CELL DIVISION
MITOSIS AND CYTOKINESIS
Mitosis involves the division of one cell into two identical daughter cells, i.e. each daughter
cell contains the same number of chromosomes as the original cell. Strictly speaking,
mitosis is the nuclear division of the cell, and cytokinesis is the process of cytoplasmic
division.

Mitosis occurs in the somatic cells of organisms. It is the process by which growth and
repair of tissues occurs in animals, and growth (eg. in root and shoot tips) occurs in
plants. It is also a process by which asexual reproduction occurs (eg. budding in yeast
and hydra, use of runners in some plants).

It is the reproductive process that occurs in tissue culture, skin grafts and cloning
organisms.
THE FOUR PHASES OF MITOSIS IN A CELL (2n = 4)
PROPHASE
Chromosomes shorten and thicken (condense). In animal cells, centrioles move to the
poles. The nucleus disappears, chromosomes continue to shorten and thicken, and
chromatids becoming distinct. Spindle fibres begin to form. Chromosomes attach to spindle
fibres.
METAPHASE
Chromosomes are arranged along the equator (middle) of the cell. Each chromosome is
attached to a spindle fibre by the centromere.
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ANAPHASE
The chromatids split (perform disjunction) at the centromere and daughter chromosomes
migrate to either end of the cell; spindle fibres begin to disappear.
TELOPHASE
The cytoplasm begins to constrict and the cell starts to pinch in half (in animal cells).
Cell plate forms in plant cells. Spindle fibres disappear. The nucleus reappears and the
chromosomes are no longer visible.
Kinetochores are protein structures located on chromosomes, and are the sites where
spindle fibres attach during division to in order to pull the chromosomes apart.
CYTOKINESIS
Cytokinesis (cytoplasmic division) follows telophase. Cytokinesis is not part of mitosis.
Daughter cells grow to reach their regular size during this period.

In animal cells, a cleavage furrow forms between dividing cells at cytokinesis.

In plant cells, a cell plate forms from materials supplied by golgi vesicles. This will
ultimately form the cell wall.
Mitosis ensures that cells maintain a large surface area to volume ratio. It allows for:
1. Growth of an organism.
2. Replacement of cells which have died or been damaged.
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MITOSIS SUMMARY
Interphase is not
part of mitosis
(Roberts MBV, 1987)
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ASEXUAL REPRODUCTION
Asexual reproduction does not involve the fusion of two gametes (sex cells).
eg: binary fission in bacteria, plant division using cuttings, bulbs etc.

The alleles (genotypes) of the parent and the offspring are identical.

It still requires some type of DNA replication.
BINARY FISSION
Binary fission is a type of cell division by which prokaryotes reproduce. The single
chromosome replicates prior to cell division, so that each daughter cell receives a single
copy of the parent cell chromosome.
(power-chemicals.com)
SEXUAL REPRODUCTION
Sexual reproduction involves the fusion of gametes. Gametes are typically produced in a
process of cell division called meiosis.

The term gamete refers to either an egg or sperm.

Gametes are haploid (n), i.e. they do not contain chromosomes in pairs. They only
contain one set of chromosomes (and therefore genes) in their nuclei. Hence, gametes
contain half the number of chromosomes as somatic (diploid) cells. In humans, somatic
cells contain 46 chromosomes, and gametes 23 chromosomes.

Hence, when the egg and sperm join during fertilisation, the newly formed cell will have
the diploid number of chromosomes restored, enabling the cell to function.
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MEIOSIS AND CYTOKINESIS
Meiosis is a reduction division involving one diploid parent cell dividing to produce four
haploid daughter cells, i.e. each daughter cell has half the number of chromosomes of the
parent cell.
stash.norml.org
Meiosis involves the random assortment of chromosomes into their homologous pairs.
A dramatic increase in variation within a species results as gametes from the one individual
are not all identical.

The daughter cells are known as gametes (sex cells). Each gamete has a single copy
of each type of chromosome.

All cells involved in the process of meiosis, including the gametes, are known as
germline cells.
Meiosis takes place in sex organs (gonads).

The gonads of male organisms are known as ________________________________

The gonads of female organisms are known as _______________________________

A human somatic cell contains 46 chromosomes. There are in 23 pairs of homologous
chromosomes in females, and 22 pairs (as well as an X and Y chromosome) in males.

As mentioned earlier, homologous chromosomes carry the same genes at
corresponding loci, however, they needn’t carry the same alleles for those genes.
eg: If a gene is coding for height, the allele on one of the pair of chromosomes could
be coding for tallness while the other allele on the other chromosome could be coding
for shortness.

During meiosis in humans, a diploid cell containing 46 chromosomes divides twice to
produce four gametes, each containing 23 chromosomes. When two gametes fuse
during fertilisation, a zygote is formed, containing the full diploid set of chromosomes.

Meiosis involves two divisions (PMAT I and PMAT II). The first division is the
reduction division, as the number of chromosomes per cell has been halved after this
division.
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INTERPHASE (PRIOR TO MEIOSIS)
As with mitosis, interphase occurs prior to meiosis. DNA replication occurs once only prior
to the two divisions in meiosis, hence, four haploid daughter cells will be created.
PROPHASE I
Nucleus disappears, chromosomes shorten and thicken and become visible. Homologous
chromosomes pair (synapsis) and spindle fibres form. Chromosomes attach to spindle
fibres. Crossing over (see page 32), which is a very important source of genetic
variation in the gametes, can take place at this time.
METAPHASE I
Spindle fibre formation complete, homologous chromosomes line up in pairs along equator
of cell.
2
Note: You will never observe this situation during metaphase of mitosis.
Independent Assortment of Chromosomes (see page 34): Maternal and paternal
chromosomes of each homologous pair line up independently of all other homologous
pairs. This is a very important source of genetic variation within the species!!
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ANAPHASE I
Each homologous pair separates (performs disjunction) as double-stranded chromosomes
move to opposite poles. Spindle fibres begin to disappear.
TELOPHASE I AND CYTOKINESIS
Spindle breaks down, nucleus reforms, cell pinches into two daughter cells each containing
half the original number of chromosomes (however, each chromosome still consists of two
chromatids).
A resting phase for both cells occurs in many species. No duplication of chromosomes takes
place before the second division.
The second division of meiosis can now proceed. Meoisis II is very similar in process to
mitosis.
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PROPHASE II
In both cells the nuclear membrane breaks down, spindle fibres form and chromosomes
become distinct.
METAPHASE II
In both cells chromosomes move to the centre of the cell (notice that the chromosomes are
not aligned in pairs). Centromeres start to divide.
ANAPHASE II
As centromeres divide, chromatids separate to form daughter chromosomes. In both cells
the daughter chromosomes move to their respective poles (again performing disjunction);
spindle fibres disappear.
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TELOPHASE II AND CYTOKINESIS
Both cells divide in two resulting in the production of four haploid daughter cells. Nucleus
reforms and each cell pinches in half (cell plates appear in plant cells).
CYTOKINESIS (NOT PART OF MEIOSIS)
Cytokinesis (cytoplasmic division) begins during telophase II. Remember, cytokinesis is
not part of meiosis.
In animal cells, a cleavage furrow forms and deepens until the cell is pinched in two.
In plant cells, golgi vesicles, laden with cell wall materials, fuse to form a bigger and bigger
cell plate, until the plate’s membrane fuses at its edges with the cell membrane, dividing the
cell in two. Contents of the cell plate form the new cell wall.
In either case, four haploid daughter cells have formed.
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IMPORTANT SOURCES OF
VARIATION DURING MEIOSIS
1. CROSSING OVER AT PROPHASE I
When homologous chromosomes pair during Prophase I, they often make contact and
become intertwined. During this time, a section of the genetic material on a maternal
chromatid can be swapped for the corresponding material on the homologous paternal
chromatid. The point of crossing over is called the chiasma (plural = chiasmata). This can
lead to new combinations of genetic material on a chromosome, referred to as
recombination.
The crossover value between two genes located on the same chromosome can be
determined by the formula:
The number of recombinant offspring (due to crossing over)
Total number of offspring
x 100
eg: If crossing over occurs during 4% of meiotic divisions between two particular genes,
the two genes are said to be four map units apart.
The further apart two gene loci are on the same chromosome, the greater the chance of
crossing over occurring between the two genes.
eg: In the diagram below, crossing over would occur more frequently between genes A and
C than between genes A and B or B and C.
Crossing over and recombination during meiosis
http://www.evolutionpages.com/homo_pan_divergence.htm
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CHROMOSOME MAPS
A chromosome map is a diagram showing some of the gene loci on a chromosome. Using
the crossing over frequency (crossover value), you can determine the relative position of
genes on a chromosome and create a chromosome map.

The amount of crossing over is a direct link to the distance between genes on a
chromosome. Two distant genes on a chromosome will have a higher rate of crossing
over than two genes that are close together.

A crossover value of 1% = 1 genetic map unit.

eg: If genes A and C have an 18% crossover value, genes B and C have a 3%
crossover value and genes B and A have a 15% crossover value, a chromosome map
for these genes can be drawn in the following manner:
18 mu
3 mu
15 mu
C B

A
Remember, the crossover value between two genes can be determined by the formula:
The number of recombinant offspring (due to crossing over)
Total number of offspring
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x 100
Page 36
2.
INDEPENDENT ASSORTMENT OF CHROMOSOMES AT METAPHASE I
Pairs of homologous chromosomes assort independently of all other pairs at
Metaphase I, resulting in significant gamete variation.
eg: There are (2)23 different gametes possible from a human as a consequence of
independent assortment!
An organism with a diploid no. of 2n = 6, can produce 23 genetically different possible
types of gametes as a consequence of independent (random) assortment of
chromosomes at Metaphase I:
Parent cell
Gametes
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MITOSIS & MEIOSIS SUMMARY
Feature
Mitosis
Site of division
All somatic cells except brain
and nervous tissue.
2
No. of divisions
No. of daughter cells
Meiosis
2
No. of chromosomes /
daughter cells
Types of daughter cells
Somatic cells.
Separation of chromatids
At anaphase.
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RNA (RIBONUCLEIC ACID)
Ribonucleic acid (RNA) is a polymer of nucleotides. The nucleotides are composed of the
sugar ribose, a phosphate unit and one of four nitrogenous bases.
The nucleotides in the single nucleotide strand of an RNA molecule are very similar in
structure to DNA nucleotides:
Label the following RNA nucleotide:
A
C
B


An RNA base can be one of four types:
1.
Adenine
2.
Uracil
3.
Guanine
4.
Cytosine
3 forms of RNA exist:
Messenger RNA (mRNA)
____________________________________________
____________________________________________
Transfer RNA (tRNA)
____________________________________________
____________________________________________
Ribosomal RNA (rRNA)
____________________________________________
____________________________________________

Complementary base pairing can exist within the RNA molecule (eg. within a viroid, a
tRNA or rRNA molecule or within retroviral RNA), between the RNA molecule and a
DNA molecule, or between two different RNA molecules (eg. between tRNA and mRNA
during protein synthesis).

When bases meet, Uracil pairs with Adenine and Guanine pairs with Cytosine.
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eg: Complete the following tables:
DNA
A
A
C
G
C
C
T
A
T
T
A
C
G
U
G
G
C
G
A
U
G
A
A
A
U
U
RNA
RNA
RNA
DNA VERSUS RNA
DNA
No. of Nucleotide Strands
2
Sugar
Deoxyribose
Bases
A, T, C, G
Functional Location
Nucleus
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RNA
Page 40
GENE ACTION
INTRONS AND EXONS
The coding regions of genes contain sections known as introns and exons.
Gene
Exons
Parts of the coding
region of a gene that
are both transcribed
and translated
Introns
Parts of the coding
region of a gene that
are transcribed
Exon 1
Intron 1
Exon 2
Intron 2
Exon 3
DNA
Transcription
(RNA synthesis)
Exon 1
Intron 1
Exon 2
Intron 2
Exon 3
PrePre-mRNA
Post transcription
modification
Intron 2
Intron 1
PolyPoly-A tail
Methyl cap
Exon 1
Exon 2
Exon 3
AAAA
Messenger RNA
but not translated
Nuclear membrane
Ribosome
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Translation
(protein synthesis)
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FLANKING REGIONS
Flanking regions are regions of DNA on either side of the coding region of a gene. Two
flanking regions exist for any gene:
i.
Upstream Region
The flanking region is the section of DNA before the start of the coding region of a
gene. It includes the promotor sequence (a section that binds RNA polymerase and
indicates where to start transcribing RNA).
The upstream regions have survived millions of years of evolution. If the upstream
region is mutated it can affect the activity of the coding region (gene) that follows
it. Hormones can attach to specific upstream regions, thereby affecting gene action.
ii.
Downstream Region
The downstream flanking region is located after the end of the coding region of a gene.
It includes an “end transcription signal” which terminates transcription.
If the downstream region is mutated, gene action can be altered, so this region also
plays a role in transcription.
Promotor/Upstream
region
Start
Coding region
Stop
Downstream
region
Promotors

The promoter is a control region of the upstream region that exists to switch the gene
on or off. Promoters are non coding base sequences that signal the start of a gene.
The promoter commonly includes a TATA box. RNA polymerase binds to the promoter
region, thus activating the gene and initiating transcription.

The promoter of gene A may be repressed by a protein produced by another regulatory
gene, gene B. The repressor protein product of gene B turns off gene A.
In prokaryotes: A prokaryote promoter has two essential sequences. One is the sequence
recognised by RNA polymerase, to which it binds. The second, closer to the initiation site, is
the TATA box where the DNA begins to separate (dissociate) so that the template strand
can be exposed.
In eukaryotes: Eukaryote RNA polymerase cannot simply bind
to the promoter and initiate transcription. In eukaryotes a
collection of regulatory proteins called transcription factors
mediates the binding of RNA polymerase and the initiation of
transcription. A protein binds to the TATA box then other
regulatory proteins, called transcription factors bind to that
protein and each other. Transcription complexes form, then
RNA polymerase can bind.
National Geographic Society
Once the polymerase is firmly attached to the promoter DNA
the two DNA strands unwind there and the enzyme starts
transcribing the template strand.
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GENE EXPRESSION
THREE BASE SEQUENCE TERMINOLOGY
A three base sequence of DNA coding for an amino acid is known as a triplet.
A three base sequence of mRNA coding for an amino acid is known as a codon.
A three base sequence of tRNA coding for an amino acid is known as an anticodon.
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TRANSCRIPTION
When a gene becomes active or is switched on in a eukaryotic organism, it must make a
mobile copy of the genetic instructions to be received by the ribosomes. This is because
DNA (instructions) cannot move out of the nucleus.
The DNA is copied (transcribed) into mRNA.
Transcription is the synthesis of RNA from a DNA template. It occurs in the nucleus of
eukaryotic cells and the cytoplasm of prokaryotic cells.
Step 1: The enzyme RNA polymerase attaches to the promoter region of DNA just
upstream of the gene.
Step 2: The double stranded DNA making up this gene is unwound by the RNA
polymerase by the breaking of the weak hydrogen bonds existing between its two
strands. Unpaired bases on the template strand are now exposed.
Step 3: RNA polymerase constructs mRNA by collecting free complementary RNA
nucleotides to the exposed DNA template strand. It assembles the RNA
nucleotides according to complementary base pairing rules. The nucleotides in
eukaryotic cells are joined to form a single stranded pre-mRNA molecule.
Pre-mRNA contains both introns (non-coding “interruption” nucleotide sections)
and exons (“expressed” nucleotide sections). The DNA molecule reforms a double
helix.
The RNA transcript is assembled in the 5’ to 3’ direction.
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POST-TRANSCRIPTION MODIFICATION
Step 4: Enzymes cut out the introns or non-coding regions from the pre-mRNA. The
remaining exons are joined together. A methyl cap is added to one end of the
molecule, and a poly-A tail at the other. The resulting shorter mRNA molecule
leaves the nucleus via a nuclear pore to perform its function at the ribosomes.
Example:
DNA template:
ATGCCTGAAT
The mRNA copy is:
UACGGACUUA
As you can see the Thymine (T) in DNA is replaced with Uracil (U) in RNA.
Complete the following table:
DNA antisense
strand
DNA template
strand
T
A
C
G
C
C
G
T
A
A
C
C
G
G
T
G
C
mRNA

The human genome contains 20,000 – 25,000 genes, however, there may be as many
as 400,000 proteins translated from these genes. Originally it was believed that one
gene coded for one protein, but obviously the maths does not work!

Research now shows that genes are regulated in different ways and that one gene can
in fact code for more than one protein.

The above differences may be due to differences in intron retention, i.e. differing
amounts and types of introns are cut out from pre mRNA in different situations,
resulting in the massive variety of proteins from a much smaller number of genes.
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A