Meiosis

What is meiosis?

Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells. It involves two consecutive divisions, Meiosis I and Meiosis II, which together result in the formation of four non-identical daughter cells called gametes. These gametes in plants and animals are used in sexual reproduction.

Where does meiosis occur?

Meiosis occurs:

1. In the testes of male animals and the ovaries of female animals
2. In the anthers and ovaries of flowering plants

Why is meiosis important?

Meiosis is a vital process involved in sexual reproduction. It increases genetic diversity through the shuffling and recombination of genetic material. This results in different combinations of alleles in the gametes produced. Having genetically unique offspring can be beneficial for a population in terms of species survival.

What are the stages of meiosis?

It is really important for your exams that you know the different stages of meiosis. Meiosis is divided into two main stages: meiosis I and meiosis II. Each stage is then further divided into several sub-stages, as described below:

Stages of Meiosis I:

1. Prophase I: The chromosomes condense and become visible under a microscope. The homologous chromosomes pair up. Crossing over and the exchange of genetic material between homologous chromosomes can occur during this stage.
2. Metaphase I: The homologous pairs line up along the equator of the spindle, with one homologous chromosome on each side, oriented randomly with respect to the poles.
3. Anaphase I: The homologous chromosome pairs separate and whole chromosomes move towards opposite poles of the cell, pulled by the microtubules.
4. Telophase I: Chromosomes arrive at the opposite poles and then decondense.Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
5. Cytokinesis: the cytoplasm divides, resulting in two daughter cells

Stages of Meiosis II:

1. Prophase II: The chromosomes condense again, and a new spindle apparatus forms.
2. Metaphase II: Single chromosomes line up along the spindle equator.
3. Anaphase II: Sister chromatids are separated and pulled towards opposite poles by microtubules.
4. Telophase II: Nuclear membranes form around each group of chromosomes.
5. Cytokinesis: the cytoplasm divides resulting in the formation of four haploid daughter cells.

The acronym PMAT is very useful for helping you remember the different stages.

What does meiosis look like?

What are the key meiosis terms I need to know?

• Homologous chromosomes: a pair of chromosomes that are the same size and shape, and have the same gene loci.
• Haploid: cell or nucleus containing single, unpaired chromosomes (a single copy of chromosome).
• Diploid: cell or nucleus containing paired chromosomes (two copies of each chromosome).
• Gamete: reproductive (sex) cell that fuses with another gamete during fertilisation.
• Allele: a specific version or form of a gene that occupies a particular position, or locus, on a chromosome.
• Chromatid: one of the two strands of a chromosome that are joined together by a single centromere prior to cell division.
• Crossing over: the exchange of DNA material between non-sister homologous chromatids.
  Independent assortment: the random orientation and distribution of homologous chromosomes during meiosis I.

What is the difference between chromosomes and chromatids?

Understanding the difference between chromosomes and chromatids isn’t always easy, especially during meiosis. We count chromosomes by the number of centromeres present. So when the 46 human chromosomes duplicate during interphase and the amount of DNA in the cell doubles there are still only 46 chromosomes present because there are still only 46 centromeres present. However, there are now 92 chromatids, which are strands of replicated chromosomes.

What happens when meiosis goes wrong?

Meiosis is controlled and regulated by several complex molecular mechanisms. Errors that occur during meiosis can lead to genetic disorders or abnormalities in the offspring produced from the affected gametes.

How was meiosis discovered?

You don’t need to know this for your exams but you may be wondering: “How on earth the process of meiosis was discovered?” Like many scientific processes and mechanisms, it was discovered through the power of observation! In 1876 German biologist, Oscar Hertwig made some important observations about sea urchin eggs during fertilization in relation to the number of chromosomes in different cells.

Then in the early 1900s, two scientists had the same idea at the same time, which happens more often than you think! American geneticist Sutton and the German biologist Boveri independently proposed the chromosomal theory of inheritance. This theory implies that chromosomes are the physical unit of heredity and that their behaviour during meiosis is responsible for the distribution of genetic traits to offspring.

Their proposals were based on their studies of various organisms, such as grasshoppers and sea urchins. They noticed that the homologous chromosomes pair up during meiosis I and then separate independently during the subsequent stages, leading to the formation of haploid cells with varying combinations of alleles. Be careful not to get mixed up here, it is important to remember that the daughter cells all have the same number of genes but different combinations of alleles.

After these initial discoveries and observations, further research revealed the complex molecular mechanisms underlying meiosis. It was found that proteins and regulatory molecules play a vital role in regulating the different stages of meiosis. The discovery of meiosis has been crucial in improving our knowledge of genetics, inheritance, and evolution. It has allowed for major advancements in other fields such as medicine and agriculture.