HL Meiosis
Syllabus Statements 10.1
HL Understandings of Meiosis
1. Chromosomes replicate in interphase before meiosis.
2. Crossing over is the exchange of DNA material between nonsister homologous chromatids.
3. Crossing over produces new combinations of alleles on the
chromosomes of haploid cells.
4. Chiasmata formation between non-sister chromatids can
result in an exchange of alleles.
5. Homologous chromosomes separate in meiosis 1.
6. Sister chromatids separate in meiosis II.
7. Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis 1
Applications and Skills
• 1. Draw diagrams to show chiasmata
formed by crossing over. Diagrams of
chiasmata should show sister chromatids
still closely aligned, except at the point
where crossing over occurred and a
chiasma was formed.
Syllabus statements
10.1.1 Describe the behavior of the chromosomes in the
phases of meiosis.
10.1.2 Outline the formation of chiasmata in the process of
crossing over. Use diagrams
10.1.3 Explain how meiosis results in an effectively infinite
genetic variety in gametes through crossing over in
prophase I and random orientation in metaphase I.
10.1.4 Distinguish between autosomes and sex chromosomes.
10.1.5 Explain how crossing over between non-sister
chromatids of a homologous pair in prophase 1 can result in
an exchange of alleles. Use diagrams
10.1.6 State Mendel’s law of independent assortment.
10.1.7 Define linkage group.
Retro:
• 1. Compare diploid chromosome numbers of Homo sapiens,
Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris
equorum.
• 2. Define genome size (total length of DNA in an organism).
• 3. Compare genome size of T2 phage, Escherichia coli,
Drosophila melanogaster, Homo sapiens and Paris japonica.
• 4. Describe Cairns’ technique for measuring the length of
DNA molecules by autoradiography.
• 5. Description of methods used to obtain cells for karyotype
analysis e.g. chorionic villus sampling and amniocentesis and
the associated risks.
• 6. Draw diagrams to show the stages of meiosis resulting in
the formation of 4 haploid cells.
1. Compare diploid chromosome numbers of Homo
sapiens, Pan troglodytes, Canis familiaris, Oryza sativa,
Parascaris equorum.
The number of chromosomes is a
characteristic feature of members of a
species. The diploid number is the number
of chromosomes present in normal body
cells (not gametes).
The diploid number varies with some
species having fewer large chromosomes
and others having a greater number of small
chromosomes.
Five examples of chromosome
number
Homo sapiens (humans) 46
Pan troglodytes (chimpanzee) 48
Canis familiaris (dog) 78
Oryza sativa (rice)
24
Parascaris equorum
4
(Horse threadworm)
1. Explain why none of the species has an odd number of
chromosomes.
2. Discuss, using these data, the hypothesis that the more complex
an organism is, the more chromosomes it has.
3. Explain why the size of the genome of a species cannot be
deduced from the number of chromosomes.
4. Suggest, using this table, a change in chromosome structure that
may have occurred during human evolution.
3. Compare genome size of T2 phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and Paris japonica.
• Genome size (total length of DNA in an
organism). This varies by a huge amount
with the smallest genomes those of
viruses. Given in million base pairs.
• T2 phage (virus that attacks E. coli): 0.18
• Escherichia coli (gut bacterium): 5
• Drosophila melanogaster (fruit fly): 140
• Homo sapiens (humans): 3000
• Paris japonica (woodland plant) : 150,000
Trends
• The prokaryote has the smallest genome.
• Genome size of eukaryotes depends on
the size and number of chromosomes.
• It is correlated with the complexity of the
organism, but is not directly proportional.
• The reason for this is that the proportion of
DNA that acts as functional genes is very
variable and also the amount of gene
duplication varies.
4. Describe Cairns’ technique for measuring the
length of DNA molecules by autoradiography.
Stolen from Allott and Mindorf (2014)
• John Cairns produced images of DNA molecules from E. coli using this
technique:
• Cells were grown for 2 generations in a culture medium containing tritiated
thymidine (thymine linked to deoxyribose). Tritium is a radioactive isotope
of hydrogen…radioactively labelled DNA produced by replication.
• Cells placed in dialysis membrane and their cell walls using lysozyme.
Cells gently burst with DNA released onto surface of dialysis membrane.
• Thin film of photographic emulsion applied to the surface of the membrane
and left in darkness for 2 months. Some of the atoms of tritium in the DNA
decayed and emitted high energy electrons, which react with the film.
• At end of 2 months, film was developed and examined under a microscope.
Dark grains appeared every place tritium atom decayed indicating position
of DNA.
• Images of E coli showed that chromosome was single, circular molecule
with a length of 1,100 um. (length of E. coli is 2 um).
Autoradiography
John Cairns 1963
Chromosome of E. coli
5. Description of methods used to obtain cells for
karyotype analysis e.g. chorionic villus sampling and
amniocentesis and the associated risks.
Preparation of Human Karyogram
(Karyotype is a property of a cell – the number and type of
chromosomes present in the nucleus)
The Human Life Cycle
Meiosis Overview
Review of Definitions
1. Somatic cells: non sex cells.
2. Sex cells (gametes): egg (ova), sperm
3. Autosome: chromosome that is not a sex
chromosome (does not determine gender).
4. Sex chromosomes: dissimilar chromosomes
that determine an individual’s sex. In humans:
X, Y
5. Homologous chromosomes: pairs of
chromosomes that have the same size,
centromere position, staining pattern, location
of genes.
Definitions continued
6. Fertilization: the union of two gametes to
form a zygote.
7. Zygote: a diploid cell that results from
the union of 2 haploid gametes.
8. Meiosis: special type of cell division that
produces haploid cells and compensates
for the doubling of chromosome number
that occurs at fertilization.
Key Differences with Mitosis
1. Meiosis is a reduction division: gametes
have ½ number of chromosomes as
parent cell.
2. Meiosis creates genetic variation: 4
daughter cells genetically different from
parent cell and from each other.
3. Meiosis is 2 successive nuclear
divisions.
Meiosis I
Meiosis II
Stages of Meiotic Cell Division
a) Interphase I: precedes meiosis
i. Chromosomes replicate
ii. Each duplicated chromosome consists
of 2 identical sister chromatids attached
at their centromere.
iii. Centriole pairs in animal cells also
replicate into two pairs.
Meiosis I:
Segregates the 2 chromosomes of each
homologous pair and reduces the chromosome
number by one-half.
b)Prophase I: 90% of the time required for
meiosis.
i. Longer and more complex than prophase of
mitosis.
ii. Chromosomes condense – start to coil up and
become shorter and thicker.
iii. Synapsis occurs: homologous chromosomes
come together as pairs. Uses the synaptenemal
complex to accomplish this.
Chromosome coiling (nucleosome) and pairing of
bivalents with synaptonemal complex
Prophase I continued
iv.
Each chromosome has 4 chromatids, so that each
homologous pair in synapsis appears as a complex of
4 chromatids of a tetrad.
v. In each tetrad, sister chromatids of the same
chromosome are attached at their centromeres.
vi. Nonsister chromatids are linked by X-linked
chiasmata, sites where homologous strand exchange
or crossing over occurs.
vii. Chromosomes thicken further and detach from the
nuclear envelope.
viii. Centriole pairs move apart, toward poles, and spindle
microtubules form between them.
ix. Nuclear envelope and nucleoli disperse.
Chromosome Tetrad
Meiosis I continued
c)
Metaphase I: tetrads (bivalents) are aligned on the
metaphase plate, having moved there during this
stage.
i. Chromosomes continue to shorten and thicken.
ii. Spindle microtubules attach to the kinetochore region
of the centromeres.
iii. Bivalents line up on the equator so that centromeres
of homologues point towards opposite poles.
iv. Chiasmata slide towards the ends of the
chromosomes causing the shapes of the bivalents to
change.
v. At the end, chromosomes start to move.
Metaphase 1: 2n = 4; 2n = 6
Chiasmata terminalizing
Meiosis I continued
d) Anaphase I: homologues separate and are
moved towards the poles by the spindle
apparatus. (Bivalents separate). This halves
the chromosome number.
i. Each chromosome consists of 2 chromatids
(sister chromatids remain attached at the
centromeres).
ii. Because of crossing over, the 2 chromatids
are not identical.
iii. At the end, chromosomes reach the poles.
Meiosis I continued
e)
Telophase I and cytokinesis: spindle apparatus
continues to separate homologous chromosome pairs
until the chromosomes reach the poles.
i. Each pole has a haploid set of chromosomes
composed of 2 sister chromatids.
ii. Nuclear membranes form around the groups of
chromosomes at each pole.
iii. Chromosomes uncoil partially.
iv. Cytokinesis occurs simultaneously forming 2
daughter cells. Cleavage furrows form in animal cells
and cell plates form in plant cells.
Meiosis I: reduction division – separation of
homologous chromosomes
Meiosis II: Separation of sister
chromatids
Interphase II: May not really
exist per se
f) Two cells either enter a brief period of
interphase or immediately proceed to the
second division of meiosis.
i) DNA is not replicated.
Meiosis II: separates sister
chromatids of each chromosome
g) Prophase II: chromosomes become
shorter and thicken again by coiling.
i. Centrioles move to poles (animal cells).
ii. Nuclear membrane breaks down.
iii. Spindle apparatus forms and
chromosomes move towards the
metaphase II plate.
Meiosis II continued
h) Metaphase II: chromosomes align singly
on the metaphase plate.
i. Spindle microtubules attach to the
centromeres.
ii. Chromosomes line up on equator.
iii. Centromeres divide
iv. Kinetochores of sister chromatids
point towards opposite poles.
Metaphase 2: 2n = ?
Meiosis II continued
i)
Anaphase II: sister chromatids separate and
move toward opposite poles of the cell.
j) Telophase II and Cytokinesis: nuclei form at
opposite poles of the cell.
i) nuclear membranes reform around the
groups of chromatids at each pole.
ii) Cytokinesis produces 4 cells total.
iii) Chromosomes uncoil, nucleoli appear
iv) In most organisms, develop into gametes
Comparison of mitosis and meiosis
Summary Comparison
Meiosis and fertilization
Primary sources of genetic variation in
sexually reproducing organisms.
Sexual reproduction provides genetic
variation by:
1. Independent Assortment
2. Crossing over during Prophase I
3. Random fertilization by gametes
Independent Assortment of
chromosomes
1. Orientation of the homologous pair of
chromosomes (one maternal and one paternal)
is RANDOM. 50-50 chance that each gamete
receives maternal or paternal derived
chromosome.
2. Each homologous pair of chromosomes
orients independently of the other pairs at
metaphase I. Ist meiotic division results in
independent assortment of maternal and
paternal chromosomes.
Alternative arrangements of 2
homologous chromosome pairs
Definition: Independent
Assortment
Independent Assortment: The random
distribution of maternal and paternal
homologues to the gametes.
OR
Independent Assortment: The presence of
an allele of one of two genes in a gamete
has no influence over which allele of
another gene is present in the same
gamete.
Independent Assortment leads
to Genetic Variation
Process produces 2n possible combinations
of maternal and paternal chromosomes in
gametes.
If n = 2 then 22 = 4.
If n = 23 (human) then 223 or 8,000,000
different possibilities before crossing over
Crossing Over
Definition: the exchange of genetic material between
homologues (homologous portion of 2 non-sister
chromatids trade places).
X-shaped chiasmata are the visible evidence of this
process.
Produces chromosomes that contain genes from both
parents.
In humans, an average of 3 crossovers/chromosome
pair.
Crossing Over continued
Most of the time, crossing over can occur
without loss of genetic material because of
the precise base to base pairing of
homologues involving the formation of the
synaptonemal complex, a protein structure
that brings the chromosomes into close
association.
The results of crossing over during
meiosis
Process of Crossing Over: 10.1.2
Benefits of Crossing Over:
10.1.5
1. Chiasmata hold
together
homologues during
Pro. & Met. I.
2. Allows
recombination of
linked genes.
Breaks up linkage
groups/parental
combinations.
Crossing over results in an
exchange of alleles
• Parental combinations of linked
genes cannot be broken up without
crossing over.
• Crossing over occurs between nonsister chromatids of a homologous
pair in prophase 1 between the loci
of the 2 linked genes .
• Parentals: abc, ABC
• Crossing over occurs between B
and C
• Position of chiasma formed by
crossing over
• The 4 chromatids separate into 4
nuclei produced by meiosis.
• Recombinants produced = abC, ABc
Linkage groups
• Definition: All the genes that have their
loci on the same chromosome type form a
linkage group.
Example of Gene Linkage
Random Fertilization: Result of
sexual reproduction
In humans an egg cell with 1 of 8,000,000
different possibilities will be fertilized by a
sperm cell that is also 1 of 8,000,000
possibilities, resulting in a zygote that can
have
1/64,000,000,000,000 possible diploid
combinations.
Evolutionary Advantages of
Genetic Variation in Populations
Basis of the theory of natural selection is
inheritable variation. Darwinian
mechanism for evolutionary change is
natural selection:
Most fit organism leaves behind the most
young
a) Increases the frequency of inheritable
variations that favor the reproductive
success of some individuals over others.
Adaptation continued
b) Results in adaptation, the accumulation of
inheritable variations that are favored by the
environment.
c) Genetic variation allows populations to cope
with new environmental conditions.
In the Red Queen, Matt Ridley, suggests that
the major reason that sex has been selected
for is that genetic variation allows species
(including humans) to adapt to evolving
parasites and pathogens.
d) Two sources of genetic variation: sexual
reproduction and mutation (rarer).