OCR A2 UNIT F215 MEIOSIS AND VARIATION
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
· The daughter cells (gametes) contain the haploid number of
chromosomes compared with the parent cell, with the diploid
number
· This halving of the chromosome number is important so that the
diploid number can be restored during fertilisation of gametes
· Each parent cell divides to form 4 gametes
· 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
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
· They are usually the same size and shape
· Their centromeres are in the same position
· They have the same genes w/ same gene loci (position along the
chromosome)
· 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
(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
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
· Anaphase I
· Telophase I
· Prophase II
· Metaphase II
· Anaphase II
· Telophase II
Details of Events in Each Stage
Interphase
· 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
· 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
· 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
· Chromosomes separate along the spindle fibres that shorten to
pull homologous chromosomes apart
· The centromeres do not divide
· Chromatids, modified by crossing over, remain modified
· Independent separation of homologous chromosomes is a source
of genetic variation
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
Metaphase II
Metaphase II
· 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
· 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
· 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
· 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
non-sister 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
· 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
· 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
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
Independent Assortment of Chromatids at Metaphase II and
Independent Separation of Chromatids during Anaphase II
· 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
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
· 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
· Chromosome mutations may occur during meiosis such as
non-disjunction
· 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
MITOSIS
MEIOSIS
Involves DNA replication in interphase
X
Involves organelle replication in interphase
X
Involve spindle formation
X
X
Only one division of the nucleus
X
Two divisions of the nucleus
X
Important during growth of an organism
X
Produces clones of cells
X
Important in asexual reproduction
X
Introduces genetic variation
X
Occurs in the sex organs
X
X
Homologous chromosomes pair up in prophase I
X
X
Daughter nuclei have the same number of chromosomes as the
parent nucleus
X
Crossing over may occur in prophase I
X
Chiasmata are never formed
X
Four daughter cells are produced
X
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