OCR A2 UNIT F215 MEIOSIS AND VARIATION
Specification:
a) Describe, with the aid of diagrams and photographs, the behaviour of
chromosomes during meiosis, and the associated behaviour of the
nuclear envelope, cell surface membrane and centrioles. (names of the
main stages are expected, but not the subdivisions of prophase)
b) Explain the terms allele, locus, phenotype, genotype, dominant,
codominant and recessive
c) Explain the terms linkage and crossing-over
d) Explain how meiosis and fertilisation can lead to variation through the
independent assortment of alleles
IMPORTANCE OF MEIOSIS:

Nuclear division that occurs in the sex organs to produce gametes

Process involves a reduction in the chromosome number of a diploid
cell
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The daughter cells (gametes) contain the haploid number of
chromosomes compared with the parent cell, with the diploid number
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This halving of the chromosome number is important so that the
diploid number can be restored during fertilisation of gametes
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Each parent cell divides to form 4 gametes
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Meiosis is a source of genetic variation in the gamete cells
SOURCES OF GENETIC VARIATION IN MEIOSIS:

Chiasmata and crossing over in prophase I

Independent assortment of homologous chromosomes in
metaphase I
Important Definitions
Haploid (n) refers to cells or organisms with only one set of chromosomes
per cell
Diploid (2n) refers to cells or organisms with two sets of chromosomes per
cell
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Homologous Chromosomes

Diploid (2n) cells of an organism have the full complement of
chromosomes

In diploid cells, the chromosomes are in pairs called homologous pairs

Each species has a specific diploid number. Human cells have a
diploid number of 23 pairs
Features of Homologous Chromosomes
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They are usually the same size and shape
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Their centromeres are in the same position
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They have the same genes w/ same gene loci (position along the
chromosome)
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The homologous chromosomes of each pair are not genetically
identical since the alleles of the genes could be different

In the first cell of the organism produced by sexual reproduction, one
chromosome of each homologous pair was derived from the mother
and the other from the father
The left hand diagram below is from a photograph of all the
chromosomes from a human male diploid cell
The right hand diagram shows the same chromosomes arranged into
homologous pairs (note the X and Y sex chromosomes No 23)
Identifying Homologous Chromosomes
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(1) State the diploid number of this cell...6..........................................
(2) How many pairs of homologous chromosomes are in this cell? 6...
(3) Using 3 different coloured pencils, indicate the different homologous pairs
Gene
A gene is a length of DNA that codes for the synthesis of a polypeptide. A
gene determines a specific feature of an organism
Gene Locus
The position of the gene on a chromosome is called the gene locus
Alleles
Alleles are alternative forms of a gene. Each gene locus can only contain one
allele
Crossing-over
The process by which a chromatid breaks during prophase I of meiosis I and
rejoins to a non-sister chromatid of its homologous chromosome, so that the
non-sister chromatids exchange alleles
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LIFE CYCLE OF A HUMAN TO REPRESENT HAPLOID AND DIPLOID
STAGES
State the number of chromosomes and whether haploid or diploid, in the
following cells;
Human sperm cell there is 46 chromosomes and it is diploid
Human ovum (egg cell)……………………………………………………..
Human zygote………………………………………………………………..
Human embryo cell………………………………………………………….
Human testis/ovary tissue cell……………………………………………..
CELL CYCLE OF CELLS IN REPRODUCTIVE ORGANS THAT UNDERGO
MEIOSIS
A cell in the ovary/ testis/anther/ovule that undergoes meiosis will go through
a cell cycle including interphase as follows

Interphase (G1, S and G2)

Meiosis I

Cytokinesis

Meiosis II

Cytokinesis
Meiosis I and meiosis II represent a double division of the nucleus to produce
4 haploid cells from 1 diploid parent cell
The stages of meiosis are as follows:
 Prophase I
 Metaphase I
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Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Details of Events in Each Stage
Interphase
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Chromosomes not visible because they are
decondensed and not supercoiled
Cell is preparing for meiosis and cytokinesis
DNA replication occurs in S phase
Organelles are synthesised, increased
protein synthesis and ATP synthesis occur
in G1 and G2 phases
Prophase I
Early Prophase I
 Chromosomes take up stains and can be seen with a light
microscope
 They become more visible as they condense by
supercoiling, becoming shorter and thicker
Early Prophase I
 As the chromosomes thicken, each chromosome is seen
as a pair of sister chromatids, joined by a centromere
Late prophase I
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Homologous chromosomes pair up, by synapsis,
forming bivalents
Each member of the homologous pair has the same genes
at the same loci
Each homologous pair consists of one maternal and one
paternal chromosome
The non-sister chromatids may wrap around each other
and attach at points called chiasmata.
The non-sister chromatids may exchange sections of
chromatids with each other – a process called crossing
over. This is one source of genetic variation
The centrioles (present in animal cells only) migrate to
the poles of the cell and the spindle fibres form from the
centrioles
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The nuclear envelope and nucleolus break down
Metaphase I
Metaphase I

The bivalents line up at the equator
(middle) of the cell, with centromeres
attached to the spindle fibres

The bivalents are arranged randomly.
Each member chromosome of an
homologous pair face opposite poles

Independent assortment of
homologous chromosomes is also a
source of genetic variation
Anaphase I
Anaphase I

Separation of homologous
chromosomes, to opposite poles of the
cell
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Chromosomes separate along the
spindle fibres that shorten to pull
homologous chromosomes apart
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The centromeres do not divide
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Chromatids, modified by crossing over,
remain modified
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Independent separation of homologous
chromosomes is a source of genetic
variation
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Telophase I
Telophase I

Chromosomes arrive at the poles of
the cell (one of each homologous pair
at each pole)

The chromosomes may decondense
and become less visible

Spindle fibres disintegrate

In animal cells, nuclear envelopes and
nucleoli reform at each end of the cell
Cytokinesis I

In animal cells, division of the cytoplasm produces 2 haploid cells, each
containing one of each of the homologous pairs of chromosomes

Cytokinesis in animal cells involves the pinching in of the plasma
membrane at the equator forming a cleavage furrow. The initial
formation of this furrow can be seen in the cell diagram under
telophase I above
Note that in most plants cells, the cell goes from Anaphase I to Prophase II
directly
Meiosis II
In Meiosis II, the equator of the cells and the poles are at right angles to their
positions during Meiosis I
Prophase II
Prophase II

Chromosomes condense and become
visible
Each chromosome is seen as a pair of
chromatids joined by a centromere
If they reformed in telophase I, the
nuclear envelopes and nucleoli
disintegrate
The centrioles migrate to opposite poles
(animal cells only)
Spindle fibres form, at right angles to the
previous fibres in meiosis I
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Metaphase II
Metaphase II
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The chromosomes line up along
the equator with centromeres
attached to the spindle fibres
The chromatids of each
chromosome may not be
genetically identical, because of
crossing over, and are arranged
independently at the equator
Independent assortment of
chromatids at the equator is a
source of genetic variation
Anaphase II
Anaphase II
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Centromeres divide
The chromatids separate, pulled apart
by shortening of the spindle fibres
The chromatids are pulled to opposite
poles
The independent separation of ‘sister’
chromatids is a source of genetic
variation
Telophase II
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The chromatids (now chromosomes)
have reached the poles
Spindle fibres disintegrate
The chromosomes decondense
becoming less visible
The nuclear envelope and nucleolus
reform
Cytokinesis II
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Division of the cytoplasm occurs in both cells produced from
cytokinesis I, producing 4 haploid gametes, sometimes referred to as a
tetrad
Each cell has 1 of each homologous chromosome
It is highly likely that all 4 cells will be genetically different
How Meiosis leads to Genetic Variation
1. Crossing over during prophase I – exchange of alleles between nonsister chromatids
2. Independent assortment of maternal and paternal homologous
chromosomes in metaphase I
3. Independent assortment of sister chromatids during metaphase II
4. Random chromosome mutation (e.g. non-disjunction)
How Fertilisation leads to Genetic Variation
1. Random mating
2. Random fertilisation of male and female gametes
Crossing Over
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During prophase I, homologous chromosomes pair up forming
bivalents
Non-sister chromatids wrap around each other very tightly, attaching at
chiasmata. Label the chiasmata in the diagram below, indicating which
chromatids are involved in the formation of each
The chromatids may break at the position of the chiasmata. If this
happens, the broken ends may rejoin to the non-sister chromatid in the
same bivalent. This leads to crossing over – the exchange of alleles
between non-sister chromatids
Crossing over results in new combinations of alleles in the chromatids
that will become chromosomes in the haploid gametes
During metaphase I, the chiasmata remain intact and hold the maternal
and paternal homologous chromosomes together on the spindle
During anaphase I, the chiasmata break and one chromosome (made
up of 2 chromatids) of each homologous pair is pulled to opposite poles
On average 2-3 cross-over events occur on each homologous pair
Independent Assortment of Homologous Chromosomes during
Metaphase I and Independent Separation of Homologous Chromosomes
during Anaphase I
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Maternal and paternal homologous chromosomes are randomly
distributed at the spindle equator during metaphase I
This random distribution leads to independent separation of the
maternal and paternal homologous chromosomes during anaphase I
Each daughter nucleus contains a different mixture of maternal and
paternal homologous chromosomes
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The cell below contains three homologous pairs of chromosomes at
metaphase I
There are four possible outcomes when two daughter cells are produced
during meiosis I
Complete the cells below by drawing in the possible combinations of
maternal and paternal chromosomes in the two daughter nuclei after
anaphase I and telophase I, using red and blue pencils
1.
2.
3.
4.
Independent Assortment of Chromatids at Metaphase II and Independent
Separation of Chromatids during Anaphase II
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Because of crossing over in prophase I, ‘sister’ chromatids are no
longer genetically identical
The distribution of the chromatids along the equator is random
during metaphase II and this will determine the independent
separation of ‘genetically non-identical sister chromatids’ during
anaphase II
The diagram below shows one daughter cell containing three
chromosomes after meiosis I. Crossing over has occurred in all three
chromosomes
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Two haploid gametes are produced from each haploid daughter cell produced
in Meiosis I
There are four different ways in which these 3 chromosomes could be
distributed at the equator in metaphase II and therefore, four different
independent ways in which the chromatids could be separated in anaphase II
Draw these possible combinations in the cells below, using red, blue and
green pencils
1.
2.
3.
4.
Random Mating
and Fertilisation
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Any adult female of a species can mate with any adult male of the
same species. This is random mating.
In sexual reproduction, the nuclei of two haploid gametes must fuse to
restore the diploid number of the zygote.
This fusion of gametes (fertilisation) is completely random and adds to
genetic variation within a population.
Random Chromosome Mutation
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Chromosome mutations may occur during meiosis such as nondisjunction
If the mutation occurs in the production of gametes and a mutated
gamete is involved in fertilisation, the- mutation will be present in every
body cell of the offspring
Comparison of mitosis and meiosis
FEATURES
Involves DNA replication in
MITOSIS
X
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MEIOSIS
interphase
Involves organelle replication in
interphase
Involve spindle formation
Only one division of the nucleus
Two divisions of the nucleus
Important during growth of an
organism
Produces clones of cells
Important in asexual
reproduction
Introduces genetic variation
Occurs in the sex organs
Homologous chromosomes pair
up in prophase I
Daughter nuclei have the same
number of chromosomes as the
parent nucleus
Crossing over may occur in
prophase I
Chiasmata are never formed
Four daughter cells are produced
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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Answers to questions on pages 13 and 14
Four possible combinations after random separation of three pairs of
homologous chromosomes during meiosis I
Four possible combinations after random separation of ‘sister’
chromatids during meiosis II
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