Download Topic guide 14.4: Cell division

Unit 14: Cell biology
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Cell division
The process of cell division is responsible for a new life being made. When an
egg is fertilised it divides over and over again to eventually make a baby.
Children and adults continue to rely on the process of cell division for growth
and repair. Cells renew themselves all the time, for example, in the gut cells
are replaced at a very fast rate.
Cell division can, however, also cause terrible medical problems such as
cancer. If a cell divides uncontrollably, a mass of tissue known as a tumour can
form. If the tumour is malignant the person will need to undergo treatment
for cancer to remove the tissue mass.
On successful completion of this topic you will:
•• understand the growth and development of cells (LO3).
To achieve a Pass in this unit you need to:
•• compare the processes of mitosis and meiosis (3.1)
•• explain the events of the cell division cycle (3.2)
•• explain the control of the cell division cycle (3.3)
•• explain how multicellular organisms develop by a process of growth
and differentiation (3.4).
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Unit 14: Cell biology
1Mitosis
During mitosis two genetically identical nuclei are formed from one parent cell. In
this process genetic material is carefully copied and chromosomes are replicated
and separated.
Mitosis has four distinct phases – prophase, metaphase, anaphase and telophase
– and, in humans, produces cells that are diploid and contain 23 pairs of
chromosomes.
The cell cycle
The cell cycle describes the steps that the parent cell goes through in order to
produce two new, identical cells. Each time a cell divides it goes through all the
stages of mitosis plus interphase (see Figure 14.4.1).
Interphase
Interphase has three stages, G1, S and G2:
•• G1 is the initial growth phase and this occurs shortly after the cell has been
made. These new cells are small with little cytoplasm but consist of a fully
mature nucleus. During the initial growth phase the volume of the cytoplasm
increases as protein synthesis commences and the organelles required by the
cell also increase in number.
•• A cell enters the S phase after the G1 phase when it is going to divide. During
the S phase DNA replication occurs, copying exactly the genetic instructions
present in the cell’s nucleus. The restriction point is the point at which the
replication starts and, when the replication is completed, the cell is described
as restricted and will now follow the steps of cell division and complete the
cell cycle.
•• G2 takes place after the S phase but before mitosis. During this phase proteins
for cell division are synthesised. This is the second growth phase but it is
shorter than G1. Maturation-promoting factor (MPF) enables the G2 phase to
move into mitosis by phosphorylating proteins needed for mitosis. MPF must
be activated by protein kinase and cyclins – they cause phosphorylation by
adding phosphate groups where necessary. This coordinates the movement
from one stage of the cell cycle into the next.
Mitosis: prophase
There are two genetically identical chromatids, known as sister chromatids
and joined by a centromere, present during prophase. The nuclear envelope
disappears and the centrioles move to opposite sides of the nucleus to form the
protein spindle.
Mitosis: metaphase
During metaphase the chromatid pairs line up on the equator of the spindle by
the centromere.
14.4: Cell division
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Unit 14: Cell biology
Mitosis: anaphase
The sister chromatids are pulled apart during anaphase – they split at the
centromere and are now single structures called chromosomes. As the spindle
fibres shorten, these chromosomes travel to opposite poles. Each chromosome is
identical to the original one from the parent cell.
Mitosis: telophase
A new nuclear envelope is formed during telophase, around each set of
chromosomes. The spindle disappears and the chromosomes start to uncoil.
Cytokinesis
Cytokinesis is the splitting of the cell itself into two genetically identical daughter
cells.
Figure 14.4.1: The movement
of chromosomes during
prophase, metaphase, anaphase,
telophase and cytokinesis.
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
2Meiosis
Meiosis is the process of cell division where haploid gametes (sex cells) are
produced. These cells contain half the number of chromosomes in a diploid cell
and are not genetically identical because the genes have been mixed around. This
is why offspring are not identical to their parents (see Figure 14.4.2).
Meiosis has two phases, meiosis I and II. These phases consist of the same stages
as mitosis but each stage occurs twice. During interphase before meiosis I the
46 chromosomes replicate so there are 92 chromatids.
Meiosis I
Prophase I
The chromosomes come together to form homologous pairs called a bivalent and
the paternal and maternal chromatids attach at the chiasmata. During this phase
sections of genetic material (alleles – different versions of the same gene) may
swap places. This is called crossing over and it plays an important role in genetic
diversity within a population. The spindle starts to form.
Metaphase I
Bivalents are randomly lined up along the equator of the spindle fibres, attaching
by the centromeres. Random assortment of the bivalents also plays an important
part in genetic diversity within a population.
14.4: Cell division
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Unit 14: Cell biology
Anaphase I
The homologous chromosomes are pulled apart and travel to opposite poles.
The chiasmata separate but the centromeres stay intact. If any crossovers have
occurred, donated DNA will remain attached to the new centromere so chromatids
may carry a mixture of DNA from both parents.
Telophase I
Two new nuclear envelopes develop around each pole and cytokinesis occurs
producing two non-identical haploid daughter cells.
Meiosis II
Prophase II
The newly formed nuclear envelope breaks down and the spindle fibres start
to form.
Metaphase II
The chromosomes are randomly lined up on the equator of the protein spindle by
the centromeres.
Anaphase II
The chromatids are separated at the centromere and pulled to the opposite poles
by the shortening of the protein fibres.
Telophase II
In this final stage nuclear envelopes form around the four new haploid daughter
nuclei.
Figure 14.4.2: The movement of
chromosomes during the stages
of meiosis, with the final product
being four daughter cells.
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
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Unit 14: Cell biology
Genetic diversity
Genetic variation within populations allows natural selection to favour the fittest
individuals, which is the basis of evolution. Genetic variation arises by mutation,
and mutations can spread quickly in populations by recombination during sexual
reproduction. This can occur through the random assortment of chromosomes
and/or crossing over during meiosis.
Apoptosis
Link
Genetic diversity is also discussed in
Unit 7: Molecular biology and genetics.
Cell death occurs quickly in all multicellular organisms – it is programmed to
happen to ensure it is organised and tidy. Apoptosis occurs after cells have gone
through approximately 50 mitotic divisions. Enzymes break down the cytoskeleton
of the cell and the cell membrane breaks off into small bits called blebs. The cell
breaks into vesicles and phagocytosis takes place to ensure all cellular remains are
removed.
Take it further
Find out more about controlling the cell cycle by accessing the journal article: Cyclin-dependent
protein kinases: Key regulators of the eukaryotic cell cycle (Nigg, E. A., 1995), Bioessays, 17: 471–480.
doi: 10.1002/bies.950170603.
Checklist
In this topic you should now be familiar with the following ideas about cell division:
 cells divide in order that an organism can grow, and replace and repair cells
 mitosis produces two identical diploid cells
 meiosis produces four non-identical haploid cells
 genetic diversity is caused by crossing over and independent assortment that occurs during
meiosis
 apoptosis is programmed cell death.
Activity
Access the PowerPoint® slide titled ‘Stages of mitosis and meiosis’ and label the stages based on the
position of the chromosomes.
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Unit 14: Cell biology
3 Growth and differentiation
Foetal development
Approximately 24 hours after fertilisation the first cleavage division occurs. Cells
that make up a zygote are continually copied producing a mass of cells called a
morula. As the morula moves from the oviduct to the uterus, it changes to form a
blastocyst. There is no growth in the size of the embryo at this point; the divisions
create cells that are smaller than the original cell because it is restricted within a
glycoprotein shell called a zona pellucida.
Key terms
Cleavage: A series of rapid cell
divisions early in embryonic
development that does not lead to
cell growth.
Zygote: A diploid cell produced
when two gametes fuse.
Morula: A mass of cells produced
after a series of cellular divisions early
in embryonic development.
Cell junction: The site at which two
cells join together.
Gastrulation: The process where
the blastocyst is reorganised and
continues to develop.
Stem cells: Undifferentiated cells.
Pluripotent: Describes a cell that
can differentiate to form all cells of
the body but not most of the extraembryonic membranes.
Totipotent: Describes a cell that can
differentiate to form all cells of the
body and all the extra-embryonic
membranes.
Cell junctions are found in developing blastocysts – these are structures found
between two cells that are joined together. Proteins on the surface of cells called
cell adhesion molecules (CAMs) bind with other cells to join them together in the
process called cell adhesion. Tight cell junctions in the blastocyst close to the outer
surface create a seal to isolate the embryo from the external medium and gap
junctions enable communication between cells of the epithelium surrounding the
fluid-filled cavity.
When the blastocyst reaches the uterus it hatches out of the shell. Cell adhesion
molecules play an important part because they allow the trophectoderm cells
in the outer layer to stick to the cells lining the uterus. This stage is known as
implantation and usually occurs 8–10 days after ovulation. Rapid growth of the
embryo must now take place in order for body plan features to develop.
The body plan of the embryo starts to develop during gastrulation – the
blastocyst changes from a single-layered structure into a three-layered structure
known as the gastrula. Early in gastrulation the primitive streak is formed. This is
the structure that creates a mirror image or bilateral symmetry of the body. During
gastrulation a group of epithelial cells called the germ layers differentiate to form
major structures and organs. The three germ layers are the ectoderm, mesoderm
and endoderm:
•• The ectoderm differentiates to form the nervous system, brain and tooth
enamel; it also forms some of the linings in the body, for example mouth, anus
and nostrils.
•• The mesoderm differentiates to form muscle, the cartilage of the ribs and
vertebrae, blood vessels and connective tissue.
•• The endoderm differentiates to produce the one-cell-thick lining of the
digestive system and respiratory system, and organs needed for digestion.
Stem cells
Stem cells are self-renewing undifferentiated cells that can also give rise to other
cell types. The range of cell types that can be produced from stem cells depends
on the stage of development. Embryonic stem cells are pluripotent, which means
they can give rise to all of the cells in the body, but not to the membranes and
structures surrounding the embryo, such as the foetal part of the placenta. Only
very early cells that exist just after fertilisation can do this, and they are described
as totipotent.
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Unit 14: Cell biology
Figure 14.4.3: The different ways
stem cells can specialise into blood
cells, cardiac muscle and nerve cells.
Key terms
Multipotent: Describes a cell that
can differentiate into a number of
distinct cell types.
Differentiated: A cell that has
reached its final stage of structural
and functional specialisation.
Adult stem cells are found in tissues such as bone marrow, adipose tissue and even
the lining of the nose. These stem cells are restricted in the types of cells they can
produce and are described as multipotent.
Cord blood stem cells are isolated from umbilical cord tissue. They can
differentiate into many different kinds of blood cells.
Stem cells can be used later in life to help reverse degenerative diseases and repair
damaged tissue.
Differential gene expression
Link
Transcriptional factors for gene
expression are also discussed in
Unit 7: Molecular biology and genetics.
The differentiation of cells reflects the expression of genes encoding specialised
proteins (e.g. globin proteins in developing red blood cells, keratin in skin cells,
or myosin in muscle cells). The process of differentiation therefore involves the
activation of cell-type specific genes that encode the proteins necessary for each
cell type to function properly.
Case study
The NHS has been praised by health ministers for the success of the new cord blood donation
centre in London. By January 2012, 140 new mothers had donated their umbilical cord blood since
the centre opened in November 2011.
Blood that is present in the placenta and umbilical cord after a baby is born is the cord blood,
which could prove useful in the future. It contains plentiful stem cells that can help cancer patients.
For more information visit: http://www.nhsbt.nhs.uk/cordblood/.
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Unit 14: Cell biology
Checklist
In this topic you should now be familiar with the following ideas about growth:
 stem cells are cells that are not differentiated
 there are three types of stem cells: embryonic stem cells, adult stem cells and cord blood
stem cells
 specialised cells will have specific characteristics that are due to the proteins produced
depending on the genes expressed in that particular cell
 cells that make up a zygote are continually copied and produce a morula.
 the morula moves from the oviduct to the uterus, and it changes into a blastocyst.
Further reading
http://www.embryology.ch/genericpages/moduleembryoen.html
Twyman, R.M. (2002) Instant Notes in Developmental Biology, Taylor and Francis, Abingdon, UK
Acknowledgements
The publisher would like to thank the following for their kind permission to reproduce their
photographs:
Corbis: Steve Gschmeissner / Science Photo Library; Science Photo Library Ltd: Carol & Mike
Werner / Visuals Unlimited, Inc. 7
All other images © Pearson Education
Every effort has been made to trace the copyright holders and we apologise in advance for any
unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any
subsequent edition of this publication.
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