The document discusses cell division and the cell cycle. It describes the stages of interphase where the cell prepares for division. These stages are G1, S phase where DNA replicates, and G2. Mitosis is then described in its four stages: prophase, metaphase, anaphase and telophase. Cytokinesis, the final stage of cell division, is different between plant and animal cells. Factors involved in cell cycle control and cancer development are also mentioned.

1. 10. CELL DIVISION
1.6 – Mitosis is division of the nucleus into two
genetically identical daughter cells
1.6 – Chromosomes condense by supercoiling
during mitosis
1.6 - Cytokinesis occurs after mitosis and is
different in plant and animal cells
1.6 – Interphase is a very active phase of the
cell cycle with many processes occurring in the
nucleus and cytoplasm
1.6 – Cyclins are involved in the control of the
cell cycle
1.6 – Mutagens, oncogenes and metastasis are
involved in the development of primary and

2. INTRO
o Two opposing processes keep the number of cells in the body at an
appropriate level: Cell division and Apoptosis
o Cell division increases the number of somatic (body) cells and
consists of mitosis (division of the nucleus) and cytokinesis (division
of the cytoplasm)
o Apoptosis is programmed cell death which decreases the number of
cells
o Cell division and apoptosis are constantly occurring throughout your
entire life – if you have a cut, cell division will repair the injury; death
of a cell through apoptosis prevents a tumour forming
o NB: Mitosis is involved whenever cells with genetically identical nuclei are required
in eukaryotes: during embryonic development, growth, tissue repair and asexual
reproduction

3. THE CELL CYCLE
o Cell division is a part of the Cell Cycle (an orderly set of stages that
take place between the time a cell divides and the time the resulting
cells also divide). Some cells that have become specialised no longer
enter the cell cycle

4. THE CELL CYCLE - INTERPHASE
Before a cell divides it needs to
undergo preparation for division.
This phase is called Interphase.
This preparation can take a long
period of time. Interphase is
divided into three stages:
1. G1 – Growth phase 1
2. S – Synthesis phase
3. G2 – Growth phase 2
*Remember: Interphase (consisting
of the above three stages) takes
place BEFORE the cell undergoes
mitosis and divides – these are the
preparation stages!

5. THE CELL CYCLE -
INTERPHASE
Interphase (preparation for mitosis)
occurs in three stages: G1, S and G2.
G1 – Growth phase 1 – cell
enlargement, cell doubles its
organelles and accumulates materials
needed for DNA synthesis
S – Synthesis phase – DNA replicates.
At the beginning each chromosome is
composed of one DNA molecule called
a chromatid. At the end of this stage
each chromosome consists of to
identical DNA molecules – two sister
chromatids
G2 – Growth phase 2 – another growth
phase where the cell synthesizes the
proteins needed for cell division

6. THE CELL CYCLE - INTERPHASE
o Most of the cell cycle is spent in interphase. This is when a
cell carries out its usual functions and gets ready to divide:
it grows larger, the number of organelles doubles and the
amount of DNA doubles.
o For mammalian cells, interphase lasts for about 20 hours
(90% of the cell cycle)
o The amount of time the cell takes for interphase varies
widely. Some cells (nerve and muscle cells) typically do not
complete the cell cycle and are permanently stuck in G1.
These cells are said to be in a G0 phase.
o Embryonic cells spend very little time in G1 and complete
the cell cycle in a few hours.

7. THE CELL CYCLE – INTERPHASE
CONTROL
o Cyclins are a group of proteins that control the cell’s progression
through the cell cycle. They ensure that tasks are performed at the
correct time and that the cell only moves on to the next stage of the
cycle when its appropriate
o Cyclins bind to enzymes called cyclin-dependent kinases. These
kinases become active and attach to other proteins in the cell. This
attachment triggers the other proteins to become active and carry out
tasks specific to one of the phases of the cell cycle
o There are four main types of cyclins in human cells and their
concentration rises and falls depending on the stage of the cell cycle.
Unless these cyclins reach a threshold concentration the cell does not
progress to the next stage of the cell cycle.

9. DNA PREPARATION
o Before mitosis can occur, all of the DNA in the nucleus must be
replicated
o Each chromosome is converted from a single DNA molecule (a
chromatid) into two identical DNA molecules called sister
chromatids. They are held together by a centromere. One of each
of these chromatids will pass to each daughter cell
o The DNA molecules in these chromosomes are very long and it is
therefore essential to package chromosomes into much shorter
structures. This process is known as condensation of
chromosomes and it occurs during the first stage of mitosis
o Condensation occurs by repeatedly coiling the DNA molecule to
make the chromosome shorter and wider – called supercoiling. In
eukaryotes, proteins called histones help with the supercoiling.
Histones make up to 50% of the chromosome structure.

11. MITOSIS - BRIEF
o Once all the preparatory processes have taken place and the DNA has
replicated, the cell moves into mitosis or M phase
o During mitosis the replicated chromosomes separate and move to opposite
poles of the cell, thus providing the same genetic material at each of these
locations
o When the chromosomes are at the poles of the cell the cytoplasm divides to
form two cells distinct from the larger parent.
o These two cells have the same genetic material and are referred to as
daughter cells
o Mitosis involves four phases:
o Prophase
o Metaphase
o Anaphase
o Telophase

12. MITOSIS – PROPHASE (TAKEN FROM ALLOTT)

13. MITOSIS – METAPHASE (TAKEN FROM ALLOTT)

14. MITOSIS – ANAPHASE (TAKEN FROM ALLOTT)

15. MITOSIS – TELOPHASE (TAKEN FROM ALLOTT)

16. CYTOKINESIS – ANIMAL CELLS
o Once nuclear division has occurred, the cell undergoes cytokinesis
(division of the cytoplasm)
o It usually begins before mitosis has been completed and it happens
in a different way in plant and animal cells
o Animal cells – the plasma membrane is pulled inwards around the
equator of the cell to form a cleavage furrow. Proteins that contract
are used in this process to create the furrow. When the furrow
reaches the centre, the cell is pinched apart into two daughter cells

17. CYTOKINESIS – PLANT CELLS
o The rigid cell wall that surrounds plant cells does not permit
cytokinesis by furrowing. Instead, cytokinesis involves the building of
new cell walls between daughter cells
o A small flattened disk appears between the two daughter plant cells.
The golgi apparatus produces vesicles which move to the region of
the disk. More vesicles arrive and the vesicles fuse together to form a
cell plate.
o The cell plate extends across the whole of the equator and becomes
the plasma membranes of the two daughter cells
o Each of the daughter cells will bring cellulose to the equator and
deposit it by exocytosis so each cell builds its own cell wall

19. CELL DEATH
Apoptosis – nucleus
collapses, DNA
fragments, plasma
membrane ‘blebs’
and the cell splits
into fragments
which are then
phagocytised by
WBCs. It is
programmed cell
death – i.e. a
controlled and
planned for
sequence of steps
Necrosis is not
programmed cell
death. It is caused by
infection or injury.
Autophagy – another
type of controlled cell
death. It has a roll in
recycling the contents
of the cell, often to
help the cell survive. It
gets rid of damaged
organelles or unused
proteins. E.g. during
starvation parts of the
cell will be broken
down and recycled as a
source of energy using
this process.

20. CANCER
o Tumours are abnormal groups of cells that develop at any stage of
life in any part of the body.
oIn some cases the cells adhere to each other and do not invade
nearby tissues or move to other parts of the body (i.e. they’re a
primary tumour) – these are classified as benign
o In other tumours the cells can become detached and move
elsewhere in the body and develop secondary tumours – malignant
(very life threatening). This is referred to as metastasis – the
movement of cells from a primary tumour to set up secondary
tumours in other parts of the body
o Diseases due to malignant tumours are commonly known as cancer
and have diverse causes.

21. CANCER
o Mutations are random changes to the sequence of genes. They can
occur randomly or may be triggered by an outside agent – a mutagen
(e.g. chemicals, radiation etc)
o Most genes do not cause cancer if they mutate. The few genes that
can become cancer-causing after mutating are called oncogenes.
o In a normal cell oncogenes are involved in the control of the cell
cycle and cell division. This is why mutations in them can result in
uncontrolled cell division and therefore tumour formation.
o Several mutations must occur in the same cell for it to become a
tumour cell. The chance of this happening is extremely small, but
because there are vast numbers of cells in the body, the total chance
of tumour formation during a lifetime is significant

22. SMOKING AND CANCER
o A correlation in science is a relationship between two variables. There
are two types of correlation. With a positive correlation, one factor
increases as the other factor increases. With a negative correlation,
one factor increases while the other decreases
o There is a positive correlation between cigarette smoking and the
death rate due to cancer. This has been shown consistently in many
independent studies.
o The more cigarettes smoked per day the higher the death rate due to
cancer.
o Finding that there is a positive correlation between smoking and
cancer does not prove that smoking causes cancer. However, in this
case the causal links are well established. Cigarette smoke contains
many different chemicals and twenty of these have been shown in
experiments to cause lung tumours.

23. THE MITOTIC INDEX
o The mitotic index is an important tool for predicting the response of
cancer cells to chemotherapy. It is the ratio of the number of cells (in
a tissue or tumour) undergoing mitosis compared with the number of
cells not undergoing mitosis.
o It can be calculated using the following:
Mitotic index = number of cells in mitosis
total number of cells
o A higher mitotic index indicates a more rapid proliferation of cells
o It is likely that tumours with higher mitotic indices will be more
difficult to control, and a patient with such a tumour may be given a
poorer prognosis than a patient with a tumour that has a lower
mitotic index

24. PRAC – OBSERVING
MITOSIS
1.6 – Using prepared
slides to view and draw
the stages of mitosis

25. PRAC – CALCULATING THE
MITOTIC INDEX
1.6 – Determination of a
mitotic index from a
micrograph