LIFE SCIENCES
GRADE 12
TOPIC: MEIOSIS
PRESENTERS : RAEDENE KRUGER & FAWZIE RAHIMAN
21 June 2022
Exam Guidelines: Meiosis
• Differentiate between replicated
and unreplicated chromosomes
• Discuss the different stages of
meiosis
• Compare mitosis and meiosis
• Differentiate between meiosis 1
and meiosis 2
• Discuss non-disjunction and Down
syndrome
Meiosis : 14%
21 Marks in Paper 2
Meiosis in Animals
MALES in the
Testes
FEMALES in the
Ovaries
Meiosis in Plants
Mitosis
A type of nuclear
division that
produces identical
daughter nuclei,
each having the
same number of
chromosomes as
the parent
nucleus.
Meiosis
Meiosis is a cell
division whereby a
diploid cell undergoes
two cell divisions and
divides to from four
different haploid cells
(Gametes)
Haploid (n) vs Diploid (2n)
Every species has a specific number of chromosomes in the nucleus
Haploid
Diploid
Having just one set of chromosomes i.e.
chromosomes are unpaired.
Having a double set of chromosomes i.e.
the chromosomes are paired
Sex cells / gametes (sperm and ova) only
have 23 chromosomes
Our somatic cells (body cells) have 23
pairs of chromosomes
When a female
gamete (n) and a
male gamete (n)
fuse, the
resultant zygote
is diploid (2n)
Interphase
• DNA replication occurs.
• Each chromosome will now consist of
two chromatids joined by a
centromere.
• DNA replication helps to double
genetic material to be shared equally
during cell division
Unreplicated chromosome: single
chromosome as it appears in a cell
before DNA replication takes place.
Replicated chromosome: double
chromosome with two chromatids after
DNA replication.
Homologous Pair of Chromosomes
Chromosomes that are IDENTICAL in
• Shape
• Size/length
• Position of genes/alleles
• Genes coding for same
characteristic
• Location of centromere
There are two chromosomes of each
kind, one from the mother and one
from the father.
Replicated vs Unreplicated Chromosomes
Chromatids
Unreplicated
chromosome
Replicated
chromosome
Terminology
Unreplicated Chromosome
This refers to a chromosome as it appears before DNA replication
takes place
Replicated Chromosome
This refers to a chromosome as it appears after DNA replication takes
place. Because of DNA replication all chromosome material is
doubled, hence, each replicated chromosome is made up of two
chromatids, joined by a centromere
Chromatid
This refers to each of the two threads of a replicated chromosome
Daughter Chromosome
This refers to each chromatid after it splits from its sister chromatid
during anaphase 2 and is moving towards the poles.
Process of Meiosis
Prophase - Preparation
Prophase 11
Prophase 2
• Nuclear membrane and nucleolus
disintegrate
• Chromosomes have a patched
appearance (that is a difference
between prophase 1 and 2)
• No crossing over.
• The spindle starts to form.
• Homologous pairs of
chromosomes
• Bivalents
• Crossing over occurs at chiasmata
NB. Crossing over causes variation
15
Crossing Over
Process that takes place in prophase 1
of meiosis,
involving the exchange of genetic material
between sister chromatids of each homologous
pair of chromosomes
16
Crossing Over
New “patchy” look
maternal chromosome contains a piece
of the paternal chromosome and vise
versa
Crossing Over
• Homologous pair of
chromosomes are known as
Bivalents only when they are
crossing over.
paternal
maternal
•
Chiasma
At this stage they function as
one unit, connected to each
New “patchy” look
Maternal chromosome
contains a piece of the
paternal chromosome
and visa versa
other.
paternal
maternal
Metaphase - Middle
Metaphase I
Metaphase II
• Chromosomes align singly at equator
• Pairs of chromosomes align at
• Random arrangement of
equator
chromosomes takes
• Random arrangement of
place
chromosomes takes place – source
• source of variation
of variation
NB. Random arrangement of
chromosomes causes variation
Anaphase - Apart
Anaphase I
Anaphase II
• Spindle fibers contract.
• Chromosomes move to opposite poles.
• Centromeres split.
• Chromatids move to the opposite
sides.
NB. If spindle fibers fail to separate pair of chromosome, non-
disjunction occurs and at pair 21 it results in Down’s syndrome
Telophase - Two
Telophase I
•
•
•
•
Telophase II
Nuclear membrane re-forms.
Chromosomes are replicated
Cell membrane constricts.
Cytokinesis occurs.
•
•
•
Single stranded or unreplicated chromosomes.
Each daughter cell has half the
number of chromosomes.
Four haploid daughter cells
that are genetically different.
Meiosis vs Mitosis
Meiosis I and Meiosis II
MEIOSIS I
MEIOSIS II
Chromosomes arrange at the equator of the cell
in homologous pairs
Chromosomes line up at the equator of the cell
individually
Whole chromosomes move to opposite poles of
the cell
Chromatids move to opposite poles of the cell
Two cells are formed at the end of this division
Four cells are formed at the end of this division
The chromosome number is halved during
meiosis 1 diploid to haploid (2n to n)
The chromosome number remains the same or
haploid (n) during meiosis II
Crossing over occurs
Crossing over does NOT occur
Activity 1
1.1
How many pairs of chromosomes occur in a
normal human cell?
(1)
23
1.2
Give labels for:
(a) Structure X
(1)
Centromere 
(b)
Area Y
(1)
Chiasma/ Chiasmata
1.3
Name the organ in the human female,
where meiosis occurs.
Ovary
(1)
Activity 1
1.4
Name the:
(a)
Process occurring in diagram B
(1)
Crossing over
(b)
Phase represented by the diagrams
above
(1)
Prophase 1
(c)
Type of cells that would result from
meiosis of this cell.
Ova / gamete / sex cell
(1)
Activity 2
The diagram below represents various phases of Meiosis
2.1
Identify the phase of meiosis in the diagram
(a)
(b)
A
Prophase 1
B
Anaphase 1
Activity 2
2.2
Draw a labelled diagram to show the
cells that will be formed at the end of meiosis
from the cell in diagram C
CRITERIA FOR MARKING
Two cells have been drawn
MARKS
(D)
1
Each cell contains two un-replicated
chromosomes
(C)
Each chromosome is the correct size
and correctly shaded
(S)
1
Any TWO correct labels
2
(L)
1
Activity 3
2.1. Which type of cell division is illustrated in the graph? (1)
Meiosis
2.2. Which phase occurs between points 1 and 2?
Interphase
nucleus
the nucleus of a cell during the cell cycle.
DNA content per
The graph alongside shows the varying amounts of DNA in
(1 )
Time
Activity 2
between points 2 and 3.
During meiosis I /telophase I homologous
chromosomes are separated into different/two
nuclei when the cell divides.
2.
between points 3 and 4.
During meiosis II/ telophase II daughter
chromosomes are separated into different/two
nuclei when the cells divide.
nucleus
1.
DNA content per
2.3. Describe the decrease in DNA content per nucleus…
Time
Activity 3
The diagram alongside represents TWO phases of
meiosis.
1. Identify Part A
Centriole / Centrosome
2. Identify the phase
Anaphase 1
3. Describe the events that took place in the phase before
the one represented in diagram 2.
The spindle fibres contract
The centromeres split
Each centromere is pulled to the opposite pole
Activity 3
4. Name the process that causes the chromosomes to
have a combination of genes as shown in the
diagrams.
Crossing over
5. Give ONE reason why the process named in Q4
above is important.
It leads to genetic variation
6. If this was a human cell, how many chromosomes
would be present in the cell during the phase
represented in diagram 1?
46 / 23 Pairs
7. Structure B and C are BOTH chromosomes, Explain
why they are structurally different.
‒ Structure B consists of two DNA molecules
‒ because of DNA replication
‒ Structure C consists of one DNA molecule because it
is unreplicated / as a result of the splitting of the
chromosome during anaphase 2.
Activity 4
Figure below represents a cell in a phase during Meiosis I.
1. Identify the stage of Meiosis shown.
(1)
Metaphase I
2. Give a visible reason for your answer to Q1.
(2)
‒ Chromosomes are in homologous pairs
‒ along the equator
3. Provide labels for structures 1 and 2 respectively
1 - Homologous Chromosomes
2 - Sister Chromatids
(2)
Activity 4
Figure below represents a cell in a phase during Meiosis I.
4. Would you consider the cell shown in the figure to be
haploid or diploid?
(1)
diploid
5. Explain your answer to Q4 above.
Homologous pairs have not yet been
separated / are still present
(1)
6. How many chromosomes would be present in each
of the daughter cells resulting from THIS cell at the
end of Meiosis II?
2
Activity 5
The diagrams below represents a phase during meiosis.
1. Identify structures labelled (a)
W
(b)
X
(2)
(a) W- Centriole
(b) X- Spindle Fibre
2. Identify the phase which is represented.
Anaphase 1
(1)
3. Give a reason for your answer to Q2 above.
(1)
Each chromosome of each homologous pair is being
pulled to the opposite poles
4. State the number of chromosomes that each daughter cell will
contain at the end of meiosis.
(1)
THREE / 3
Activity 5
5. Give evidence from the diagram which indicates that the
resulting daughter cells will be genetically different
(2)
The chromosomes show swapped segments of genetic
material
6. Explain the significance of the daughter cells being genetically
different.
(2)
Introduces genetic variation in offspring thereby
improving the chances of survival
Importance of Meiosis
• Production of Haploid
Gametes
• The halving effect of
meiosis overcomes the
doubling effect of
fertilization, thus
maintaining a constant
chromosome number
from one generation to
the next.
Significance of Meiosis
Meiosis is important to introduce
genetic variation through:
• Crossing Over (Prophase 1)
• Random arrangement of
chromosomes at the equator
(Metaphase 1 &2)
Consequences
of
Abnormal Meiosis
38
Non-Disjunction
• The failure of chromosomes to separate
during meiosis.
It can happen during:
• Anaphase 1, when the homologous
chromosomes may not separate
• Anaphase 2, when the sister chromatids do
not separate
Non-Disjunction
LIFE SCIENCES PAPER 1 REVISION
2 SEPTEMBER 2021
Non-disjunction of
chromosome 21
Non-Disjunction during ANAPHASE 1
•
Non-disjunction in
Anaphase 1 results in:
• 2 gametes receive an
extra copy of
chromosome 21 and
• 2 gametes lack
chromosome 21
(All cells are affected!)
40
Non-disjunction of
chromosome 21
Non-Disjunction during ANAPHASE 2
Non-disjunction in
Anaphase 2 results in
• One cell without
chromosome 21
and
• One with an extra
chromosome 21
41
Non-Disjunction
•
•
•
Spindle fibre fails to separate
during meiosis
Members of one pair of
homologous chromosomes fail
to become separated during
Anaphase 1 and may happen in
Anaphase 2
This results in:
• 2 gametes receive an
extra copy of affected
chromosome
• 2 gametes lack the
affected chromosome
Karyotypes of Normal People
Autosomes
Gonosomes – XY
Karyotype
Autosomes
Gonosomes – XX
Activity 1
The diagram below represents the chromosomes from the human somatic cells of two individuals who are
twins.
2. Name the specific type of chromosomes
numbered 1 to 22.
(1)
Autosomes 
3. Each of the pairs shown is a homologous pair
of chromosomes.
a) State the origin of each chromosome in a
homologous pair during zygote formation. (2)
‒ One chromosome comes from the sperm  /father
‒ and the other comes from the ovum  /mother
1. What are somatic cells?
(1)
Body cells/cells in the body except the sex cells
Activity 1
The diagram below represents the chromosomes from the human somatic cells of two individuals who are
twins.
3. Each of the pairs shown is a homologous pair
of chromosomes.
b) List THREE characteristics that chromosomes in
a homologous pair have in common.
(3)
‒ Shape 
‒ Size  /length
‒ Position of genes  /alleles
‒ Genes coding for same characteristic 
‒ Location of centromere 
(Mark first THREE only)
4. Explain ONE observable reason why the two
individuals are not identical twins.
(3)
‒ Gonosomes are not identical  /chromosomes at position 23 are not
identical
‒ Individual 1 has XY gonosomes  /is a male
‒ Individual 2 has XX gonosomes  /is a female
46
Karyotype of Down Syndrome
• If the gamete (ovum) with 2
copies of chromosome 21, is
fertilized by a normal gamete
(sperm cell),
• the zygote will have three
copies of chromosomes 21
• Individual will have DOWN
SYNDROME / Trisomy 21
47
Explain the cause of Down Syndrome
•
In meiosis 1/Anaphase 1 the chromosome
pair 21 does not separate or
•
In meiosis 2 /Anaphase 2 the chromatids of
chromosome 21 do not separate /centromere
does not divide.
•
Referred to as non-disjunction 
•
One gamete will have an extra copy of
chromosome number 21
•
If this gamete fuses with a normal gamete (23
chromosomes)
•
The resulting zygote will have 3 copies  of
chromosome 21 instead of 2, (47
chromosomes in that karyotype instead of 46)
leading to Down’s syndrome / Trisomy21
Down syndrome- trisomy 21
Down Syndrome
Amniocentesis
Activity 2
Sometimes an error occurs in Oogenesis resulting in an
ovum which carries an extra chromosome.
Part of such a process is shown in the diagram alongside.
1. Name STAGE 1 in the diagram.
(1)
Meiosis 1
2. Identify the error that resulted in ovum R.
(1)
Non - Disjunction 
3. How many gonosomes are there in the diploid cell? (1)
TWO/ 2
4. If Ovum S is fertilized by a normal sperm:
(a) What will the number of chromosomes in
the zygote be?
(1)
47
(b) What genetic disorder will result?
(1)
Down Syndrome/ Trisomy 21
Question 2
Activity 3
Study the diagram alongside and answer the questions
that follow.
2.1. What is this type of diagram called?
(1)
Karyotype 
2.2. What is represented in this diagram?
(1)
Chromosomes 
2.3. Name the syndrome that the individual
is suffering from.
(1)
Down’s Syndrome / Trisomy 21
2.4. What gave rise to this syndrome?
Non – Disjunction
(1)
Question 2
Activity 3
Study the diagram alongside and answer the questions
that follow.
2.5. Describe how this syndrome is caused.
‒ During anaphase the chromosome pair 21 did not
separate.
‒ This led to one cell having two chromosome number 21
instead of one. 
‒ When this gamete fused with a sperm cell the resulting
zygote had 47 chromosomes instead of 46 
‒ there are three chromosome 21 instead of only 2 
Activity 4
The diagrams represent the distribution of
chromosome pair 21 as it appears in gametes
at the end of meiosis II in a human male.
1. Explain why the diagrams represented by
C and D do not have any chromosome (3)
‒ Due to non-disjunction / Non – separation of chromosome pair
‒ during Anaphase 1
‒ Two chromosomes moved to the one pole and NONE moved to the
other pole √
Activity 4
The diagrams represent the distribution of
chromosome pair 21 as it appears in gametes
at the end of meiosis II in a human male.
2. If gamete A is involved in fertilization,
describe how this may result in Down’s
Syndrome
(3)
‒ Gamete A will have 24 chromosomes/ an extra chromosome
‒ and when it fertilizes with a normal ovum / gamete with 23 chromosomes,
‒ the zygote will have 3 chromosomes at position 21 / 47 chromosomes
Activity 5
An investigation was conducted on the
chances of women of different ages having a
baby with Down Syndrome as a result of
errors in Meiosis I and Meiosis II.
The results of the investigation are shown in
the table below.
Maternal
Age in
Years
Incidence of Down
Syndrome (per 1000
Births)
Error in
Meiosis I
Error in
Meiosis II
<25
0.4
0.1
25 - 29
0.5
0.2
30 – 34
0.8
0.3
36 - 39
1.2
0.5
40+
5.9
1.9
1. Name the error during meiosis that could
eventually result in a child with Down Syndrome.
(1)
Non- disjunction 
2. According to the information in the table, the error
mentioned in Qu1 above is more likely to occur
during Meiosis I or Meiosis II?
(1)
Meiosis I 
Activity 5
3. Over a five – year period, a hospital recorded a total of
44 Down Syndrome babies born to mothers who were
40 years and older.
How many of these women were likely to have the error
that caused Down Syndrome , occurring during
Meiosis II? Show ALL working.
(3)
Number of Down Syndrome babies = 1.9 / 7.8  X 44 
= 10.71
= 11 babies 
Maternal
Age in
Years
Incidence of Down
Syndrome (per 1000
Births)
Error in
Meiosis I
Error in
Meiosis II
<25
0.4
0.1
25 - 29
0.5
0.2
30 – 34
0.8
0.3
36 - 39
1.2
0.5
40+
5.9
1.9
Activity 5
4. Draw a histogram to represent the information in the
table below regarding the error occurring in Meiosis II
that leads to Down Syndrome.
(6)
Maternal
Age in
Years
Incidence of Down
Syndrome (per 1000
Births)
Error in
Meiosis I
Error in
Meiosis II
<25
0.4
0.1
25 - 29
0.5
0.2
30 – 34
0.8
0.3
36 - 39
1.2
0.5
40+
5.9
1.9
THANK YOU