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 Dividing we stand
Cell division is one of the most crucial processes of any living organism. It is necessary for
mitosis, which is responsible for the growth of a multicellular organism, and for reproduction of
a eukaryotic single-celled organism. Mitosis is also required for the repair of damaged tissues,
as new cells are made to replace any that are lost, old or damaged. Another type of cell division
is meiosis, the process by which sex cells are made.
The cell cycle is an organised series of events that take place in a cell. The longest stage of the
cell cycle is interphase, which can be split into G1, S phase and G2. In the G1 phase, proteins
required for cell division are made. This is followed by the S phase, in which chromosomes are
duplicated, and then the G2 phase, in which more proteins are synthesised. Interphase is
followed by mitosis, and then the cycle begins again. The cell cycle must be tightly regulated
for cell division to take place, as events such as protein synthesis or chromosome replication
must occur at the correct stage.
This resource first appeared in ‘The Cell’ in January 2011 and reviewed in September 2015.
Published by the Wellcome Trust, a charity registered in England and Wales, no. 210183.
bigpictureeducation.com
Mitosis
Mitosis is the process of making new eukaryotic cells. This is divided into stages called
prophase, metaphase, anaphase and telophase and is followed by cytokinesis. During
interphase, a cell’s chromosomes (organised, thread-like structures composed of a mixture of
DNA and proteins) are duplicated so that the nucleus contains twice the original DNA. The
chromosomes are now X-shaped structures, composed of two identical sister chromatids.
At the start of mitosis, in a stage called prophase, the chromosomes condense and can be
viewed under a microscope, and spindles (protein structures that will later attach to sister
chromatids to separate them) begin to form in the cytoplasm.
Stages of mitosis
•
In prophase, the nuclear envelope breaks down.
•
During metaphase, the pairs of sister chromatids then line up along the equator of the
cell.
In anaphase, spindle fibres attach to chromosomes via a protein complex (several
proteins that are associated with each other) called a kinetochore and separate the
sister chromatids to opposite poles of the cell.
In the last stage, telophase, new nuclear envelopes form around each set of
chromosomes.
•
•
Finally, in cytokinesis, the cell cytoplasm splits to form two identical daughter cells, each with
exactly the same DNA as the original cell.
Regulation of mitosis
This process is highly regulated by a series of biochemical reactions to ensure that mitosis only
occurs after sufficient cell growth and DNA replication have occurred. CDKs (cyclin-dependent
kinases) are enzymes that add phosphate groups to proteins to activate them. One of their
roles is to activate proteins that initiate DNA replication before mitosis, to ensure this happens
at the correct stage.
In cancer cells, growth is incorrectly regulated, and this is often due to mutations in genes that
control cell division. A tumour-suppressant gene called p53 is mutated in more than 50 per cent
of all human cancers. When regulation mechanisms for mitosis cannot function, mitosis
becomes unregulated and this causes a mass of cancer cells called a tumour.
This resource first appeared in ‘The Cell’ in January 2011 and reviewed in September 2015.
Published by the Wellcome Trust, a charity registered in England and Wales, no. 210183.
bigpictureeducation.com
If we can find out more about the mechanisms of regulating mitosis, it might provide us with
more information about how to prevent or cure cancer. In particular, lots of research is going
into finding ways to target only the cells that divide abnormally; the targets of anti-cancer
drugs can often also be found in ordinary cells, so they cause unpleasant side-effects.
Meiosis
Meiosis is the process in which haploid sperm and egg cells are generated. In humans, instead
of containing 46 (2n) chromosomes, haploid cells only contain 23 (n) chromosomes. Each sex
cell contains half of the DNA required to create an embryo.
During meiosis, the cell undergoes two rounds of division, named ‘meiosis I’ and ‘meiosis II’. As
in mitosis, the DNA is replicated before the first division, and spindles begin to form.
Stages of meiosis I
•
In prophase I, the nuclear envelope breaks down and chromosomes condense into Xshaped structures. A process called ‘crossing-over’, or recombination, occurs. This involves
the exchange of DNA between paternal and maternal chromosomes, so the chromatids
are no longer identical and are referred to as non-sister chromatids.
•
In metaphase I, chromosome pairs line up along the equator of the cell and spindle fibres
attach to each chromosome.
•
In anaphase I, chromosomes do not separate in the same way as they do in mitosis. Nonsister chromatids remain attached to one another, but pairs of homologous chromosomes
separate. Homologous chromosomes are chromosomes of a similar size and structure and
contain the same genes. Each chromosome in the pair may contain a different allele of the
gene, as one homologous chromosome is inherited from the mother and the other is
inherited from the father.
•
In telophase I, a new nuclear membrane forms around each set of chromosomes
Cytokinesis then occurs, resulting in two non-identical haploid cells. Before meiosis II, DNA is
not duplicated, so prophase II begins with 23 chromosomes in each cell.
This resource first appeared in ‘The Cell’ in January 2011 and reviewed in September 2015.
Published by the Wellcome Trust, a charity registered in England and Wales, no. 210183.
bigpictureeducation.com
Stages of meiosis II
•
In prophase II, the chromosomes condense and the nuclear membrane breaks down.
•
In metaphase II, the chromosomes line up along the equator and spindle fibres attach.
•
In anaphase II, the sister chromatids are pulled to opposite ends of the cell.
•
In telophase II, a new nuclear envelope forms around each group of chromosomes.
Cytokinesis is then repeated. At the end of this division, there are four non-identical haploid
cells.
Genetic variation and errors in meiosis
Meiosis introduces genetic variation into populations, which is important because more diverse
populations are more likely to withstand disease or illness. Variation is further increased when a
sperm and egg cell combine during sexual reproduction, as the embryo inherits half of the DNA
from each parent, creating a unique combination of DNA.
Down’s syndrome (trisomy 21) is caused when non-sister chromatids of chromosome 21 fail to
separate in meiosis II. The result of this is a sex cell that contains an extra version of
chromosome 21. After fertilisation, this extra chromosome will be copied into every cell in the
embryo as it grows, meaning that each of its cells will contain an extra chromosome.
Meiosis is a very complex topic to study and can provide information on fertility, plant and
animal breeding, and sources of genetic variation. In particular, research into meiosis can
provide information about why age-related fertility issues arise.
This resource first appeared in ‘The Cell’ in January 2011 and reviewed in September 2015.
Published by the Wellcome Trust, a charity registered in England and Wales, no. 210183.
bigpictureeducation.com
Differences between mitosis and meiosis
Mitosis
Meiosis
Number of cell divisions
One
Two
Number of daughter
cells produced
Similarity to dividing
cell
Genetics of daughter
cells
Organisms in which it
occurs
Type of cells created
Two
Four
Identical
Genetically unique
Diploid (2n)
Haploid (n)
All organisms except viruses
Only animals, plants and fungi
Somatic cells (all body cells
excluding egg or sperm cells)
No
Only germ cells (egg or sperm)
Sister chromatids are
separated to opposite poles
During anaphase I sister
chromatids move to same pole.
In anaphase II they are separated
to opposite poles
Does recombination or
crossing over occur?
What happens during
anaphase?
Yes, during prophase I
Lead image:
Human cells showing the stages of cell division
Matthew Daniels, Wellcome Images
This resource first appeared in ‘The Cell’ in January 2011 and reviewed in September 2015.
Published by the Wellcome Trust, a charity registered in England and Wales, no. 210183.
bigpictureeducation.com