Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell (Fig. 1.2). Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During this replication phase the chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope. With the onset of mitosis the chromosomes begin to coil, contract, and condense; these events mark the beginning of prophase. Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. Throughout prophase the chromosomes continue to condense, shorten, and thicken (Fig. 1.2A), but only at prometaphase do the chromatids become distinguishable (Fig. 1.2B). During metaphase the chromosomes line up in the equatorial plane,
and their doubled structure is clearly visible (Fig. 1.2C). Each is attached by microtubules extending from the centromere to the centriole, forming the mi-totic spindle. Soon the centromere of each chromosome divides, marking the beginning of anaphase, followed by migration of chromatids to opposite poles of the spindle. Finally, during telophase, chromosomes uncoil and lengthen, the nuclear envelope reforms, and the cytoplasm divides (Fig. 1.2, D and E). Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell.
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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.