Meiosis

Meiosis

Mitosis provides a method for the asexual reproduction of singular cellular organism and a means of growth and cell replacement in multicellular organisms. An alternative to asexual reproduction, where the offspring are identical to the parent, is sexual reproduction where the genetic material from reproductive partners combines to produce offspring. Clearly such combination cannot involve all the genetic material of the parents, otherwise, with each successive generation we would see a doubling of the genetic material within each cell. To accommodate this the parents produce sex cells (eggs/sperm) called gametes that each contain half the parental genetic material, these fuse to produce offspring.

The gamete cells, then, are unusual in that they contain half the number of chromosomes of 'normal' cells and, as such, are termed haploid (normal cells, containing two sets of chromosomes, are termed diploid).

The normal number of chromosomes in human cells is 46 (in 23 pairs). These pairs originate from parental gamete cells containing 23 single chromosomes, 23 in the mother's egg and 23 form the father's sperm. These two sets of chromosomes are called homologues in that they carry genetic code for the same function, that is a particular chromosome from the mother may carry the genetic coding for hair colour, this is the partner chromosome for the paternal chromosome carrying the same genetic information. Each of the 23 chromosomes can be distinguished by its size, the position of its centromere and the pattern of dark bands. The process of identifying specific chromosomes is called karyotyping and you will undertake a karyotyping exercise in class - external visitors to this page can find the karyotyping exercise on the links page. You can learn more about karyotyping from the set.

The process of meiosis can be considered in two stages; Meiosis I and Meiosis II.

Meiosis I

As with mitosis, meiosis I is preceded by an interphase stage. During this stage chromosomal replication takes place resulting in two identical chromatids, attached by centromeres. Centrosomes are also replicated during this stage.

Interphase is followed by Prophase (I). Homologous pairs of chromosomes pair up and the chromatids of these (of which there are four) cross over one another (see diagram, set book page 140). This coming together of chromatids is important in understanding genetic inheritance. The alignment of chromatids seems random and during this cross over phase there is an exchange of genetic material from one chromatid to the other. In other words, segments of genetic code are exchanged - see Figure 8.18b, page 145 in the set book. This grouping of four chromatids is termed a tetrad. This exchange of genetic material results in chromatids that are uniquely different from that of the parent and ensures genetic variation from one generation to the next.

As this exchange of genetic material takes place, the rest of the cell prepares for division. The centrosomes, with their spindle mictrotubules, separate and the nuclear membrane dissolves. The chromosomes now move to the metaphase plate, as in mitosis, except now they arrive as pairs. Spindle fibres from one pole of the cell (cf mitosis where fibres from both poles) attach to one chromosome of each pair.

Anaphase I

As in mitosis, anaphase I sees the movement of chromosomes to opposite poles of the cell. Paired chromatids are attached by their centromere and so move together towards the poles, in effect the homologous pairs of chromosomes are separated and we have two individual sets of chromosomes at either end of the cell. This process of reducing the amount of genetic material to be passed onto the daughter cells is called reduction division.

Following Anaphase I are the stages of Teleophase I and Cytokinesis. Cell division begins with the formation of a cleavage furrow (in animals) or cell wall (in plants) and the two sets of chromosomes are separated into daughter cells. This is similar to mitosis but, unlike mitosis, there is no further duplication genetic material during the subsequent interphase.

Meiosis II

Meiosis II can follow meiosis 1 immediately or can be delayed by a period of interphase during which time the chromosomes decondense and the nucleus reforms. As in meiosis I, interphase is followed by stages called prophase, metaphase, anaphase, telophase and cytokinesis.

Prophase II

The chromosomes (containing two chromatids) condense, the spindle apparatus forms and the chromosomes move to the metaphase II plate.

Metaphase II

The chromosomes position themselves along the metaphase plate ready for separation.

Anaphase II

The sister chromids separate into individual chromosomes and move in opposite directions to the poles of the cell.

Teleophase II

The chromosomes decondense and nuclei of the (now four) cell(s) reforms. The new cells separate during cytokinesis.

Interphase II

Normal cellular activity continues but there is no further DNA replication and the gamete awaits fertilization.

Advantages of meiosis

The advantage of meiosis (sexual reproduction) lies in the shuffling and combination of genetic material. We have seen that homologous chromosomes code for certain characteristics - by exchanging DNA and combining different sources of genetic material we create a new set of genetic code. It is this generation of new code that results in variation within a population (such as eye colour etc.). This variation enables a population to evolve. Genetic diversity also enables populations to adapt and overcome threatening circumstances.