3B Scientific Cell Division II Chart, Meiosis - Manual (Page 3)

®
English
Meiosis, or reduction division, is a special type of cell division. Depending on the organism and cell type,
it can take anything from several days to years, resulting in the production of sex cells (gametes). Each
gamete precursor cell produces four gametes through reduction division. In general, there are two types
of gametes. Large, immobile cells are referred to as egg cells or oocytes, and small, mobile gametes as
sperm cells or spermatocytes.
Egg cells are formed by meiotic division from precursor cells in the ovaries. Human egg cells already begin
to mature in the embryo (3rd to 8th month of pregnancy), however, the cells remain in a meiotic interme-
diate phase until sexual maturity is reached. From then on, some of the immature egg cells complete the
meiotic division at regular intervals under the control of hormones. The maturation of human sperm cells
occurs regularly in the testes when sexual maturity has been reached. In this case, a complete meiotic
division takes 20-24 days.
Usually, body cells (e.g. precursor cells of the gametes) contain a double (diploid) chromosome comple-
ment, with one half of the chromosomes originating from the mother and the other half from the father.
Therefore, a twin copy exists of each chromosome, i.e. as a matching (homologous) pair of chromosomes.
By contrast, the gametes contain only a single (haploid) chromosome complement. In other words, egg or
sperm cells only contain half of the mother’s or father’s genetic information, so that when both cells join,
a new cell (zygote) with a complete diploid chromosome complement can originate.
The purpose of meiosis is therefore to reduce the normally diploid chromosome complement of a gamete
precursor cell to the haploid complement to establish the basis of any sexual reproduction. A further
important function of meiosis is to mix the genetic information. This is achieved by two mechanisms:
1. a random distribution of the maternal and paternal chromosomes to the sex cells being produced
2. the exchange of genes between the homologous chromosomes (genetic recombination)
(The underlying procedures of both mechanisms are explained below in the description of the individual
phases).
In humans, who have 23 chromosomes in the haploid complement, the random distribution of the
chromosomes alone allows for 2
23 , i.e. 8.4 x 10 6 different genetic possibilities of variation. The number
of variations is furthermore increased by the exchange of genes between the chromosomes.
Prior to the meiotic division, the gamete precursor cells are in the interphase, which refers to the period
between two (mitotic or meiotic) cell divisions. The interphase comprises three stages:
• G
1
phase (presynthesis) The stage where the cell grows.
• S phase (synthesis) In this phase, the centrioles and the DNA (deoxyribonucleic acid) begin to duplicate.
• G
2
phase (postsynthesis) This phase separates the end of the DNA synthesis from the phase of division.
Furthermore, the duplication of the centrioles is completed.
Meiosis, the phase following the interphase, comprises two successive maturation (meiotic) divisions, which
are separated by a short, specific interphase (interkinesis). As in mitosis, several stages of division are diffe-
rentiated in each meiotic division:
First meiotic division:
• Prophase I (four subsections: leptotene, zygotene, pachytene and diplotene with diakinesis)
• Metaphase I
• Anaphase I
• Telophase I
• Cytokinesis I
Meiosis
3