Chromosomes

The DNA molecule in the cell nucleus is wrapped up in special coverings known as chromosomes (See, DNA). The total length of the DNA molecule packaged in the chromosomes in a single cell reaches 1 meter (3.3 feet). The total thickness of the chromosome is 1 nanometer, or 1 billionth of a meter. The one-meter-long (3 feet, 3 inches long) DNA molecule is twisted and folded into this tiny volume.

Inside the nucleus of every human cell (except for reproduction, or germ cells) there are 46 chromosomes. If we compare every chromosomes to a book made up of pages of genes, then we can compare the cell to a six-volume encyclopedia containing all a person’s characteristics. The information in this “encyclopedia” is equivalent to that found in a 32-volume edition of the Encyclopedia Britannica.

The chromosomes containing the DNA molecule actually consist of much smaller special packaging systems. This DNA molecule is first tightly surrounded by special proteins known as histones, just like cotton wound round a spool. Those parts of the DNA attached to the histone spools are known as the nucleosomes, which have been designed to protect the DNA from any harm. When nucleosomes are combined end to end, they constitute chromatins, which cling tightly to one another and fold over, forming dense coils. Thus it is that the DNA molecule is able to be squeezed so perfectly into an area just 1 millionth of its actual length.