A biological cell is transparent and so is difficult to see with a microscope. Biologists usually dye cells before they use a microscope to examine them. The dyes they use are called stains and the dyeing process is called staining.
A stained animal cell then shows the features in the picture above – it is surrounded by a membrane and contains a region that takes up more stain, called the nucleus. Books often say that this is a typical animal cell. But different types of cell have their own characteristic appearance so that, for example, an osteoblast (a type of cell in bone) does not look like a nerve cell. However, all cells (apart from red blood cells) have a nucleus.
The cells in our bodies can divide, when we are growing, to replace cells, for example, in damaged parts of the body. When a cell divides the nucleus begins to divide and then the whole cell divides into two, as shown in the picture above. During this process, objects called chromosomes appear in the nucleus.
Most humans have 46 chromosomes. Women have 23 identical pairs of chromosomes. Men have 22 identical pairs and a pair in which the two chromosomes are different. Chromosomes can be identified by their appearance. The two non-identical chromosomes in men are called X and Y; women have two X chromosomes.
During the process of cell division, each chromosome forms a pair of chromatids. The chromatids then separate to form a complete set of chromosomes for each of the new cells. The process is shown, schematically in the picture above. To keep the picture simple, I’ve drawn only two pairs of chromosomes that I’ve coloured red and blue.
The result of this process is that each new cell has the same number of the same types of chromosomes as the original cell. So, in a man each new cell has an XY pair, and, in a woman each new cell has an XX pair. This type of cell division is called mitosis.
Why is it important that each new cell has the same chromosomes as the original? Could the chromosomes contain the genes that ensure that each of our cells belongs to us and not to somebody else? To get more information about this possibility, we will need to look at another form of cell division (in the next post) called meiosis, in which sperm and ova (the plural of ovum) are produced.