2. Cells arise from pre-existing cells, and cell division leads to
an increase in cell number.
There are two types of cell division in eukaryotic cells
- Meiosis
- Mitosis
Before any cell division the amount of DNA in the cell
must double = semi-conservative replication.
Prokaryotic cells divide by binary fission
The DNA in a bacterium is often held in a singular
circular chromosome (plasmid)This needs to replicate
to form an identical copy
3. BINARY FISSION
Bacteria divide by binary fission – dividing in half. Single
chromosome replicates itself.
Chromosomes move so that they are at either end of
the cell.
As the process continues the cell membrane pinches
inwards and two new identical cells are formed.
Binary fission occurs at a rapid rate – which is important
when cloning DNA or other forms of genetic
manipulation.
4.
5.
6. MITOSIS
All cells from the first time they are formed go through
what is called a cell cycle.There are three stages
1. Mitosis – division of the nucleus
2. Cytokinesis – division of cytoplasm and formation of
two new cells
3. Interphase
a) time of growth (G1)
b) DNA replication (S)
c) preparation for division (G2)
7.
8. Mitosis accounts for the division of a cell to form two identical
“daughter” cells.This process can be divided into four phases.
Prophase
in the nucleus chromatid fibres become tightly coiled and
visible
Each chromosome is duplicated forming two identical
chromatids joined at the centromere
At end of prophase the nucleolus and nuclear membrane
disappear and the chromosomes move into the cytoplasm
Microtubules appear as spindle fibres and centrioles move
towards the poles of the cell.
9. Metaphase
Spindle fibres radiate from the poles towards cells
equator.
Chromosomes become aligned along the metaphase
plate
Microtubules become attached to the centromeres
(spindle).
10. Anaphase
Centromeres of the chromosome divide and chromatids
are drawn apart
Each daughter chromosome is drawn towards opposite
poles of the cell by the microtubules
Telophase
Daughter cells form nuclei and nucleoli reappear
Chromosomes uncoil to form chromatin and cytoplasm
begins to pinch in
Cell begins to cleave (divide) and microtubules act as a
ligature = cytokinesis
13. DIVISION IS CONTROLLED BY
INTERNAL AND EXTERNAL
FACTORS
It is vital that cells control timing of their
division In a mature human, red blood
cells divide continually while nerve and
muscle cells do not divide.
The cell cycle is controlled by a set of
molecules in the cytoplasm of the cell
and “cycle” in concentration
14. INTERNAL FACTORS
There are two main types of molecules
1. Enzymes – kinases – Cdks (cyclin dependant
kinases)
2. Cyclin
Cyclin combines with kinase to form a complex
protein called Mitosis Promoting Factor (MPF)
An increase in MPF is the stimulus to initiate
cells to divide
15. The control of the cell cycle is checked at three
check points
1. At the end of G1 in which the cell growth is
assessed
2. At the end of G2 where progress of DNA
replication is assessed before cell proceeds to
mitosis
3. Progress of mitosis is checked during
metaphase, M
16.
17. COMPARE CELL CYCLE AND A
WASHING MACHINE
The function of the washing machine is to take in
water and detergent, wash the clothes, rinse
them, and spin them dry.
The essential processes, such as DNA replication
and mitosis and cytokinesis, are triggered by a
central cell-cycle control system. By analogy with
a washing machine, the control system is drawn as
an indicator that rotates clockwise, triggering
essential processes when it reaches specific points
on the outer dial.
Feedback is essential
18. EXTERNAL FACTORS
Physical and / or chemical triggers external to the cell
which impact on the internal factors controlling
division.
Nutrient dependence
If certain nutrients are unavailable in the extracellular
fluid, a cell will not divide.
Anchorage dependence
Cells will only divide if they are attached to a
substrate
19. Density dependence
Cells which are in close contact with each
other (membranes touching) inhibit cell
division.
Hormones can also regulate cell division
Growth hormone (GH) stimulates cells in the
liver to release a growth factor
Sex hormones (FSH) stimulates gamete
production
Cytokinins stimulate plant cell division
20.
21. If cells are deprived of growth factors they will
not divide.
They are thought to stop at the G1 checkpoint
and are said to be in the G0 phase. Mature nerve
and muscle cells do not divide and are said to be
in G0 phase.
An example of a growth factor which has been
identified is PDGF ( platelet derived growth
factor).This stimulates division of cells termed
fibroblasts which are important for clotting of
blood.
22. Growth factors travel in the blood of animal and
the sap of plants.They become attached to the
specific receptor molecules in the plasma
membrane of the target cells.
These target cells relay messages across the
membrane and to a series of proteins called
“relay proteins”
These relay proteins activate the process of
transcription and in turn translation to produce a
protein that stimulates cell division.
Growth – inhibiting factors have the opposite
effect producing non-functional proteins
inhibiting cell division.
23. Growth factors in plant cells are called
“auxins”
Synthetic auxins such as “2,4,5T” and
“glycol” are active ingredients of “Round-
up”
They stimulate cells of plants to divide
rapidly thereby causing death of a plant.
24. CARCINOGENS AND CELL
DIVISION
Mutations are permanent changes to DNA and the
rate is increased by radiation, mutagenic chemicals
and heat.
Carcinogens are mutagenic substances that induce
mutations that control normal cell division, leading
to the development of oncogenes.
Cancer cells have faulty cell cycles
Cancers tend to be associated more with ageing as
more and more damage accumulates in DNA over
time
26. CANCER
It appears that between four and seven mutations in
a single cell may cause a cancer
These mutations interfere with the production of
MPF’s, cyclins and kinases.
There are a relatively small numbers of genes which
control the development of cancers which are called
oncogenes. Oncogenes are faulty counterparts of
normal genes that control functioning in the cell.
27. A cancer is an abnormal growth of cells which
invades other tissues. Several environmental
factors can increase the rate of cancer.These
factors are referred to as mutagens or
carcinogens.
Asbestos fibre
Cigarette smoke
Free radicals – found in food additives and
personal care products (soap, skin lotions)!!!
electromagnetic radiation – x-rays / UV rays
mobile phones (microwaves)
28. New products and substances are now tested for
their “mutagenicity” by exposing microbes grown
on an agar plate.
Types of cancer
Benign growth
Growth which remains at its original site – generally
not life threatening unless in a confined space
(skull)
Slow growing and removed surgically
29. Malignant growth
Grow quickly and cells split off and spread
(metastasis) to other parts of the body and
can lead to secondary tumours.
