3.2 – Meiosis
3.2.1 – State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei
Meiosis consists of two divisions. In the first division, the diploid cell replicates its chromosomes then divides, forming two diploid cells. However, in the second division, there is no DNA replication, so the resultant cells only contain half the DNA. Therefore, each diploid cell that undergoes meiosis will produce four haploid cells.
Diploid cells contain two sets of chromosomes, whilst haploid cells only contain one set.
Chromosomes in a diploid cell which contain the same sequence of genes, but are derived from different parents. During meiosis, the homologous chromosomes will pair up.
They share the same structural characteristics, such as length, shape and the loci of their genes. However, they may have different alleles of each gene.
3.2.3 – Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which result in four haploid cells
In this stage, the nuclear membrane is intact and the chromosomes are not densely
wound. In the G1 Stage, the chromosomes are still single DNA molecules with their
histones. In the S1 Stage, there is replication of chromosomes. Sister chromatids are held together at the centromere.
The DNA condenses by super coiling and become visible. Homologous chromosomes
pair up. Crossing over occurs as there is breakage and reunion of parts of chromatids
– this is the exchange of genetic material between the non-sister chromatids. The nuclear membrane breaks down and spindles form from the microtubules at opposite ends of cell, organised by the centrioles
The pairs of homologous chromosomes line up along the equator. The spindle fibres attach to the centromere. The random orientation of the chromosomes means that the maternal or paternal chromosome may move to either pole.
Spindle fibres shorten, pulling the chromosomes towards the opposite poles. Sister chromatids remain attached at the centromere. Each pole will have a complete haploid set of chromosomes consisting of one member of each homologous pair.
The spindle breaks down and the nuclear membrane reforms. Each daughter nucleus contains two sister chromatids for each chromosome, attached at the centromere. Crossing over means that the two sister chromatids are not identical. The cell divides and the two resulting cells are haploid cells
The nuclear membrane breaks down and a new spindle forms. The chromosomes appear as two chromatids joined at the centromere.
The chromatids arrange at the equator and the spindle fibres bind to both sides of the centromeres
The spindle fibres contract, causing the centromeres divide. Sister chromatids move to opposite poles.
Spindle breaks down and nuclear envelopes reform around the sets of daughter chromosomes. The cells divide again through cytokinesis, resulting in four haploid cells.
Meiosis contributes to genetic variability as it reduces chromosomes by half, permitting fertilisation and combination of genes from two parents. There is random assortment of maternal and paternal chromosomes during meiosis I, meaning that the genes from either parent have an equal chance of entering a cell. There is also recombination of segments of individual paternal and maternal homologous chromosomes due to crossing over.
3.2.4 – Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome
Non-disjunction is the term for the failure of a pair of chromatids to separate and go to opposite poles during the division of the nucleus. In meiosis, this results in gametes with more than and less than the haploid number of chromosomes.
A pair of number 21 chromosomes fails to separate during the formation of an egg or sperm, called nondisjunction, or trisomy 21. When the egg is fertilised to form an embryo, three copies of chromosome 21 are present, which is copied to every cell in the baby’s body. The risk of nondisjunction increases as women get older.
3.2.5 – State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure
Karyotype – the chromosome complement of a cell or whole organism
Karyotypes show the number, size and shape of chromosomes during metaphase of mitosis, They are prepared from the nuclei of cultured white blood cells. Scanning electron micrographs take a picture of the chromosomes. Chromosomes are arranged into pairs on the basis of length, pattern of banding and shape, and will appear as pairs of sister chromatids.
This shows a male human (Y) with no visible chromosomal abnormality
3.2.6 – State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities
Chorionic Villus sampling
A sample is taken from the fetal tissue part-buried in the wall of the uterus in the period 8 10 weeks. The purpose is to diagnose any conditions or diseases in the unborn child. These cells have the same genotype as the embryo. A sample is taken from the chorion, an extra embryonic membrane. A catheter tube is inserted via the vagina and a sample is taken. The sample is cultured to produce cells for karyotyping
Remove , produced by the amnion membrane and contains cells . The cells are removed by inserting a needle into the abdominal wall, myometrium and into the amniotic fluid. The fluid is centrifuged, cells are incubated and then karyotyped. The fluid (supernatant) can be used to test for neural tube disorder such as spina bifida.
Preparation of the Karyotype
A sample is taken. The cells are centrifuged and treated to make the cells swell up and chromosomes spread out. White blood cells are treated with a drug that causes them to go into mitosis, then one to halt the process at metaphase. Cell suspension is placed on a slide, dried and stained to show the banding. A photograph of chromosomes is taken and is cut up so that the chromosomes can be arranged in homologous pairs
Karyotyping can be carried out when chromosomes from the metaphase are available. Appropriate staining techniques are used to reveal characteristic banding patterns. The number of
chromosomes is counted. The chromosomes are then organised into pairs based on length, position of the centromeres, banding patterns and the satellite ends.