it came (the parent cell). This number is the number for the species. This is the way cell division takes place in almost every part of your body (with one exception). It is termed mitosis (Figure 3.6). Figure 3.6 Mitosis – cell division for growth If the cell is dividing over and over how does the number of chromo- somes remain exactly the same ? Before mitosis each chromosome makes an exact copy of itself to form a chromosome with two strands held together at a single point. Each strand in the double- stranded chromosome is called a chromatid. 196 FD12A F F M M Cell divides into two: one strand from each chromosome goes to each daughter cell The cells grow, but the DNA strand in each chromosome will not replicate into two strands unless the cell is going to divide Each chromosome replicates to form two identical strands of DNA (chromatids) in prepar- ation for division. Parent cell – 4 chromosomes present in pairs (in this case, 2 pairs): one of each pair from father (F); one from mother (M) F F M M
Cell division for gamete formation (meiosis) In mitosis you get back exactly what you start with! A cell divides to form two exact copies of itself. Cell division for gamete forma- tion is somewhat different, (the exception mentioned above). Gametes are reproductive structures, in our case, the male sperms and the female eggs. Remember that our body cells have 46 chromo- somes. Sperms and eggs have only 23 chromosomes i.e. half the number found in body cells. There is a very practical reason for that which should be fairly obvious if you think about it. Cells with the capacity to produce gametes divide in a two-step process. Figure 3.8 Meiosis – reduction division Step 1: One cell becomes two, but each daughter cell gets one of each chromosome pair, and so has 23 unpaired chromosomes. Note that one will get the X and the other the Y chromosome (see above). This step is called a reduction division because of the halv- ing of the chromosome number. Step 2: These two cells divide to become four, each with 23 chromo- somes. In a male, these four cells become the gametes (sperms), half FD12A 197
having X chromosomes and half Y. The process is called meiosis (see Figure 3.7). By convention, we represent this half or haploid number as n , and the full diploid number in the body cells as 2n . The haploid number of humans is 23 and the diploid number is 46, thus we have n chromosomes in our gametes and 2n in all our other body cells. Interestingly, in females Step 1 in meiosis starts in the ovaries of the foetus i.e. before birth, then stops. About 1 million of these “pre- eggs” survive until after birth, remaining dormant until puberty when the process restarts. Then one is selected for release at ovula- tion each month. In her lifetime a woman will release only some 400–500 eggs. The rest degenerate.
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