1. 2.5 Cell Division (Mitosis)
Topic 2 Cells

2. Cell Division
2.5.1 Outline the stages in the cell cycle, including interphase (G1,
S, G2), mitosis and cytokinesis.
2.5.2 State that tumours (cancers) are the result of uncontrolled
cell division and that these can occur in any organ or tissue.
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.

3. Cell Division
2.5.4 Describe the events that occur in the four phases of mitosis
(prophase, metaphase, anaphase and telophase).
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.

4. The Continuity of Life
In 1855, Rudolf Virchow claimed:
“Where a cell exists, there must have been a preexisting cell”
He summarised with the Axiom:
“Omnis cellula e cellula” meaning “All cells from cells”
The continuity of life is based on the reproduction of cells, or Cell
Division.

5. The Cell Cycle
Cells go through repetitive stages of growth and
division.
This is called the Cell Cycle.
The cell cycle consists of a number of stages:
– Interphase
– Mitosis
– Cytokinesis

6. The Cell Cycle
Ref: Biology, Campbell

7. Events occurring in the Cell Cycle
Stage of cell cycle Phase Major events
Interphase G1 Period of high metabolic activity resulting in
cell growth.
New proteins synthesised.
Cell organelles are produced.
S DNA Replication occurs.
G2 Mitochondria and chloroplast divide.
Centrioles (if present) divide.
Mitosis Prophase Chromosomes form/coil up.
Metaphase Chromosomes line up along equator of cell.
Anaphase Chromosomes move to each end of cell.
Telophase New nuclear envelope forms.
Cytokinesis Division of cytoplasm and cell organelles.
into two approximately equal halves.

8. Mitosis
Mitosis is used in eukaryotes whenever genetically identical
cells are needed:
– during growth;
• In areas where organisms are getting bigger.
– tissue repair;
• In areas of an organism that have been damaged and need to be repaired.
– asexual reproduction;
• Some organisms reproduce without sex.

9. The Products of Mitosis
Mitosis produces:
– Two daughter cells with genetically identical nuclei.
ie: each cell has the same number and type of
chromosomes as the parent.
The two cells are considered genetically identical to
each other and the parent cell.

10. Mitosis
Mitosis occurs in organisms where there is a need for
growth or repair.
When Eukaryotic organisms reproduce Asexually.
Mitosis is a part of the cell cycle.
Mitosis is usually divided into 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase

11. Prophase (before phase)
In the nucleus, chromatin strands
condense and become clearly
visible as thick chromosomes.
Each chromosome is composed of
two identical strands called
Chromatids (sister chromatids),
which are joined at the
Centromere.
Late in the phase, the nuclear
membrane and nucleolus begin to
disappear.
In animal cells, the centrioles
move to opposite sides of the cell
(spindle poles).
Spindle fibres begin to form.

12. Metaphase (between phase)
By this phase, the nuclear
membrane has totally broken
down.
The nucleolus has disappeared
The chromosomes, previously
randomly arranged within the cell,
become aligned at the equator of
the cell, half way between the two
spindle poles (centrioles in animal
cells).
Spindle fibres stretch from the
chromosomes (attached to the
centromere) to the spindle poles.

13. Anaphase (back phase)
At the beginning of this phase,
each centromere divides, releasing
the attachment of sister
chromatids.
Each chromatid is pulled along the
spindle fibres to opposite spindle
poles, lead by its centromere.
Once near the opposite ends of the
cell, Cytokinesis begins.

14. Telophase (end phase)
During this phase, the
spindle fibres break down
and each chromosome
unwinds as the chromatin
that forms it again becomes
extended and thin.
The nuclear membrane and
nucleolus reform in each
daughter cell.
Cytokinesis nears
completion.

15. Cytokinesis
This is the process of cytoplasmic division and enclosure of the
cytoplasm of the two new daughter cells.
In animals, cytokinesis occurs by the pinching of the cytoplasm (a
cleavage furrow), pinching the cell into two.
In plant cells, a cell plate forms between the 2 new daughter
nuclei, dividing the cell into two.
Cytokinesis may begin as early as Anaphase.

16. Cancer
A Cancer is an abnormal growth of cells which invades other
tissue and may move to other parts of the body.
A cancer begins with just one abnormal (mutated) cell.
The cell divides by mitosis to form a lump or tumour.
They can occur in any organ or tissue.
Ref: Biology Key Ideas

17. IBO guide:
2.5.1 Outline the stages in the cell cycle, including interphase (G1,
S, G2), mitosis and cytokinesis.
2.5.2 State that tumours (cancers) are the result of uncontrolled
cell division and that these can occur in any organ or tissue.
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.

18. IBO guide:
2.5.4 Describe the events that occur in the four phases of mitosis
(prophase, metaphase, anaphase and telophase).
Include supercoiling of chromosomes, attachment of spindle
microtubules to centromeres, splitting of centromeres, movement
of sister chromosomes to opposite poles, and breakage and re-
formation of nuclear membranes.
Textbooks vary in the use of the terms chromosome and
chromatid. In this course, the two DNA molecules formed by DNA
replication are considered to be sister chromatids until the splitting
of the centromere at the start of anaphase; after this, they are
individual chromosomes. The term kinetochore is not expected..

19. IBO guide:
Aim 7: Students could determine mitotic index and fraction of
cells in each phase of mitosis. Individual groups could paste data
into a database. Pie charts could be constructed with a graphing
computer program. If a graphing computer program is used in
DCP for internal assessment, it should be according to the IA and
ICT clarifications.
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.