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Meiosis and Variation
Introduction
You should already be aware that living organisms reproduce and that this might be asexual (without
sex) or sexual (with sex). Asexual reproduction produces offspring that are genetically identical and
involves either mitosis (in eukaryotes) or binary fission (in prokaryotes). In contrast sexual reproduction
produces offspring that are genetically different, both from each other and from the parents. The
parents produce gametes or sex cells that then fuse during fertilisation, bringing together characteristics
from each parent when the zygote is formed.
You should be familiar with this schematic
from the Cells Exchange and Transport Unit:
Q. The terms haploid and diploid refer to
number of sets of chromosomes present
in the cell (nucleus in a eukaryotic cell).
What do the terms haploid and diploid
mean?
• Haploid:
• Diploid:
Meiosis
Meiosis
Sperm
n=23
Egg
n=23
Mitosis
leading to
growth
Fusion of
gametes
Mitosis
leading to
growth
Zygote
n=46
Mitosis
Q. Which of the cells are haploid?
Q. Which of the cells are diploid?
Clearly meiosis has a key role in halving the chromosome number in the gametes so that when
fertilisation happens the normal diploid number is restored – without meiosis the chromosome number
would double in successive generations. As we will see, meiosis also has an important role in introducing
genetic variation into gametes and hence the offspring of sexually reproducing organisms. Variation is
also introduced by the mixing of genetic information from the two parents and from mutation.
Q. Why is genetic variation so important?
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Cell Division
From the Cells Exchange and Transport Unit you will be aware that there are two types of cell division:
mitosis and meiosis. However, you will only have studied mitosis in any depth. The following is intended
as a reminder of what you should already be familiar with.
Mitosis
Stage
Description/Explanation of Key Events
Early prophase
During prophase the chromosomes become more distinct as
they coil up, shorten, thicken and take up stain more
intensely. The centriole divides and the nucleolus becomes
less prominent.
Late prophase
The chromosomes have become more distinct and are
clearly seen to consist of two chromatids joined by a
centromere.
The centrioles migrate to opposite poles of the cell.
The nucleolus continues to shrink and disappears.
The nuclear envelope disintegrates.
Metaphase
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Each centriole is at a pole and produces spindle fibres which
attach to the centromere of the chromosomes – note that
each centromere is attached to both poles.
Anaphase
Spindle fibres contract, the centromere divides and
chromatids (daughter chromosomes) are pulled centromere
first to opposite poles of the cell.
Telophase
Chromatids (daughter chromosomes) reach the poles of the
spindle, begin to uncoil and so become less distinct.
Nuclear envelope starts to reform.
Cytokinesis
The cell divides!
In animal cells, this begins by constriction from the edges of
the cell (invagination). In plant cells a cell wall is laid down.
The resultant cells have the same chromosome number as
the parent cell and the same genetic make-up. This is
because DNA replication preceded mitosis.
Q. Why is mitosis biologically important? (Think: growth, repair/replacement, asexual reproduction.)
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Meiosis
Meiosis involves two divisions: Meiosis I and Meiosis II. Essentially, during Meiosis I genetic variation is
introduced by ‘crossing over’ and by ‘independent assortment’. In addition the chromosome number is
halved. During Meiosis II chromosomes (pairs of chromatids) are split, and in total four haploid cells are
produced. The key events are summarised in the diagrams. Label and annotate them to describe the
main features, and complete the descriptions by inserting the missing words or phrases.
Meiosis
Stage
Prophase I
Description/Explanation of Key Events
Homologous chromosomes pair up. The pairing is called
__________ and results in paired chromosomes sometimes
called ___________ or ___________.
The centriole divides (animal cells only) and starts migrating
to opposite poles of the cell.
The nucleolus is present.
Late Prophase I
Crossing over (chiasma formation) may occur, especially in
the longer chromosomes. The adjacent chromatids can
break and reconnect with another chromatid. This
introduces ______________ _________________.
The nuclear envelope and the nucleolus _____________. At
the end of Prophase I the spindle is formed.
Metaphase I
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Bivalents or tetrads line up at the _______________ plate.
This is because each bivalent is attached to a spindle fibre
from opposite poles of the cell. Rather like in a tug of war,
the paired chromosomes get pulled to the metaphase plate
or ______________.
Anaphase I
Contraction of the _________ fibres results in separation of
whole chromosomes to opposite poles of the cell. This
contrasts with mitosis ______________________________
__________________.
Note: the orientation of one bivalent at metaphase is
independent of the orientation of other bivalents. Thus,
during anaphase either the maternal or paternal
chromosome can pass into either cell. This introduces yet
more _______________.
There are 2n possible combinations (where n=haploid
number).
Telophase I
Following telophase, animal cells usually divide – the
nucleolus and nuclear envelope reform, followed by
cytokinesis. This may not be the case in plant cells, which
may go straight into the second meiotic division.
Prophase 2 and Metaphase 2
The centriole divides, the nucleolus and nuclear envelope
disappear. Spindle fibres attach to the centromeres (one to
each pole) and pull the chromosomes to the metaphase
plate or ______________.
Anaphase II
The centromere divides and contraction of the spindle
microtubules pull the chromatids to opposite poles of the
cell. The chromatids are now sometimes referred to as
daughter _______________.
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Telophase II
The daughter chromosomes (chromatids) reach the poles of
the spindle and start to decondense. The nuclear envelope
and nucleolus reform. Cytokinesis follows and results in the
formation of four ________ daughter cells.
Q. What are the key markers/events for each of the stages?
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Q. How does meiosis differ from mitosis?
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Genetic Terminology
You may already be familiar with some of the key genetic terms from GCSE. Match the term to its
description.
1
Refers to cells containing a single set of chromosomes/genes, e.g. sperm and egg
cells
2
Refers to cells containing two sets of chromosomes/genes, e.g. human body cells
3
A length of DNA (chromosome) that carries the instructions for a particular protein
or polypeptide
4
The position on a chromosome that carries the instructions for a particular protein
5
An alternative form of a gene, e.g. in the human ABO blood group there are three
alternative forms coding for A, B or O
6
The genetic make-up of an individual
7
The physical expression of the genes – this may be influenced by the environment
8
An allele which is expressed when it is present in the genotype
9
An allele which is only expressed when the dominant allele is absent – in reality
when it is homozygous
10
Alleles which are partially expressed in the heterozygous state and result in a
mixing of the character, e.g. AB blood group
11
Both alleles of a particular gene are the same
12
The two alleles of a gene are different
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A
Diploid
G
Haploid
B
Allele
H
Genotype
C
Phenotype
I
Gene
D
Recessive
J
Codominant
E
Homozygous
K
Heterozygous
F
Dominant
L
Locus
Sources of Variation
Sexual reproduction is an important source of variation upon which natural selection operates. Meiosis
introduces variation into the daughter cells (gametes) by crossing over or chiasma formation and by
independent assortment. Fertilisation brings together male and female gametes which are genetically
different and so introduce yet more variation into the offspring.
Crossing over occurs between the chromatids of adjacent homologous chromosomes. The chromatids
become entangled, break and then fuse resulting in swapping of genetic material between the adjacent
chromosomes.
Independent assortment means that the inheritance of one chromosome is independent of the
inheritance of others – either chromosome from each homologous pair can pass into the gamete. If
there is one homologous pair there are 21 possibilities; if there are two homologous pairs there are 22
possibilities; if there are three homologous pairs there are 23 possibilities… In humans there are 23
homologous pairs, resulting in 223 possible chromosome combinations during metaphase I / anaphase I.
Fertilisation joins gametes from two different individuals – each of which has been subjected to crossing
over and independent assortment! Consequently even closely related individuals (siblings, i.e. brothers
and sisters) are genetically unique unless they are identical twins, produced by the cleavage of a single
fertilised egg cell.
Q. So, what are the sources of genetic variation in sexually reproducing organisms? List them and give a
brief (one-line) description.
Q. Are there any sources of genetic variation in asexually reproducing organisms?
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