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Number 237
Mitosis vs Meiosis
Students often get overwhelmed by the details when they first begin to study mitosis and meiosis.
This Factsheet:
• Gives a simple overview of the processes
• Summarises and compares mitosis and meiosis
• Reviews some of the more straightforward exam questions which pop up on these topics
Note: For a much more detailed account of mitosis and meiosis see Factsheets:
50: Sources of genetic variation
76: Eukaryotic cell cycle
168: Gamete formation in animals
Mitosis vs. Meiosis
The processes that your body must perform every day to keep you alive are carried out by millions of different cells, many with individual,
specialised functions. However, these cells age and eventually die – so in order for you to survive, these cells must be continually
replaced.
To ensure this happens, the cells in your body are able to divide and produce copies of themselves. There are two different ways a cell can
divide, either by mitosis or meiosis – and this Factsheet outlines how, when and why each process happens.
Why are there two different ways a cell can divide?
When a cell is preparing to divide, it first copies everything inside it – including your DNA. Your DNA is divided into 46 pieces called
chromosomes, and is a mixture of information from your parents: you have 23 chromosomes from your father, and 23 from your mother.
Every cell in your body has 46 chromosomes apart from your red blood cells (that don't contain DNA) and your gametes. These cells
(sperm in males or eggs in females) only have 23 chromosomes. This means that during fertilisation, when a sperm cell fuses with an egg
cell, the fertilised egg has 46 chromosomes in total. Every other cell type that forms from the fertilised egg as it develops into an embryo
therefore has 46 chromosomes. If sperm and egg cells had 46 chromosomes each, then the cell formed when they fused would have 92
chromosomes in total, and would die.
.
So, you have two groups of cells in your body: those with 23 chromosomes (sperm or egg cells) or those with 46 chromosomes (most other
cell types). Cells with 23 chromosomes are referred to as haploid, whereas cells with 46 chromosomes are referred to as diploid. Because
these cells have different numbers of chromosomes, the way in which they divide is also different. This is why there are two different ways
a cell can divide: mitosis maintains the number of chromsomes in a cell, meiosis halves the number.
Typical Exam Question
(a) Complete the table to indicate the processes in which mitosis
occurs in humans.
Tick the box to indicate that mitosis does occur, put a cross if it
does not. (4).
(b) Epithelial cell 48;
Sperm 24;
Brain cell 48;
Repair of tissue
Cancer
Cell growth
Replacement of skin cells
Process
9 if mitosis does occur
X if mitosis does not occur
9
9
X
9
9 if mitosis does occur
X if mitosis does not occur
Repair of tissue
Cancer
Cell growth
Replacement of skin cells
Process
(a)
Markscheme
(b) The diploid number of chimpanzees is 48. How many
chromosomes would the following cells contain? (3)
Epithelial cell
Sperm
Brain cell
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237. Mitosis Vs Meiosis
Bio Factsheet
www.curriculum-press.co.uk
Fig 1 shows why sperm and egg cells have 23 chromosomes, how they fuse to form a cell with 46 chromosomes and how this cell can either
divide by mitosis to produce more diploid cells, or divide by meiosis to form haploid cells.
Fig 1. Mitosis maintains ploidy but Meiosis results in haploid cells
Haploid gametes
Sperm
23 chromosomes
(5 shown)
Egg
23 chromosomes
(5 shown)
Diploid
Fertilised egg
46 chromosomes
(10 shown)
Mitosis
Meiosis
46 chromosomes
DIPLOID
23 chromosomes
HAPLOID
Diploid
Diploid
Diploid
Diploid
Haploid
Haploid
Diploid
Haploid
Haploid
Meiotic cell division ONLY produces
sperm cells and egg cells
Mitotic cell division produces all types of
cells EXCEPT sperm cells and egg cells
The next two sections describe the key events that occur during the processes of mitosis and meiosis. These events are illustrated in Fig
2 and 3.
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237. Mitosis Vs Meiosis
Bio Factsheet
www.curriculum-press.co.uk
Mitosis
1. The 46 chromosomes in your cells are found in the nucleus in pairs. In each pair, one chromosome is from your father, and one from
your mother. The chromosomes in each pair are called homologous chromosomes. Because you have 46 individual chromosomes in
total, you have 23 pairs of homologous chromosomes.
2. When the cell is ready to divide, each chromosome is copied. The two copies of each chromosome are called sister chromatids, and
are held together in the middle by a structure called the centromere.
3. The centromere fixes sister chromatids to fibres within the cell. These fibres separate the sister chromatids by pulling them to opposite
sides of the cell. This process results in identical copies of the original homologous chromosomes being located at opposite sides of
the cell (Fig. 2).
4. The cell then physically divides to form two new diploid cells, each with the same number of chromosomes as the original cell.
Fig 2 Mitosis
One pair of homologous chromosomes
= from father
= from mother
Sister chromatids
Each chromosome copies itself to
form sister chromatids
Centromere forms
Centromere
Extract from Chief Examiner’s Report
It was surprising to find a significant
number of candidates that confused
the terms chromatid, centrosome and
centromere. Weaker candidates were
confused about which membrane was
breaking down – some thought it was
the cell membrane…
The sister chromatids attach to
fibres by the centromere and are
pulled to opposite ends of the cell
The fibres break down and the
cell begins to divide
Two identical DIPLOID
cells are formed
(46 chromosomes)
N.B. Only one pair of homologous chromosomes are shown per cell for simplicity – the same process occurs to all 23 pairs found in the
nucleus of a cell.
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237. Mitosis Vs Meiosis
Bio Factsheet
www.curriculum-press.co.uk
Meiosis
1. As in mitosis, when the cell is ready to divide it first copies its chromosomes. However, there is a key additional step in meiosis at this
point that does NOT happen in mitosis. After the chromosomes have been copied, the sister chromatids of each homologous chromosome
are found next to each other in the nucleus (Fig. 3). At this point, sister chromatids from DIFFERENT chromosomes in a homologous
pair can cross over each other, forming a structure called a chiasma.
2. The reason chiasma occur in meiosis and not mitosis is because the formation of chiasma results in mixing of genetic information.
When cells divide by mitosis they want to produce identical copies of themselves – so they do not want to mix chromosomes. However,
meiosis produces sex cells which pass on your genetic information to the next generation. The mixing of genetic information in these
cells introduces variation into a species, and is why you are not identical to either of your parents, but a mixture of characteristics from
them both.
3. After the formation of the chiasma, the chromatids are pulled to opposite ends of the cell and two diploid cells are produced, as in
mitosis. However, instead of ending here, during meiosis the two chromosomes of a homologous pair in the newly formed cells are
reattached to the fibres, and again pulled to opposite ends of the cell (Fig. 3).
4. Because this time the chromosomes have not been copied, when the cell then divides each daughter cell only has one chromosome
from each pair, and therefore the cell now has only 23 chromosomes. It is haploid – it only has half the number of chromosomes as the
original cell.
Fig 3. Meiosis
Two diploid cells are produced by mitosis after mixing of genetic
information by chiasma formation
Chromatids reattach to fibres in the cell and are pulled to opposite
ends without copying themselves
Chiasma
Four HAPLOID sex cells are
produced
(23 chromosomes)
N.B. Only one pair of homologous chromosomes are shown per
cell for simplicity – the same process occurs to all 23 pairs
found in the nucleus of a cell.
Summary: Mitosis vs. Meiosis
MITOSIS
MEIOSIS
Number of chromosomes in daughter cells
46
23
Genetic definition
Diploid
Haploid
Type of cell produced
Cells required for growth,
maintenance and repair
Sex cells (sperm or egg cells)
No. of times cell division occurs
Once
Twice
No. of daughter cells produced
Two
Four
Formation of chiasma?
No
Yes
Acknowledgements:
This Factsheet was researched and written by Bryony Graham
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