin replicates and does all of it's twisting and coiling, we then have our 23 chromosome pairs. Yes?
Before replication there were 23 pairs of molecules, 23 of maternal origin and 23 of paternal origin, plus the maternally-contributed mitochondrial chromosome (which I'll ignore for the rest of this post). Counting chromosomal centromeres, there are 46 centromeres total. After replication there are still 46 centromeres, but the mass of DNA and number of chromatids have doubled (and so you get the X-shaped chromosomes you typically see in textbook cartoons). At this point you have twice the number of DNA molecules, because instead of each centromere binding a single molecule of DNA you have two molecules bound at each centromere after replication. During cell division, the centromeres separate into two parts, each travelling with one molecule of DNA into a daughter cell. For a moment, before cytokinesis, the dividing cell contains 92 chromosomes, but when the cell membrane seals up each of the daughter cells is back to 46 chromosomes (the 23 pairs, each pair consisting of a paternal and a maternal chromosome). OK, so I promised to ignore it but I can't - you still have that mitochondrial chromosome as well.
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