Biology A Level
Year 1
Module 2: Foundations in Biology
2.1.6: Cell division, cell diversity and cellular organisation
The cell cycle: a highly ordered sequence of events that takes place in a cell, resulting in the
division of the cell and the formation of 2 genetically identical daughter cells
In eukaryotic cells, the cell cycle has 2 main phases: interphase and
mitotic (M) phase
Interphase
● Cell spends majority of time in this phase
● Sometimes referred to as the ‘resting phase’, but this is
inaccurate as although cells are not actively dividing (as they do not
divide continuously) it is still a very active phase of the cell cycle - the
cell carries out all its major functions and normal metabolic processes
eg. cellular respiration while also preparing for cell division. The 3 stages of interphase:
1. G1 - 1st growth phase
- Proteins from which organelles are synthesised are produced in the cytoplasm
(cell also prepares mRNAs, proteins and the centromeres - required for future
steps)
- Organelles replicate eg. mitochondria and chloroplasts grow and divide, in the
cytoplasm, increasing in number
- Cell increases in size
2. S - synthesis phase
- DNA is replicated in the nucleus
3. G2 - second growth phase
- Energy stores are replenished
, - Preparation and reorganisation of cytoplasmic components for division eg.
assembling microtubules that will be used to mobilise the chromosomes in M
phase
- Replicated DNA (duplicated chromosomes) checked for errors in the nucleus
- Cell continues to increase in size
- Cell prepares for mitosis; G2 ends when mitosis begins
Mitotic phase
● Period of cell division. Involves 2 stages:
1. Mitosis - nucleus divides
2. Cytokinesis - cytoplasm divides and 2 genetically identical cells are produced
● Mitosis and cytokinesis occupy only a small percentage of the cell cycle - approx 5%
G0: phase when the cell leaves the cell cycle, temporarily (‘quiescent state’ ) or permanently. The
cell is neither dividing nor preparing to divide. Numerous reasons for a cell to enter G0 eg.
● Differentiation - a cell which has differentiated is specialised for a particular function
which it will carry out indefinitely. It is no longer able to divide and therefore won’t
enter the cell cycle again (permanent) eg. muscle and nerve cells
● DNA damage - a cell with damaged DNA is no longer viable and can’t divide anymore,
so it enters G0 for a permanent period of stable cell cycle arrest called cellular
senescence
- The majority of normal cells only divide a limited number of times and eventually
become senescent. Therefore as you age, the number of senescent cells in your
body increases (linked with age related diseases eg. arthritis)
● A few types of cells enter G0 and can be stimulated to go back into the cell cycle and
start dividing again eg. lymphocytes in an immune response
How the cell cycle is regulated
Checkpoints
● The control mechanisms of the cell cycle
● The passing of a cell cycle checkpoint occurs when enzymes called kinases bind to a
variety of checkpoint proteins called cyclins , to form a CDK complex. CDK complexes
catalyse the activation of key cell-cycle proteins by phosphorylation, ensuring the cell
progresses through the different phases of its cycle at the appropriate times
● Monitor and verify whether the processes at each phase of the cell cycle have been
accurately completed before the cell is allowed to progress into the next phase
, ● This is vital in ensuring the order, integrity and fidelity of cell division (stops errors
being passed onto daughter cells)
● If the cell satisfies the requirements of the checkpoint, it continues onto the next phase,
but if not, it enters a resting phase (G0)
● Checkpoints occur at various stages of the cell cycle.
G1 checkpoint occurs at end of G1 - checks for cell size, nutrients, growth factors, DNA
damage. If the checkpoint mechanisms detect damaged DNA, the cell cycle would not proceed
until the DNA is repaired. Has a checkpoint called the ‘restriction point’, at which the cell
irreversibly commits to the cell division process. Overall ensures that the internal and externa;
conditions are appropriate for proceeding with the cell cycle
G2 checkpoint occurs at end of G2 - checks for cell size, DNA replication, DNA damage.
Overall ensures that the chromosomes have been replicated correctly
Spindle assembly checkpoint (or ‘metaphase checkpoint’) occurs at the point in mitosis where
all the chromosomes should be attached to spindles and have aligned - so checks for
chromosome attachment to the spindle. Ensures that all the chromosomes are at the metaphase
plate with their kinetochores correctly attached to microtubules (as TMT sister chromatids will
split evenly between the 2 daughter cells when they separate). If a chromosome is not properly
aligned or attached the cell will halt division until the problem is fixed
Cancer and cell-cycle regulation
, ● A malignant tumor (abnormal mass of cells which continues to grow unchecked and
uncontrolled) is the basis of cancer
● Tumors are often the result of damage or spontaneous mutation of the genes that encode
the proteins involved in regulating the cell cycle eg. checkpoint proteins
● For example, if overexpression of a cylin gene results from mutation, the abnormally
large quantity of cyclins produced would disrupt the regulation of cell cycle, causing
uncontrolled cell division, tumour formation and possibly leading to cancer
● If the activity of CDKs can be reduced it may reduce or stop cell division and therefore
cancer formation, so CDKs are used as a possible target for chemical inhibitors in the
treatment of cancer
Role of mitosis
‘Mitosis’ is often used to describe the entire process of cell division in eukaryotic cells, but it
actually specifically refers to nuclear division, which is an essential stage in cell division. It
ensures that both daughter cells produced when a parent cell divides are genetically identical -
each new cell will have an exact copy of the DNA present in the parent cell and the same
number of chromosomes
[Prokaryotic cells do not have a nucleus and reproduce asexually by a different process known as
binary fission]
The significance of mitosis in life cycles
● Mitosis is necessary during growth, replacement and repair of tissues in multicellular
organisms such as animals, plants and fungi, as all the daughter cells have to be identical
● Mitosis is also necessary for asexual reproduction (producion of genetically identical
offspring from one parent) in multicellular organisms including plants, fungi, some
animals and in eukaryotic single-celled organisms such as Ameoba species.
Chromosomes
● Before mitosis can occur, all of the DNA in the nucleus is replicated during interphase
● TMT each chromosome (molecule of DNA) is converted into 2 identical DNA molecules,
called chromatids - was a chromosome as 1 molecule of DNA and is still a chromosome
as 2 DNA molecules now!!
