CHOROMOSES

The human genome has about 3-4 billion base pairs of DNA. (I am uncertain if that estimate is for all 23 pairs, or since each member of a pairs is almost identical, it is an estimate of the DNA in one member each of the just 23 different pairs of chromosomes. I will assume that it is for one member of each pair.) Therefore, on average, each single chromosome of a pair has about 150 million base pairs, which consists of one molecule of DNA and lots of proteins bound to it. dsDNA is a highly charged molecule, and can be view, to a first approximation, as a long polyelectrolyte with a large negative. charge. This very large molecule must somehow be packed into a small nucleus. These packing problem is solved by coiling DNA and packing it with proteins, which usually have a net positive charge. The chromosomes are usually dispersed within the nucleus and are not visible with an ordinary microscope. When the cell is ready to divide, the DNA in the chromosomes replicates, and the chromosomes condense in a fashion that they are not visible using an ordinary microscope. At this point the chromosomes can be stained with a variety of stains (hence the name chromosomes), some of which bind differentially to different chromosomes. The different chromosomes can hence be distinquished by their size, shape, and dye-binding properties, the later called a spectralkaryotype analysis of chromosomes.