- Information
- AI Chat
Grade 12 Life Science: Meiosis FET Phase
Life Sciences
FET
Recommended for you
Preview text
Meiosis
What is meiosis? Meiosis is a type of cell division whereby a diploid cell (somatic cell) undergoes two cell divisions, and divides to form four dissimilar haploid cells (sex cells). Diploid cells have two sets of chromosomes, where each chromosome has a homologous partner. Haploid cells only have one set of chromosomes. Chromosomes in haploid cells have no homologous partners. Before meiosis begins (during interphase), DNA replication takes place. The result is two sets of chromosomes consisting of two identical chromatids joined together with a centromere. This is shown in Figure 2 (below)
The process of meiosis in animal cells : Meiosis is the type of cell division used to produce gametes or sex cells (sperm and egg cells). A cell undergoing meiosis will divide twice – the first division is meiosis I and the second is meiosis II. In the first meiotic division, the number of cells is doubled, but the number of chromosomes is not. This results in half as many chromosomes per cell. In the second meiotic division, the number of chromosomes does not get reduced. The diagram alongside shows how meiosis starts with a diploid cell and divides twice (meiosis I and II), resulting in four haploid cells.
First meiotic division: Prophase 1
- Chromosomes shorten and become visible as two chromatids joined by a centromere.
- Homologous pairs of chromosomes are now visible. • The nuclear membrane and nucleolus disappear.
- The spindle starts to form.
- Chromatids from each homologous pair touch. The point where they touch is called a chiasma.
- DNA is crossed over (swopped) at the chiasma.
- The spindle continues to form.
Metaphase 1
- The spindle extends across the whole cell.
- The homologous chromosomes line up along the equator of the spindle in their homologous pairs.
- One chromosome of each pair lies on either side of the equator.
- The centromere of each chromosome attaches to the spindle fibres.
Anaphase 1
- The spindle fibres shorten and pull each chromosome of each chromosome pair to opposite poles of the cell.
Telophase 1
- The chromosomes reach the poles of the cell.
- Each pole has half the number of chromosomes present in the original cell.
- The cell membrane constricts and divides the cytoplasm in half to form two cells
Second meiotic division Prophase 2
- Each cell formed during meiosis I now divides again.
- A spindle forms in each of the new cells.
Metaphase 2
- Individual chromosomes line up at the equator of each cell, with the centromeres attached to the spindle fibres.
M
Metaphase M for middle: The chromosomes move to the ‘middle’ (equator).
A Anaphase A for apart: The chromosomes/chromatids move apart/move to the poles.
T Telophase T for terminal: The final phase of meiosis I/ meiosis II.
The significance of meiosis There are two reasons why meiosis is important.
It reduces the number of chromosomes by half, in other words from diploid to haploid. This ensures that sex cells have half the number of chromosomes of other somatic cells so that when fertilisation occurs the zygote formed has the correct number of chromosomes_. It balances the doubling effect of fertilisation._ Crossing over introduces genetic variation. Genetic variation results in offspring that are better adapted to a particular environment and ensures that they will have a better chance of survival.
Abnormal meiosis Sometimes mistakes occur during the process of meiosis.
- This can happen in Anaphase 1 where the homologous chromosomes may not separate. Also called non-disjunction.
- It can also happen in Anaphase 2 when there is non-disjunction of the sister chromatids.
- If there is non-disjunction of chromosome pair 21 in humans, it leads to the formation of an abnormal gamete with an extra copy of chromosome 21.
- If there is fusion between a normal gamete and an abnormal gamete (with extra copy of chromosome 21) it leads to Down Syndrome.
Grade 12 Life Science: Meiosis FET Phase
Subject: Life Sciences
FET
