Introduction to Cell Division
Above: Simple diagram of a general animal cell
This follows the page about the structure of an animal cell.
Living cells divide to form new cells in order to repair worn-out or damaged tissues throughout an organism, and (in the gametes only) to enable the exchange of genetic material at the initial stage of the process of sexual reproduction. (A gamete is a mature sex cell, specifically the ovum of the female or the spermatozoon of the male.)
The two types of cell division are generally called mitosis and meiosis but, strictly, these terms refer to the stages of division of the cell nucleus for somatic (non-reproductive) and reproductive cells, respectively.
Definition of Mitosis
Mitosis is the type of cell division by which a single cell divides in such a way as to produce two genetically identical "daughter cells". This is the method by which the body produces new cells for both growth and repair of aging or damaged tissues throughout the body.
Mitosis is also referred to as "binary fission". Further detail about the process of mitosis is included on the page about mitosis, and also on the illustrations of mitosis.
Definition of Meiosis
Meiosis, which is also referred to as "reduction division", is the form of cell division in which a cell divides into four "daughter cells" each of which has half** of the number of chromosomes of the original cell. Meiosis occurs prior to the formation of sperm (in males) and ova (in females). That is - meiosis only occurs in the "gametes".
**The cells return to having the normal (called "diploid") number of chromosomes after fertilization of the ova by the sperm.
Meiosis consists of two successive divisions, each of which is divided into four phases. The first meiotic division is similar to mitosis (defined above) and the second meiotic division is the "reduction" stage.
Meiosis enables the exchange of genetic material between chromosomes.
Further detail about the process of meiosis is included on the page about meiosis.
Before studying the pages about the processes of mitosis and meiosis, it is useful to understand the following terms:
A chromosome is a thread-like structure found in the nucleus of cells.
Chromosomes are composed of a long double filament of DNA (deoxyribonucleic acid) coiled into a helix together with associated proteins. Genes (the most basic units of genetic material) are arranged in a line along the length of chromosomes.
The nucleus of each human somatic cell (i.e. those relating to the nonreproductive parts of the body) contains 46 chromosomes - 23 of maternal origin (from the mother) and 23 of paternal origin (from the father).
The word "diploid" is an adjective that may be used to describe cells, nuclei or organisms in which each chromosome (except the Y sex chromosome) is represented twice, a situation sometimes summarised as 2n. This is better understood when compared with the term "haploid":
The word "haploid" (and also the word "monoploid") is an adjective that may be used to describe cells, nuclei or organisms that contain a single set of "n" unpaired chromosomes.
An example of use of these adjectives is: "In the case of the human species, the gametes are haploid following meiosis."
But what is n ?
n is a number that varies according to species. In the human species, n=23, therefore there are 46 chromosomes in all human body parts except for the gametes, which contain only 23.
The simplest complete definition of a chromatid is that it is one-half of a replicated chromosome.The more detailed version is that a chromatid is one of two identical strands of DNA that, together, form a chromosome - each chromosome being composed of two sister chromatids joined together at a centromere.
The term "chromosome" applies provided that the centromeres remain in contact. When the centromeres separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands previously called "chromosomes" are referred to as "daughter-chromosomes".
A kinetochore is another term for a centromere.
A centromere (or "kinetochore") is the part of a chromosome at which the two chromatids (see above) are attached together. This (centromere) becomes attached to the spindle during mitosis and meiosis.
When chromosome division occurs the centromere divide longitudinally.
See also a simple comparison between mitosis and meiosis: mitosis vs meiosis.