The divisions of meiosis
Meiosis is the type of cell division by which gametes, are produced. Meiosis involves a reduction in the amount of genetic material.
Meiosis is the type of cell division by which gametes, are produced. Meiosis involves a reduction in the amount of genetic material.
The divisions of meiosis
In the gamete mother cell, before meiosis begins, each chromosome replicates.
- At this stage each chromosome exists as two genetically identical chromatids attached to their centromere. This shows four chromosomes within the nucleus of the cell. Each chromosome appears as two chromatids attached to a centrometre.
- In the first meiotic division chromosomes align in homologous pairs. Points of contact form between members of the same homologous pair. The shaded chromosomes are identical to those that were originally donated by one of the parents in the ovum. The white chromosomes are identical to those that were originally donated from the other parent in the sperm that fertilised the ovum. The points of contact or crossing over between members of a homologous pair are the chiasmata.
- The homologous pairs move to the equator of the cell. Equal lengths of the chromatids of the same homologous pair have broken off and crossed over. The chromosomes align at random and independantly. Only one possible arrangement is shown.
- Homologous pairs align at random at the equator of the cell. This shows the early separation of the chromosomes of each of the homologous pairs.
- Homologous pairs are separated. This shows the cell at an early stage of meiotic division. The cell membrane is starting to pinch inwards.
- The cell divides to form two cells each with a haploid set of chromosomes. Cell division is complete and two cells containing a haploid set of chromosomes are formed.
- In the second meiotic division the chromosomes in each cell align independently and randomly at the equator of the cell.
- The chromatids in each cell are separated. The chromatids are pulled towards the opposite poles of the cell.
- The cells divide. The cell membrane is starting to pinch inwards.
- Cell division is complete, resulting in the formation of four cells each with the haploid number.
The divisions of meiosisThe following video clip gives an explanation of mitosis and meiosis.
SummaryIn meiosis the random alignment of the homologous pairs of chromosomes gives new combinations of the original maternal and paternal chromosomes present in the gamete mother cell. Crossing over between members of homologous pairs of chromosomes leads to the formation of chromosomes that contain new combinations of the alleles of the genes.
Meiosis takes place in reproductive tissue. In male animals, meiosis takes place in the testes and in females within the ovaries. In plants, meiosis occurs in the anthers to form pollen grains and within the ovary to form ovules. In meiosis a diploid cell divides in such a way so as to produce four haploid cells. These haploid cells are known as gametes. Each gamete is genetically different, as a result of both random alignment of the homologous pairs of chromosomes (independent assortment of the chromosomes) and crossing over between members of a homologous pair.
Meiosis exam skills
- DNA in meiosis
- Changes that occur in the chromosomes during meiosis are matched by change in the cell DNA content. Much of the structure of a chromatid exists as a strand of DNA. Many questions ask about change in cell DNA content and not directly about meiosis. When the chromosomes replicate before meiosis begins, the DNA content of a cell doubles. When the homologous pairs of chromosomes are separated at the first stage in meiosis, the DNA content of the cells produced is half that of the original cell. When the chromatids are separated at the second stage in meiosis, the DNA content of the cells produced is halved again.The DNA content of a gamete is half that of the gamete mother cell or of any other body cell, such as liver and nerve cells.
- Changes that occur in the chromosomes during meiosis are matched by change in the cell DNA content. Much of the structure of a chromatid exists as a strand of DNA. Many questions ask about change in cell DNA content and not directly about meiosis. When the chromosomes replicate before meiosis begins, the DNA content of a cell doubles. When the homologous pairs of chromosomes are separated at the first stage in meiosis, the DNA content of the cells produced is half that of the original cell. When the chromatids are separated at the second stage in meiosis, the DNA content of the cells produced is halved again.The DNA content of a gamete is half that of the gamete mother cell or of any other body cell, such as liver and nerve cells.
- Red blood cells
- In the growth and development of unspecialised cells into red blood cells (RBC) the nuclear content of the cells is removed. This means that a mature RBC no longer has a nucleus, chromosomes or any DNA present.
- In the growth and development of unspecialised cells into red blood cells (RBC) the nuclear content of the cells is removed. This means that a mature RBC no longer has a nucleus, chromosomes or any DNA present.
- Homologous chromosomes
- Each individual produced through sexual reproduction receives one of each member of a homologous pair of chromosomes from each of their parents. These are inherited in the sperm of one of the parents and in the ovum of the other parent. The pattern of genes on a homologous pair of chromosomes is for the same characteristics. These genes will be located on exactly the same position on the chromosome although the alleles of each of these genes can differ.
The physical appearance of a homologous pair is identical.They have the same length; the centromere is in the same position and when stained, the banding along their length will be the same.
- Each individual produced through sexual reproduction receives one of each member of a homologous pair of chromosomes from each of their parents. These are inherited in the sperm of one of the parents and in the ovum of the other parent. The pattern of genes on a homologous pair of chromosomes is for the same characteristics. These genes will be located on exactly the same position on the chromosome although the alleles of each of these genes can differ.
