Diagram 3.14: A cell with an enlarged chromosome
Diagram 3.15: A full set of human chromosomes
The nucleus is the largest structure in a cell and can be seen with the light microscope. It is a spherical or oval body that contains the chromosomes. The nucleus controls the development and activity of the cell. Most cells contain a nucleus although mature red blood cells have lost theirs during development and some muscle cells have several nuclei.
A double membrane similar in structure to the plasma membrane surrounds the nucleus (now called the nuclear envelope). Pores in this nuclear membrane allow communication between the nucleus and the cytoplasm.
Within the nucleus one or more spherical bodies of darker material can be seen, even with the light microscope. These are called nucleoli and are made of RNA. Their role is to make new ribosomes.
Inside the nucleus are the chromosomes which:
- contain DNA;
- control the activity of the cell;
- are transmitted from cell to cell when cells divide;
- are passed to a new individual when sex cells fuse together in sexual reproduction.
In cells that are not dividing the chromosomes are very long and thin and appear as dark grainy material. They become visible just before a cell divides when they shorten and thicken and can then be counted (see diagram 3.14).
The number of chromosomes in the cells of different species varies but is constant in the cells of any one species (e.g. horses have 64 chromosomes, cats have 38 and humans 46). Chromosomes occur in pairs (i.e. 32 pairs in the horse nucleus and 19 in that of the cat). Members of each pair are identical in length and shape and if you look carefully at diagram 3.15, you may be able to see some of the pairs in the human set of chromosomes.