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Eden Schools Durban
LIFE SCIENCE GRADE 12
CELL DIVISION (MITOSIS & MEIOSIS)
During sexual reproduction, a sperm cell fuses with an egg cell (ovum) to form a
unicellular zygote.
How does this unicellular structure develop into the multicellular structure which
consists of millions of cells?
It is the result of simple cell division called MITOSIS which results in the production
of new cells – 1 parent cell divides into 2 daughter cells, 2 into 4, 4 into 8, 8 into
16 and so on. In this way, the unicellular zygote can form a multicellular
organism.
REMEMBER : According to the cell theory:
“Every cell comes from another cell.”
Mitosis is a process of simple cell division which occurs in somatic (or body) cells
during which the parent cell divides into 2 daughter cells which are identical to
each other and to the parent cell – they are identical in that they have the
same number and type of chromosomes.
There are two types of cells in the body : SOMATIC or BODY CELLS (which are
genetically identical) and SEX CELLS (sperm cells and egg cells).
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46
PARENT CELL
46
2 DAUGHTER CELLS
IDENTICAL TO EACH
OTHER AND TO THE
PARENT CELL
WHY IS MITOSIS IMPORTANT?
1. It is a form of asexual reproduction during which, for example, a
unicellular Amoeba can split into two daughter cells by a process called
binary fission. Mitosis is also a form of vegetative reproduction in fungi,
mosses and ferns.
2. It results in growth of the body.
3. It repairs and replaces damaged and worn out cells.
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A REVIEW OF THE STRUCTURE OF CHROMOSOMES
A chromosome is made up of two identical chromatids joined by a region called the
centromere. Each chromatid is made up of a number of genes. Each gene is made up
of DNA molecules, each responsible for a particular characteristic.
The diagrams below simply show the structure of chromosomes.
Photograph of human chromosomes
Look for a pair of similar
looking (homologous)
chromosomes
A NORMAL HUMAN KARYOTYPE
(Karyotype refers to all the
chromosomes in the nucleus of
an individual organism)
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Each species of organisms has its characteristic number of chromosomes, for
example, in human beings, every somatic cell has 46 chromosomes and every
sex cell has 23 chromosomes.
The chromatin is composed of DNA which becomes visible as chromosomes
during cell division.
The DNA becomes wrapped around nucleosomes, histosomes and other
proteins forming the chromosome
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OUTLINE OF THE PROCESS OF MITOSIS
The process of mitosis can take place very quickly (20 minutes in certain
bacteria) or very slowly (hours in other cells). It is a continuous process which, for
convenience of study, is divided into phases.
The nucleus first divides (called karyokinesis). This is followed by division of the
cytoplasm (called cytokinesis).
The diagrams below show the cell cycle:
PROPHASE
METAPHASE
ANAPHASE
CYTOKNESIS
INTERPHASE
TELEPHASE
CYTOKNESIS
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A brief description of the events that occur during mitosis in an animal cell
follows :
INTERPHASE
The cell is undergoing its normal activities. The
chromatin material unwinds & the chromosomes
become visible as single strands. DNA replication
takes place & the chromosomes appear as
double strands each having two chromatids joined
by a centromere.
PROPHASE
The chromosomes become more
clearly visible. The nucleolus, nuclear
membrane and larger organelles
disappear. The centrosome splits and
each centriole moves to an opposite
pole of the cell. Spindle fibres arise
from each centriole.
METAPHASE
Chromosomes arrange themselves along the
middle (equatorial plane) of the cell. Each is
attached to a spindle fibre at the centromere.
ANAPHASE
The chromosomes split and each half
(chromatid) is pulled to an opposite pole. An
invagination (constriction) begins to form in the
middle of the cell.
TELOPHASE
A nucleus is formed at each pole. The
invagination deepens and divides the cytoplasm
(cytokinesis) into two. Two daughter cells
identical to each other and to the parent cell are
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formed. The chromatin network re-forms and the cells then go into interphase.
There are some differences between mitosis in plant and animal cells
(tabulated as follows):
ANIMAL CELLS
Spindle fibres radiate from the centriole.
PLANT CELLS
Spindle fibres radiate from the poles.
An invagination divides the cytoplasm A cell plate, which becomes a cross
into two.
wall, divides the cytoplasm into two.
Now look at the series of drawings below, and then read the notes which follow:
Notes:
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 Sperm and egg cells have half the number of chromosomes (23) than in
normal body cells (23 pairs, or 46). These are called male and female
gametes
 When the sperm and egg fuse during fertilisation, the zygote formed now has
the correct number of chromosomes (23 pairs, or 46)
 As you can see, 23 chromosomes came from the father (paternal
chromosomes) and 23 from the mother (maternal chromosomes). Thus you
inherit half of your chromosomes from your father, half from your mother. This
does not mean you are half like your father and half like your mother (see
genetics later)
 Male and female gametes (sperm and egg cells) are formed by a reduction
division called MEIOSIS. During this process, the number of chromosomes is
halved to form the gametes.
Note: I did not use 46 chromosomes, it would not have fitted in. I used 4 pairs to
fit
them in the drawing, you must imagine there are 23 pairs in humans
 Imagine if meiosis did not take place? 46 + 46 = 92, 92 + 92 = 184. The number
of chromosomes will just keep increasing. No! All humans must have the same
genetic information (in 46 chromosomes).
 So meiosis maintains a constant chromosome number from one generation to
the next.
THE DIFFERENCE BETWEEN MITOSIS AND MEIOSIS
MITOSIS (In humans)
MEIOSIS (In humans)
Nucleus
46
Chromosomes
(23 prs)
46
Chromosomes
both diploid (2n)
MEIOSIS
MITOSIS
23
Chrom
46
Chrom
(23 prs)
23
Chrom
46
Chrom
(23 prs)
23
Chrom
23
Chrom
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Table of differences between mitosis and meiosis
MITOSIS
MEIOSIS
1. Nucleus divides once (replication of
Nucleus divides twice (Meiosis I =
Reduction; Meiosis II = Replication)
cell)
2. Chromatids are pulled to opposite cells
(daughter chromosomes, in daughter
cells)
Chromosomes are pulled to opposite cells
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Two daughter cells formed
Four daughter cells formed
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Each daughter cell has an identical
nucleus to the parent
None of the daughter cells have the same
number of chromosomes as the parent
cell had
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Each daughter cell has the same number
of chromosomes as the parent cell (i.e.,
diploid, two sets of chromosomes)
Each daughter cell has half the number of
chromosomes present in the parent cell
(i.e., haploid)
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Takes place to grow tissues, or replace
worn or damaged cells
Takes place to make male and female sex
cells (gametes) for fertilisation during
sexual reproduction
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Ensures that all the daughter cells have
Ensures that the number of chromosomes
the same genetic complement as the
remains constant from one generation
parent cell
(parent) to the next (children)
The drawing below shows the relationship between mitosis and meiosis.
THE LIFE CYCLE INVOLVING SEXUAL REPRODUCTION
(e.g., in humans)
ADULT MALE (2n, diploid)
Testes
ADULT FEMALE (2n, diploid)
Ovary
MITOSIS
M
E
I
O
S
I
S
SPERM (n, haploid)
OVUM (egg; n, haploid)
FERTILISATION
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EMBRYO
MITOSIS
ZYGOTE (2n, diploid)
THE PROCESS OF MEIOSIS (see drawings on page
11)
T
Anaphase I
Prophase I
Metaphase I
MEIOSIS I
MEIOSIS II
MEIOSIS I
Prophase II
Metaphase II
Anaphase II
Telophase II
1.
PROPHASE I
1.1
The chromosomes condense and migrate towards the nuclear envelope.
1.2
Each chromosome is made up of two identical chromatids, known as
sister chromatids.
1.3
Formation of spindle fibers.
1.4
Pairing of homologous chromosomes takes place (also known as
bivalents).
1.5
The homologous chromosomes interchange equivalent sections of
chromatids, which is a process known as crossing over (The point at
which the chromosomes break is called the chiasmata). See page 13.
1.6
The chromosomes undergo thickening and move away from the nuclear
envelope.
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1.7
The nuclear envelope and nucleoli dissolves.
2.
METAPHASE I
2.1
The paired bivalents or homologous chromosomes line up on the
equatorial plane, that lies in the center of the cell.
2.2
The centromeres, a region in the chromosome where the chromatids are
held together, are located in the opposite poles.
3.
3.1
3.2
ANAPHASE I
The homologous chromosomes migrate to the opposite poles of the cell.
The separation is random, i.e., the paternal and maternal chromosomes
separate at random.
3.3
The sister chromatids are not separated, but remain together (each
chromosome still has 2 chromatids at this stage).
4.
TELOPHASE I
4.1
The chromosomes continue to migrate towards the poles.
4.2
Both the poles have haploid (n) number of chromosomes.
4.3
Condensation of the chromosomes and cytokinesis (division of
cytoplasm) take place.
4.4
Nuclear envelope starts forming.
4.5
Two daughter cells with haploid chromosome number are formed.
MEIOSIS II (Simlar to mitosis)
Note: The chromosomes have evidence of crossing over (this distinguishes them
from mitosis)
5.
PROPHASE II
5.1
The nuclei and nuclear membrane are separated.
5.2
The chromosomes start moving towards the equatorial plane.
5.3
The two sister chromatids are still held by the centromere.
6.
METAPHASE II
6.1
The chromosomes are aligned singly on the equator.
6.2
The centromeres are oriented towards the opposite poles.
7.
ANAPHASE II
7.1
The sister chromatids held at the centromere are separated by the
spindle fibers.
8.
TELOPHASE II
8.1
Four nuclei (two each in a daughter cell) are formed, along with the
process of cytokinesis.
8.2
Each of the four nuclei develops nuclear envelope.
8.3
Four haploid daughter cells or gametes are formed, each with
dissimilar sets of chromosomes (due to the random separation or
independent assortment in Meiosis I).
Note: the number of possible different gametes formed will be 223 due to
random separation or independent assortment in Meiosis is 8 388 608!
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Remember this is without crossing over
and just in a sperm or egg cell!!
All this (crossing over, random
assortment and an egg and sperm
fusing) mixes genetic material and
brings variety.
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Use two colours to show that the homologous chromosomes separate randomly
in meiosis I, i.e., the paternal and maternal chromosomes separate randomly.
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What is crossing over?
A process called crossing over
occurs in late Prophase I of meioses.
The homologous pairs of
chromosomes (bivalents) swap
pieces of their inner chromatids by
breaking and reforming their DNA
while they are paired up.
In this way so genes from a
maternal chromosome change
places with some genes from a
paternal chromosome.
This increases variation among the
daughter cells as there will be new
combinations of genetic material.
The point of crossing over where the
chromatids break is called the chiasma.
Why is crossing over important?
The exchange of genetic material produces chromatids with a unique
combination of genes.
During the cutting process some mistakes may occur which lead to mutations.
This introduces new genes into the genetic make-up of a species.
Why are the four gametes made during meiosis different?

Crossing over causes gametes to inherit chromatids with unique gene
combinations.

The chromosomes from each parent are distributed in the new gametes
completely at random.
Which chromosome of a given homologous pair goes to which pole is
unaffected by the behavior of the other chromosome pairs. This is called the
random assortment of chromosomes.
When these pairs separate into a human gamete a single set of chromosomes
in a gamete could consists of 17 maternal and 6 paternal chromosomes, or 14
paternal and 9 maternal chromosomes, or maybe all 23 maternal
chromosomes could go into 1gamete. Usually however, the gamete contains a
mixture of maternal and paternal chromosomes.
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REVISION QUESTIONS
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The diagrams below represent two different phases in meiosis of two
different cells.
Meiosis II: Metaphase II:
The chromosomes will split into 2
chromatids. Note that crossing over
occurred in Meiosis I (tells you that this
is NOT mitosis)
1.1
Meiosis I: Anaphase I:
The homologous chromosomes are
separating.
Give the names of the parts labelled:
(a) A
(1)
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(b) B
(1)
1.2
Identify the phase represented in:
(a) Diagram 1
(1)
(b) Diagram 2
(1)
1.3
Name the process during meiosis which is responsible for the
appearance of the chromosomes illustrated in Diagram 1.
(1)
1.4
How many chromosomes would be found in each of the resulting cells
at the end of the division of the cell shown in Diagram 1?
(1)
1.5
Explain TWO ways in which meiosis is important.
(4)
[10]
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The diagram below represents an animal cell in a phase of meiosis.
2.1
State which phase of meiosis is represented in the diagram above.
(1)
2.2
Give a reason for your answer to QUESTION 2.1.
(2)
2.3
Identify parts A and B.
(2)
2.4
How many chromosomes …
(a)
were present in the parent cell before it underwent meiosis?
(1)
(b)
will be present in each cell at the end of the meiotic division?
(1)
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2.5
State ONE place in the body of a human female where meiosis would
take place.
(1)
2.6
Could the cell represented in the diagram be that of a human?
(1)
2.7
Explain your answer to QUESTION 2.6.
(1)
[10]
Total: 20 marks
MODEL ANSWERS
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The diagrams below represent two different phases in meiosis of two
different cells.
Meiosis II: Metaphase II:
The chromosomes will split into 2
chromatids. Note that crossing over
occurred in Meiosis I (tells you that this
is NOT mitosis)
Meiosis I: Anaphase I:
The homologous chromosomes are
separating.
1.1
Give the names of the parts labelled:
(a) A – Spindle fibres√
(1)
(b) B - Centromere√
(1)
1.2
Identify the phase represented in:
(a) Diagram 1 - Meiosis II: Metaphase II√
(1)
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(b) Diagram 2 - Meiosis I: Anaphase I√
(1)
1.3
Name the process during meiosis which is responsible for the
appearance of the chromosomes illustrated in Diagram 1.
(1)
Crossing over√
1.4
How many chromosomes would be found in each of the resulting cells
at the end of the division of the cell shown in Diagram 1?
(1)
2√
1.5
Explain TWO ways in which meiosis is important.
(4)
1. Meiosis occurs during gamete formation to make male and female
sex cells (gametes) √ for fertilisation during sexual reproduction to
ensure that the number of chromosomes remains constant from one
generation (parent) to the next (children) √.
2. Crossing over√ and random assortment√ during meiosis ensures
greater genetic variety in the formation of gametes.
[10]
2
The diagram below represents an animal cell in a phase of meiosis.
2.1
State which phase of meiosis is represented in the diagram above.
(1)
Meiosis II Anaphase II√
2.2
Give a reason for your answer to QUESTION 2.1.
(2)
Single chromosomes are separated into two chromatids√ and are
moving to opposite poles.
2.3
Identify parts A and B.
(2)
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A - Spindle fibres√
B – Cell membrane√
2.4
How many chromosomes …
(a)
were present in the parent cell before it underwent meiosis?
(1)
8√
(b)
will be present in each cell at the end of the meiotic division?
(1)
4√
2.5
State ONE place in the body of a human female where meiosis would
take place.
(1)
Ovary√
2.6
Could the cell represented in the diagram be that of a human?
(1)
No√
2.7
Explain your answer to QUESTION 2.6.
(1)
Humans will have 23 chromosomes after meiosis√
[10]
Total: 20 mark