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4.2 Meiosis

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4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.
Meiosis produces gametes (sex cell, egg or sperm cell).
Chromosome number is reduced in the daughter cells.
They are haploid (n).
They contain one copy of each chromosome instead of two.
A diploid cell (2n = somatic cell = body cell) has two copies of each chromosome.
One from the dad and one from the mum.
A male (sperm) and a female (egg) gamete may then fuse (fertilisation) to form a zygote, which will have the same number of chromosomes as a somatic cell.
4.2.2 Define homologous chromosomes.
Chromosomes which have the same arrangement of genes as each other. The actual base sequence of these genes may differ, however, resulting in different alleles of those genes. For example, homologous chromosomes carry a gene coding for eye-color on the same location, the actual information carried on these genes (the base sequence) may be different.
4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.
Homologous chromosomes pair up. Nuclear membrane will break down. Spindle fibres extended from each pole of the equator.
The pairs of chromosomes line up on equator. Spindle fibres attach to different chromosomes in each pair ensuring that each is pulled to opposite poles.
The spindle fibres pull the chromosomes to opposite poles halving the chromosome number. Each chromosome still consists of two chromatids.
The cell membrane around the equator is pulled inwards to divide the cell to form two haploid cells. New spindle fibres grow from the poles to the equator.
Steps 2 and 3 repeats (separating the chromatids).
Four new haploid cells are formed. Nuclear membranes reform. Each nucleus now has half of many chromosomes as the nucleus of the parent cell.
4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).
Sometimes chromosomes that separate and move to opposite poles move to one pole instead. When chromosomes do not separate it is called non-disjunction.
When gametes that contain an extra chromosome fertilize, the zygote produces three chromosomes of one type instead of the normal two. This is called trisomy of chromosome 21. This is when there are 3 chromosome 21's instead of just 2.
4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.
- Chromosomes are arranged in pairs according to size and structure.
- Can be used as a test for gender of an unborn fetus as well as for chromosome abnormalities.
4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.
Chorionic Villus Sampling (CVS) is a pre-natal test that can be done at 8-10 weeks.
Involves taking a sample of the chorionic villi to obtain fetal cells.
Karyotype can be made.
The risk are larger (1%) than of amniocentesis (0.5%).

Amniocentesis can be done around 16th week of the pregnancy.
A sample of the amniotic fluid (containing fetal cells) is taken and a culture is made.
When sufficient cells have been obtained, a karyotype can be done to detect chromosome abnormalities.
Genetic disorders can be detected.
4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred
- Count chromosomes to ensure that there is the right number; with more than 2 chromosomes in a pair a non-disjunction has occured
- Use the sex-chromosomes to determine the sex;
- Two x chromosomes means that the karyotype is female
- One x and one y means that the karyotype is male
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