4.2: Meiosis

Question 1

State that meiosis is a reduction division of a diploid nucleus to form haploid nucleus.

Answer 1

Meiosis is a reduction division of a diploid nucleus to form haploid nucleus.

Question 2

Define homologous chromosomes:

Answer 2

Chromosomes with the same genes as each other, in the same sequence but do not necessarily have the same allele of those genes.

Question 3

Outline the process of meiosis:

Answer 3

Meiosis involves two divisions. Meiotic cells have an interphase stage before the start of meiosis I which is similar to mitosis. It includes G1, S and G2 phases. (See notes on mitosis) After meiosis I there is another brief interphase stage which is followed by meiosis II.

Meiosis I
The first stage of meiosis I is prophase I. In prophase I the chromosomes pair up so that the chromosomes in each pair are homologous. Once the homologous chromosomes are paired up, crossing over occurs. Crossing over is the exchange of genetic material between non-sister chromatids. The nuclear membrane also starts to break down and the spindle microtubules stretch out from each pole to the equator.

The second stage is metaphase I. Here the paired up homologous chromosomes line up at the equator and the spindle fibbers attach to the chromosomes in a way that ensures that for each homologous pair, one chromosome moved to one pole and the other moves to the opposite pole.

The third stage is anaphase I. This is the stage where the homologous chromosomes are separated and pulled to opposite poles. This halves the chromosome number however each chromosome is still composed of two sister chromatids. The cell membrane starts to prepare for its separation at the equator to form two cells.

The fourth stage is telophase I. Here each chromosome from the homologous pair are found at opposite poles and the nuclear membrane reforms around each daughter nucleus. The membrane then divides through citokinesis.

There is a brief interphase stage before the start of meiosis II. This stage does not include the S phase.

Meiosis II
The first stage of meiosis II is prophase II. Here the cell has divided into two daughter haploid cells however the process does not end here as these two cells immediately start to divide again. The spindle microtubules stretch out from each pole again and the nuclear membrane breaks down as in prophase I.

The second stage is metaphase II. Here the chromosomes in each cell line up at the equator and the spindle microtubules attach to the centromere of each chromosome.

The third stage is anaphase II. Here the centromere devised as a result of the spindle microtubules pulling each sister chromatid to opposite poles in both cells. Each sister chromatid then becomes a chromosome.

The fourth stage is telophase II. Here the nuclear membrane reforms around the four sets of daughter chromosomes. Cytokinesis then follows to divide the cytoplasm of the two cells and so the result is four daughter cells each with a haploid set of chromosomes.

Question 4

Explain that non-disjunction can lead to changes in chromosome number:

Answer 4

• Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy).
• The failure of the chromosomes to separate may either occur via:
  + Failure of homologues to separate during Anaphase I (resulting in four affected daughter cells)
  + Failure of sister chromatids to separate during Anaphase II (resulting in two affected daughter cells).

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21):
+ One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
+ The other parental gamete was normal and had a single copy of chromosome 21
+ When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21, leading to Down syndrome.

Question 5

State that, in karyotyping, chromosomes are arranged in pairs according to their structure.

Answer 5

In karyotyping, chromosomes are arranged in pairs according to their size and structure.

Question 6

State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Answer 6

Pre-natal karyotyping is often used to:

• Determine the gender of an unborn child (via identification of sex chromosomes)
• Test for chromosomal abnormalities (e.g. aneuploidies resulting from non-disjunction)

Amniocentesis:

• A needle is inserted through the abdominal wall, into the amniotic cavity in the uterus, and a sample of amniotic fluid containing foetal cells is taken
• It can be done at ~ 16th week of pregnancy, with a slight chance of miscarriage (~0.5%)

Chorionic Villus Sampling:

• A tube is inserted through the cervix and a tiny sample of the chorionic villi (contains foetal cells) from the placenta is taken
• It can be done at ~ 11th week of pregnancy, with a slight risk of inducing miscarriage (~1%)