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
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
Bacteria divide by binary fission – dividing in half. Single
chromosome replicates itself.
Chromosomes move so that they are at either end of
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
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
a) time of growth (G1)
b) DNA replication (S)
c) preparation for division (G2)
Mitosis accounts for the division of a cell to form two identical
“daughter” cells.This process can be divided into four phases.
in the nucleus chromatid fibres become tightly coiled and
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.
Spindle fibres radiate from the poles towards cells
Chromosomes become aligned along the metaphase
Microtubules become attached to the centromeres
Centromeres of the chromosome divide and chromatids
are drawn apart
Each daughter chromosome is drawn towards opposite
poles of the cell by the microtubules
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
DIVISION IS CONTROLLED BY
INTERNAL AND EXTERNAL
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
There are two main types of molecules
1. Enzymes – kinases – Cdks (cyclin dependant
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
The control of the cell cycle is checked at three
1. At the end of G1 in which the cell growth is
2. At the end of G2 where progress of DNA
replication is assessed before cell proceeds to
3. Progress of mitosis is checked during
COMPARE CELL CYCLE AND A
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
Physical and / or chemical triggers external to the cell
which impact on the internal factors controlling
If certain nutrients are unavailable in the extracellular
fluid, a cell will not divide.
Cells will only divide if they are attached to a
Cells which are in close contact with each
other (membranes touching) inhibit cell
Hormones can also regulate cell division
Growth hormone (GH) stimulates cells in the
liver to release a growth factor
Sex hormones (FSH) stimulates gamete
Cytokinins stimulate plant cell division
If cells are deprived of growth factors they will
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
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
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.
Growth factors in plant cells are called
Synthetic auxins such as “2,4,5T” and
“glycol” are active ingredients of “Round-
They stimulate cells of plants to divide
rapidly thereby causing death of a plant.
CARCINOGENS AND CELL
Mutations are permanent changes to DNA and the
rate is increased by radiation, mutagenic chemicals
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
Mitosis video real
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.
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
Free radicals – found in food additives and
personal care products (soap, skin lotions)!!!
electromagnetic radiation – x-rays / UV rays
mobile phones (microwaves)
New products and substances are now tested for
their “mutagenicity” by exposing microbes grown
on an agar plate.
Types of cancer
Growth which remains at its original site – generally
not life threatening unless in a confined space
Slow growing and removed surgically
Grow quickly and cells split off and spread
(metastasis) to other parts of the body and
can lead to secondary tumours.