2.5 Cell Division
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Topic 2: Cells
2.5 Cell Division
Orange book: pg.
Green book: pg.
2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2),
mitosis and cytokinesis. (pg. 36, Figure 1)
2.5.2 State that tumours (cancers) are the result of uncontrolled cell division
and that these can occur in any organ or tissue. (pg. 38)
2.5.3 State that interphase is an active period in the life of a cell when many
metabolic reactions occur, including protein synthesis, DNA replication and an
increase in the number of mitochondria and/ or chloroplasts. (pg. 36)
2.5.4 Describe the events that occur in the four phases of mitosis (prophase,
metaphase, anaphase and teleophase) (pg. 37)
2.5.5 Explain how mitosis produces two genetically identical nuclei.
2.5.6 State that growth, embryonic development, tissue repair and asexual
reproduction involve mitosis.
2.5.1 Stages in the Cell Cycle
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2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis
and cytokinesis.
Orange book  pg. 26 Figure 1
Green book  pg. 25
To do:
 Watch the Mitosis Animation
Animation taking you through the various stages of mitosis.
http://highered.mcgraw-hill.com/olc/dl/120073/bio14.swf
Use the notes below and the textbooks to summarise the cell cycle in your
green books.
Cells are not static structures, but are created and die. The life of a cell is
called the cell cycle and has three phases:
G1: Growth – synthesis of organelles for new cell
S: Synthesis – DNA is replicated
G2: Growth – preparation for mitosis
Mitosis: Nuclear division
Cytokinesis: Cell splits to form two daughter cells
In different cell types the cell cycle can last from hours to years. For example
bacterial cells can divide every 30 minutes under suitable conditions, skin cells
divide about every 12 hours on average, liver cells every 2 years, and muscle
cells never divide at all after maturing, so remain in the growth phase for
decades.
The mitotic phase can be sub-divided into four phases (prophase, metaphase,
anaphase and telophase). Mitosis is strictly nuclear division, and is followed by
cytoplasmic division, or cytokinesis, to complete cell division. The growth and
synthesis phases are collectively called interphase (i.e. in between cell division).
Mitosis results in two “daughter cells”, which are genetically identical to each
other, and is used for growth and asexual reproduction. The details of each of
these phases are shown in subsequent objectives.
2.5.2 Cancer
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2.5.2 State that tumours (cancers) are the result of uncontrolled cell division
and that these can occur in any organ or tissue.
Orange book  pg. 38
Green book  pg. 26
To do:
*I have included quite a lot of extra reading here – keep the objective in mind
as you read – you don’t need to learn all the information.
Once you have read the relevant material, look back at the objective before
summarizing it in your green books.
Cancer is not a single disease. There are hundreds of different sorts, and they
attack different parts of the body. Despite these differences, all cancers have
in common that they involve the uncontrolled growth of cells. In a cancerous
cell something goes wrong with the control of the cell cycle and the cell starts
to divide more often than it should. Eventually a mass of cancerous cells
results, called a tumour, which may give rise to secondary cancer in other parts
of the body.
A and B
A cancerous cell divides more often than normal cells and gives rise to a mass of
cancerous cells called a tumour.
C
The tumour may be carried to other parts of the body via blood vessels of the
lymphatic system. This gives rise to new (secondary) cancers some distance
from the original one. A tumour which spreads in this way is described as
malignant.
Cancer is the disruption of the cell cycle resulting in uncontrolled replication
and growth of cells.
Causes of Cancer
In a cancerous cell, specific changes or mutations have happened in the genetic
material as a result of which the controls which prevent excessive cell division
fail to work. Generalisations about cancer are difficult as there are so many
different types, though the chances of developing most cancers increase with
age.
The World Health Organisation estimates that 60% to 70% of cancer is caused
by environmental agents called carcinogens. Apart from increasing age, there
are three main cause of cancer: chemicals, radiation and viruses. As well as
these environmental causes, some people inherit a greater likelihood of
developing cancer.
Chemicals
The most notorious chemical carcinogen is cigarette tar, responsible for 30% of
cancer deaths in the United States. Today, tobacco consumption is recognised
as the UK’s single greatest cause of preventable illness and early death, with
more than 120,000 people dying each year from smoking-related diseases. In
Britain, lung cancer kills about 40 000 people per year and over 90% of these
deaths are due to smoking.
The exact extent to which our diet is responsible for cancer is less certain.
Nevertheless, there is good evidence that a low fibre intake increases the
chances of getting bowel cancers. It is also known that cancers of the breast
and bowel are linked to eating a lot of fat. Although the precise link between
these cancers and fat intake have yet to be established, the overall message is
clear. In many Western countries out diet is too low in fibre and too high in fat.
The website below is for “Cancer Research UK”, it is a very detailed site but it
gives you an idea of relationship between smoking and cancer.
http://info.cancerresearchuk.org/
Radiation
Cancer can be caused by ultraviolet radiation. X-rays, α particles and β particles.
Exposure to too much ultraviolet radiation in sunlight, especially if even mild
sunburn results, may lead to skin cancer developing, sometimes many years
later. With the damage to the ozone layer leading to more ultraviolet radiation
reaching the Earth’s surface, it is more important to prevent sunburn through
protective creams and appropriate clothing.
X-rays can damage the genetic material of a cell and cause cancer. The risks
from X-rays are particularly great for the developing foetus, which is why
pregnant women tend not to be given X-rays.
Nuclear power stations inevitably produce small amounts of radiation. There is
some evidence in Britain that the children of men who work in nuclear power
stations may have a higher incidence of cancers of their white blood cells
(leukaemia), though the evidence is still controversial.
Viruses
Some cases of uterine and cervical cancer have been attributed to the type 2
herpes simplex virus and some liver cancer to the hepatitis B virus. The human
immunodeficiency virus (HIV) promotes cancer in many AIDS patients by
destroying the immune cells which would otherwise destroy cancer cells.
Inherited
There is a tendency for certain cancers to run in families, however, it does not
necessarily mean that each generation will get the cancer. In some types of
cancer the underlying genetic cause are better understood. E.g. a mutation in a
certain gene (the BRCA 1 gene) greatly increases a woman’s chances of
developing breast cancer. However, at least 15% of women with this mutation
do not go on to develop breast cancer while many women with a history of
breast cancer in their family do not have a mutation in this gene.
2.5.3 Interphase
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2.5.3 State that interphase is an active period in the life of a cell when many
metabolic reactions occur, including protein synthesis, DNA replication and an
increase in the number of mitochondria and/ or chloroplasts.
Orange book  pg. 36
Green book  26
To do:
Once you have read the relevant material, look back at the objective before
summarizing it in your green books.
The largest part of the cell cycle in most cells is interphase. It is the longest
and most variable of the cell cycle phases. Interphase includes three phases:
G1, S and G2. During G1 the major event is growth of the cell. At the beginning
of G1, the cell is the smallest it will ever be. After G1 comes the S phase in
which the main activity is replication of the DNA of the cell, the chromosomes.
This phase is sometimes referred to as the synthesis phase. Once the
chromosomes have replicated, the cell enters its second growth phase, G2.
During this phase, the cell grows and makes preparations for mitosis or the M
phase. During G2, organelles may increase in number, DNA begins to condense
from chromatin to chromosomes, and microtubules may begin to form.
2.5.4 Mitosis
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2.5.4 Describe the events that occur in the four phases of mitosis (prophase,
metaphase, anaphase and teleophase)
Orange book  pg. 37
Green book  pg. 27-28
To do
Before trying to learn what happens at every stage in mitosis – try this quick
animation to help you become familiar with the process.
 Mitosis Animation
Animation taking you through the various stages of mitosis – there is a
worksheet to fill in as you go through the animation at the end of this
document.
http://highered.mcgraw-hill.com/olc/dl/120073/bio14.swf
 Mitosis Musical
Animation with music showing stages of mitosis. No audio voice telling you about
the stages.
As you watch the animation keep an eye on the table below so you can identify
what is happening at each stage of mitosis.
http://www.loci.wisc.edu/outreach/bioclips/CDBio.html
Notes for this will be copied from the board.
 This final animation can be used as a summary – by this stage you should be
familiar with all the staged in mitosis and recognize the changes occurring in
the cell.
Mitosis vs. Meiosis
Only concentrate on the left hand side of this animation, we do not do meiosis
until later.
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Worksheet for first Mitosis Animation
http://highered.mcgraw-hill.com/olc/dl/120073/bio14.swf
Mitosis is a process of nuclear division in which duplicate chromosomes separate
to form two genetically identical daughter nuclei. During prophase the nuclear
membrane disintegrates and the nucleolus disappears.
As prophase continues, the chromosomes condense and begin to appear. Mitotic
spindles which are made of microtubules begin to form between the poles and
(kinetochores) centromeres begin to mature and attach to the spindles
During metaphse the chromosomes become attached to the mitotic spindles at
the (kinetechores) centromeres and align along the (metaphase plate) equator
of the cell.
During anaphase the (kinetechore) centromere microtubules shorten, separating
the chromosomes to opposite poles while the polar microtubules and the cells
elongate.
During telophase chromosomes reach the poles of the cells and begin to
disappear. The polar microtubules continue to elongate. The nuclear
membranes form, the nucleoli appear and the chromosomes decondense
Telophase is followed by cytokinesis during which a cleavage furrow forms in
the centre of the cell, and the cell divides
2.5.5 Semi-Conservative Replication
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2.5.5 Explain how mitosis produces two genetically identical nuclei.
Orange book  pg. 37
Green book  pg. 28
To do
Read the relevant material in the textbooks and the information below.
Draw a diagram in your green books to illustrate the idea of semi-conservative
replication.
Genetically Identical Cells
For growth, repair and replacement it is essential that the cells being made are
genetically identical to the parent cell. All somatic cells (body cells) of an
organism contains exactly the same DNA, the characteristics of a cell depends
on which genes are ‘switched on’. As a growing child is making more bone cells, it
is important that each new cell has all the information it needs to ensure it will
carry out its function properly. The new bone cells will be identical to the
parent cells and will act as bone cells because they will have the same genes
‘switched on’ as the parent cell.
Fine Control of Replication – Semi-Conservative Replication
When a cell divides it needs to provide each new cell with an exact copy of its
DNA. Since DNA controls all cellular function, this replication process must be
very exact.
The law of complementary base pairing shows that we can predict the base
sequence of one DNA strand if we know the sequence of the other. More
importantly, it enables a cell to reproduce one strand based on information in
the other.
The basic idea of DNA replication is evident from its base pairing, but the way
in which DNA is organised in a double stranded α-helix introduces some
complications. The steps of the replication process are as follows:
1. The double helix unwinds.
2. Enzymes move along the open strands, read the exposed bases. And like
matchmakers, arrange ‘marriages’ with complementary free nucleotides. This
is known as complimentary base pairing. If the DNA polymerase finds the
sequence TCG, it assembles AGC opposite it.
3. Thus from the old DNA molecule, two new ones are made. Each new DNA
consists of a new strand from free nucleotides and an old strand conserved
from the parent DNA, hence the name semi-conservative replication.
Semi-Conservative DNA Replication
As the double helix unwinds, unzips and exposes the bases enzymes begin
assembling new bases across from the existing ones.
The result is two DNA double helices, each composed of one strand of the
original DNA and one newly synthesised strand.
Replication of DNA
The two strands of the double helix separate by breaking the hydrogen bonds
(shown as dotted lines) between nucleotides. New, complementary nucleotides
attach at the proper sites, and a new strand of DNA is synthesised along-side
each of the original stands. Arrows indicate hydrogen bonds forming again
between pairs of bases.
2.5.6 Importance of Mitosis
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2.5.6 State that growth, embryonic development, tissue repair and asexual
reproduction involve mitosis.
Orange book  pg. X
Green book  pg. 28
To do
Read the information provided.
As a summary, simply list the 3 things that involve mitosis.
Growth, reproduction and the replacement of old cells all involve the
multiplication of cells. To multiply, cells undergo cell division: one divided into
two, these two divide into four and so on.
Mitosis is the process by which a cell divides into two daughter cells with
identical copies of DNA.
Mitosis is essential to growth and repair. When a zygote is formed from two
gametes it must undergo mitosis to multiply its cells which will then
differentiate into the many cells that may be found with an organism (e.g. cells,
blood cells). Even in ‘fully grown’ organism some growth may still be required
e.g. hair and nails continue to grow throughout life and exercise may increase
muscle mass. Cells are also constantly repaired or replaced in organism. In
humans red blood cells only survive for about 120 days before they are replaces,
the intestinal wall is continually replaces and new skin is formed all the time.
Damaged tissue will also need to be replaced to maintain healthy functioning of
an organism
Asexual reproduction is the production of offspring from a single parent using
mitosis. The offspring are therefore genetically identical to each other and to
their “parent”- in other words they are clones. Asexual reproduction is very
common in nature, and in addition we humans have developed some new, artificial
methods. The Latin terms in vivo (“in life”, i.e. in a living organism) and in vitro
(“in glass”, i.e. in a test tube) are often used to describe natural and artificial
techniques.
Mitosis is essential for:
growth
repair and replacement
asexual reproduction
HW1: Cell Division Questions
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(50 marks)
This homework should be completed gradually over a one week period as we
complete cell division. Do NOT leave it until the last minute.
You can complete answers on the laptop – ensure you answer in blue so they are
clear to see.
1. The graph below shows how the quantity of DNA, measured in arbitrary units,
varies with time during the different phases of the cell cycle in an animal cell.
Interphase is made up of two growth phases, G1 and G2, separated by an
intermediate phase, S.
(a) Explain what is happening within the cell during phase S. (2)
quantity of DNA doubles; (1)
replication of DNA / chromosomes;
preparation for mitosis (1) / nuclear division / cell division / asexual
reproduction;
(b) State one process other than cell growth, which occurs during phase G2. (1)
mitochondria divide / energy stores increase / ATP produced / respiration /
duplication of centrioles / spindle begins to form / protein synthesis;
(c) Account for the changes in the quantity of DNA in the cell during mitosis.(2)
DNA content halves / returns to original level;
DNA / chromosomes / chromatids shared between (daughter) cells / nuclei;
during cell division / cytokinesis;
2. The diagrams below represent the chromosomes during stages in the process
of mitosis.
(a) Write the letters in the order that represents the sequence in which these
stages occur. (1)
B D A C F E / D A C F E B :
(b) Explain the significance of mitosis in living organisms. (3)
production of genetically identical cells;
daughter cells have same function as parent cell;
for growth / repair;
for asexual reproduction / clone formation;
rapid reproduction in favourable conditions;
3. Cell A in the diagram below has two pairs of chromosomes. Cell B, C and D
have each arisen from A by cell division.
For each of the cells labelled B and C, identify the type of cell division, which
has occurred to produce the cell. In each case give a reason for your answer.(2)
Cell B
Type of division Mitosis;
Reason = no reduction in chromosome number / same number of
chromosomes / both diploid / still diploid / identical to Cell A;
Cell C
Type of division meiosis / reduction division;
Reason = chromosome number halved / crossing over has occurred / haploid
/ chromosomes different from A;
4. Read through the following passage on the cell cycle and mitosis, then write
on the dotted lines the most appropriate word or words to complete the
passage. (6)
In the cell cycle, replication of DNA takes place during interphase / S phase.
At the beginning of prophase the chromosomes become visible and can be seen
to consist of two chromatids joined at the centromere. The nucleolus / nucleoli
and nuclear membrane disappear and a spindle develops in the cell. The
chromosomes become attached to the spindle at the equator during metaphase.
At anaphase one copy of each chromosome is pulled towards pole / end of the
spindle. The final phase, called telophase, involves the formation of two new
nuclei.
5. The diagram below shows a plant cell which is undergoing mitosis.
(a) Name the parts labelled A, B, C, and D. (4)
A
cell membrane/ wall;
B
C
D
spindle (fibre) / microtubule:
centromere;
chromatid / (daughter) chromosome;
(b) Name the stage of mitosis shown in this diagram. (1)
Anaphase
6. The diagram below shows the structure of a chromosome as it might appear
at the end of prophase of mitosis.
(a) Name the parts labelled A and B. (2)
A - chromatid;
B- centromere
(b) During metaphase of mitosis, the chromosomes become attached to the
equator of the spindle. Name the stage of mitosis that follows metaphase and
describe the events that occur in this stage.(3)
Anaphase;
Chromatids separate / centromere splits
Move / pulled to (opposite) poles / ends of cell / ends of spindle to
centrioles ;
By spindle fibres / microtubules
(c) Explain the significance of the stage you have named and described in (b). (1)
Daughter cells genetically identical (to parent cell) /maintains chromosome
number/eq;
(d) Mitosis forms part of the cell cycle. Name one other stage of the cell cycle
and state what occurs in the stage that you have named. (2)
Interphase / G1 / S / G2 / cytokinesis / cleavage
If interphase or named stage - growth/synthesis of organelles
synthesis / replication of DNA / division of organelles
OR
If cytokinesis - division of the cytoplasm / formation of cell plate in plants
;
Points linked. [if give ‘telophase’and then describe division of cytoplasm
allow second mark]
7. The diagram below shows an animal cell, in which 2n is 4, undergoing nuclear
division.
(a) Name the structures labelled A, B and C on the diagram. (3)
A = chromatid ;
B = centromere ;
C = centriole
(b) Name the type of nuclear division which is taking place and give a reason for
your answer. (1)
Mitosis (Check spelling)
8. The graph shows the movements of chromosomes during mitosis. The curve
shows the mean distance between the centromeres of the chromosomes and the
corresponding pole of the spindle.
(a) At what time did anaphase begin? (1)
10 minutes
(b) Explain how the graph supports your answer. (2)
Distance between centromere and pole gets less / curve falls; as
chromosomes are pulled apart / spindle fibres shorten
9. The diagram shows four stages in mitosis. Only one pair of homologous
chromosomes is shown.
(a) Place stages A, B, C and D in the correct order. (1)
D–B–A–C
(b) Name the structures labelled X. (1)
Spindle / spindle fibres / microtubules
(c) Describe the part played by the structures labelled X in this type of cell
division. (2)
Contract / shorten; to separate chromatids move chromatids / chromosomes
towards poles
10. The diagram shows the main stages of the cell cycle. The letters A to D
represent the four stages of mitosis.
(a) Identify the stage when each of the following events is taking place.
DNA replication. (1)
S / synthesis stage
(b) Individual chromatids from a chromatid pair move to opposite poles of the
cell. (1)
Anaphase / C
(c) What is happening during Stage X? (1)
Division / cleavage of cytoplasm / cytokinesis
11. Vinblastine is an anti-cancer drug that prevents the formation of a spindle.
(a) What is the function of the spindle? (1)
Pull chromatids apart / attachment for centromeres;
(b) How would a drug like vinblastine help prevent the growth of a tumour? (2)
Cells cannot complete cell division; (therefore) number of cells does not
increase;
(c) Chemotherapy involves using drugs to kill tumour cells. The graph shows the
effect of different doses of a drug used in chemotherapy on tumour cells and
on healthy body cells.
(i) Which dose of the drug, A or B, would you use to treat a patient with a
tumour?
Give the reason for your answer. (2)
A as it kills a high proportion of cancer cells;
but has less effect on healthy cells;
so would cause fewer side effects;
(ii) Explain why it might be necessary to give the patient several sessions of
treatment with the drug. (1)
A single treatment would not kill all cancer cells