Multicellular eukaryotes and single-celled sex cells reproduce by a process called meiosis. In this process, the number of chromosomes in a mother cell is halved to produce gametes, cells that fuse with other cells. This happens in plants, animals, and fungi. German biologist, Oscar Hertwig first discovered meiosis in sea urchin eggs in 1876.
A mother cell contains chromosomes in its nucleus. A chromosome contains coils of deoxyribonucleic acid (DNA) and is structurally made up of two long chromatids which are linked in the centre by a centromere.
Phases of Meiosis
Meiosis takes part in two main stages, meiosis I and meiosis II. In Interphase, the phase before the start of meiosis, two identical sets of chromosomes are created from the DNA in the cell. There is a G1 phase when the proteins for cell growth are created. This is followed by an S phase for synthesis when the cell’s chromosomes duplicate to produce identical sister chromatids linked at the centromere. Meiosis I then begins with Prophase I.
The chromosomes form homologous pairs. Long strands of protein (microtubules) start to form as the meiotic spindle at the poles of the cell. The spindle fibres will separate the chromosomes later in Metaphase I.
In the meantime, the pairs of chromosomes (tetrads) then exchange sections containing DNA through homologous recombination. The barrier around the nucleus disintegrates to set these chromosomes free.
The homologous chromosome pairs then line up across one another with an equator between them. The spindle fibres then attach themselves to one of the chromosomes in each of the pairs.
The spindle fibres pull the chromosomes apart in different directions, to opposite poles. However, the sister chromosomes stay together.
Complete sets of the chromosomes are created at each pole.
The membrane starts to split around each set of these chromosomes to create two nuclei. Two new haploid daughter cells form with only half the number of chromosomes of the original cell.
Following the formation of these two new cells, Meiosis II starts in a process similar to mitosis in that chromosomes create two identical sets of chromosomes each with their own nucleus:
Each chromosome in the daughter cells transitions to an X-shape. The nucleus constraining the chromosomes disintegrates, freeing them. Spindle fibres start to form and the chromosomes begin to move to the equator of the cell.
The chromosomes line up across each respective cell equator randomly.
The spindle fibres grab the chromosomes and they separate at their centromeres, pulling away from each other to the poles.
The process of nucleus creation starts again.
Once again the membrane divides up and now four granddaughter cells are created. For female cells, these contain an egg cell and three polar bodies unable to create eggs. Male cells contain all sperm cells. The process of meiosis is now complete.
Occasionally there are malfunctions in the meiosis process and miscarriages can occur in the case of sex cells or they can cause disabilities.