Laura Marie Wendling, age 15, of San Diego, California, for her question:
How does mitosis differ from meiosis?
Without a doubt, life is the greatest miracle on earth. Living things can keep going in all sorts of wondrous ways and even the most simple cells can multiply themselves. Life can perpetuate itself by handing on life to the next generation. This multiplication is done either by mitosis or meiosis small miracles that take place inside the nuclei of living cells.
Most living cells multiply by the process called mitosis. Changes occur in the nuclei which cause it to duplicate itself. When this process is complete, the parent cell divides into two and becomes a pair of identical twins.
As a rule, the single celled plants and animals multiply by mitosis. So do all the cells in the bodies of larger animals. In the human body, about three billion worn out cells die every minute. They are replaced as three billion other cells multiply by mitosis. All these replacements, are identical copies of the parent cells.
If the only method of cell multiplication were mitosis, then every human baby would be an exact copy of its parent. This is not so because life has another method of multiplication. It is called meiosis. In this miraculous process, each offspring is unique because it inherits half its features from one parent and half from the other.
A living cell is a tiny nucleus inside a jellyfied mass of busy cytoplasm. The secret of life is in the genes and chromosomes packed inside the nucleus. The genes are tiny beads made of DNA, the magic blueprint biochemical that directs the activities of the cell. Every living human cell has 23 pairs of chromosomes. Each chromosome may have 1,200 beady genes and each gene bears features inherited from one parent or the other.
In simple mitosis, the chromosomes duplicate themselves and form strings. The duplicate sets separate and pull to opposite ends of the cell. As they pull apart, the cell also is pulled apart. It becomes thin at the waist and then divides into two. Each identical half becomes a new cell, exactly like the original.
The process of meiosis is more complicated because the new cell must produce a completely original offspring. This can happen only with special cells called sex cells. It can happen only when the sperm cell of a male parent meets and fertilizes the egg cell of a female parent.
New sex cells are produced by ordinary mitosis. Then they go through the process of meiosis. In the case of humans, the 23 pairs of chromosomes divide into two sets, each with a different group of genes. A half cell of this sort has no future unless it meets up with a half cell from an opposite parent. Together they form a complete and original cell which multiplies to form an entirely original person.
Mitosis is fine for duplicating worn out body cells. But meiosis is necessary to make sure that every offspring is original. Every person inherits genes from both sides of the family because the original fertilized egg took half its chromosomes from one parent and half from the other. The opposite sex cells, with half the usual number of chromosomes were produced by meiosis.