Genetics
4.2 : Meiosis
DP Biology
HL
Mitosis occurs in somatic (non-reproductive) cells and is the division of a diploid parent cell to produce two,
identical, diploid cells.
Meiosis occurs in reproductive cells and is the division of a diploid parent cell to produce four, non identical,
haploid cells. It is termed reduction division as the chromosome number is reduced by half.
A haploid cell contains a single complete set of chromosomes. These chromosomes carry all the genes needed by the
cell, one gene for every polypeptide that has to be made.
A diploid cell contains two complete sets of chromosomes. This means there are two genes for
every polypeptide that needs to be made ie two genes for eye colour, two for blood group etc.
A pair of chromosomes carrying genes for the same polypeptide, or characteristic, is known as a
homologous pair.
Meiosis (reduction division)
Meiosis results in the formation of haploid gametes and is therefore known as
reduction division. The process involves two cycles of cell division, one following
the other. These are known as meiosis I and meiosis II. In each of these cycles you
get the same sequence of: prophase, metaphase, anaphase and telophase.
Prior to meiosis, during interphase,
the DNA is replicated and each
chromosome becomes a pair of
chromatids joined by a centromere.
Meiosis I
In prophase I of meiosis the DNA condenses, the nuclear envelope breaks down, the
homologous chromosomes pair up and the spindle fibres form. Each homologous
pair is termed a tetrad and is visible as a tight cluster of 4 chromatids. At various
places along their length are crisscrossed. These crossings are called chiasmata.
These are the points at which genetic material is exchanged between homologous
chromosomes
During metaphase I the homologous pairs of chromosomes line up along the equator.
Spindle fibres attach to the centromere of each homologous pair.
During anaphase I the homologous pairs are pulled apart by the spindle fibres. (no centromeres are split)
During telophase each pair of homologous chromosomes reaches the pole of the cell. The cell now divides into two,
this process is called cytokinesis . Nuclear envelopes normally do not reform.
The daughter cells produced by meiosis I are officially haploid, since they only contain a single set of chromosomes,
although each chromosome consists of a pair of chromatids. The aim of meiosis II is to separate the chromatids
Meiosis II
This is virtually identical to meiosis I except that there is no duplication of DNA.
During prophase II spindle fibres reform
Chromosomes line up along the equator of the cell during metaphase II. Spindle fibres attach to centromeres.
During anaphase II the sister chromatids separate, now individually termed chromosomes, move towards opposite
poles of the cell.
At telophase II nuclei form at opposite poles of the cell and cytokinesis occurs.
Meiosis Activities
Each of the daughter cells produced by meiosis is haploid. This means that it will receive one chromosome from
every pair of homologous chromosomes in the diploid parent cell. Which chromosome it gets from each pair is
entirely a matter of chance., and depends on the way the pair lien up during metaphase I. Because of this
Independent assortment of the parental chromosomes there are many different types of daughter cells that can be
produced. Imagine a diploid parent cell with three pairs of chromosomes; A1 and A2, B1 and B2, and C1 and C2. A
haploid daughter cell might end up with chromosomes A1, B1 and C2 or with A2, B1 and C1, or any other
combination. (humans have 23 pairs of chromosomes and therefore the re are 223 different possible combinations –
this is over 8 million different combinations).
The variety of daughter cells that can be produced in meiosis is further increased by a process known as crossing
over. During prophase and metaphase of the first meiotic division, when the homologous chromosomes are lying
side by side they often become entangled. When they are pulled apart in anaphase I, they end up exchanging lengths
of DNA. This results in the production of chromosomes that contain new and unique combinations of genes.
Comparison of mitosis and meiosis
Karyotyping and its uses
The figure on the right shows a photograph of human chromosomes. If the
chromosomes are cut out they can be arranged into matching pairs (homologous pairs)
according to their size, position of the centromere and the pattern of banding (shown
below). Apart from the sex chromosomes (X and Y) the pair contain the same genes ie
eye colour. There may, however, be different forms of the gene for example blue or
brown eyes.
Human cells each have 46 chromosomes (23 pairs), other species have
different numbers, for example chimpanzee cells each have 48
chromosomes (24 pairs) and cabbage plants have 18 chromosomes
(19 pairs).
Down’s Syndrome and trisomy
Advances in DNA technology have brought a new era in preventative medicine. We can
now detect a large range of inherited diseases before birth, one of the most common
of which is Down’s syndrome.
Down’s syndrome is the most common single cause of learning disability in children of
school age. Children with the syndrome typically have a round, flat face, and eyelids
that appear to slant upwards. They also have a higher rate of infection and heart
defects.
The syndrome is named after John Langdon Down who first described the condition in 1866. In 1959 Lejeune
showed that Down’s Syndrome is caused by an extra chromosome 21. Having one extra chromosome is known as
trisomy, hence Down’s syndrome is known as trisomy 21.
The extra chromosome usually comes from the
egg cell due to non-disjunction of chromosome
21. About 70% of the non-disjunction occurs
during meiosis I, when homologous
chromosomes fail to separate and 30% during
meiosis II when chromatids fail to separate.
Genetic screening: amniocentesis and chorionic villus sampling
Genetic screening refers to procedures used to examine an individual for the presence of a genetic disease. The most
widely available genetic screening procedure for Down’s syndrome is amniocentesis. This disease is more common in
pregnant women over the age of 35.
The risk of Down’s syndrome varies with maternal age:
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1:1500 at the age of 20
1:800 at the age of 30
1:270 at the age of 35
1:100 at the age of 40
1: 50 at the age of 45+
Amniocentesis is usually carried out at 15-16 weeks of
pregnancy. It involves passing a very fine needle into the
uterus, observed with an ultrasound image, and withdrawing a
sample of amniotic fluid containing fetal cells. The karyotype
of the fetal cells is then analysed to test for Down’s syndrome.
There is a 0.5 – 1.0% risk of spontaneous miscarriage after the
procedure. Therefore amniocentesis is usually recommended
only for those at high risk of carrying a Down’s syndrome baby.
Chorionic villus sampling (CVS) involves the extraction of a sample of cells
from the chorionic villus (small finger-like processes which grow from the
embryo into the mother’s uterus). The sample is obtained by inserting a fine
catheter via the vagina and cervix into the actively dividing cells of the chorion.
Fetal cells are then analysed as for amniocentesis. This procedure can be
carried out between the 8th and 12th week of pregnancy.
If the tests show the fetus to have Down’s syndrome a decision about
abortion can be made. CVS allows earlier detection, this is an advantage as
early abortions are less difficult both physically and mentally. However a
higher risk of miscarriage is associated with CVS than amniocentesis.
Genetic counselling
Suppose that a couple has been identified as being at risk of having a child with a genetic disorder. A specially
qualified genetic counsellor provides advice, both before and after screening.
Counselling aims to make sure that the parents have a proper understanding of the probability of the risk that that
they have of producing an affected child. The severity of the disorder concerned is also explained. The options
available to the couple are considered in the light of their religious and moral beliefs and their cultural background.
The hope is that the couple can then make a decision in an informed manner. The results of any tests and the
discussions are confidential, so that a couple has a free choice of what to do next.
Using karyotypes to identify genetic disorders
Genetic screening raises a number of questions:
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Who should be screened?
Who should have access to information derived from screening?
Do parents have the right to terminate the pregnancy of a fetus with a genetic disorder?
Should an individual who is found to be a carrier of an inherited disorder tell other members of the family
who might also be affected?
What would be the advantages and disadvantages of screening everyone for one or more inherited disorders?
Other trisomy diseases:
Trisomy 13
Trisomy 16
Trisomy 18
Trisomy XXX
Kleinfelter’s
Trisomy XYY