A closer look at what makes up the human genome...
CHROMOSOMES
Having reviewed the chemical basis of heredity in DNA, let us now examine how the “genetic code” is packaged into living cells.
What is a chromosome?
As you might have guessed, chromosomes (Figure B-9) are indeed bundles of DNA. However, in most cells they are present only for a brief moment. In fact, most of the time DNA is spread out in a large, diffuse mass – something like a big plate of spaghetti. When a cell needs to produce more cells, it does so by dividing in two. Think of the problems that this “spaghetti” might cause during cell division, when a dividing cell must bestow each of its successor cells with its own complete set of DNA. Imagine trying to separate and transport a tangled mass of noodles! For this reason, the DNA condenses before cell division into the thick, rod-like form that we recognize as chromosomes.
Chromosomes have several important features. First of all, the DNA packs so tightly that one can see it under a simple light microscope. Secondly, recall that because the cell is getting ready to divide in two, the DNA of a visible chromosome has already been duplicated, so that each successor cell will have its own copy. This means that, on close inspection, a cell that is ready to divide will have four strands of DNA, two helices of two strands each. Each of these double strands of DNA condenses into a single rod called a sister chromatid (as in Figure B-9). The two chromatids are therefore exact replicas of one another, and the center of each is joined together prior to the division of the cell. As a result, most chromosomes take on the appearance of the letter X.
Last Modified: 9-16-02
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