Mitosis and Meiosis
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genetics concepts.
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This presentation contains diagrams of:
• Mitosis
• Meiosis
• Meiotic non-disjunction
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
What is the purpose of mitosis?
Cell division
Products genetically identical
Growth of organism
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
The stages of mitosis
See next slides for
individual stages
Fig. 2.6 ©Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Meiosis
•
Function
Reduction division (23 chromosomes per gamete) reassortment of genes by:
• crossing-over
• independent segregation of chromosomes
•
Mechanism
Each homologue (e.g. “chromosome 7”) replicates to give two sister chromatids
Homologues pair (e.g. maternal chromosome 7 and paternal chromosome 7)
Exchange of material between non-sister chromatids: crossing-over, recombination
Chiasmata (visible cytologically) are the physical manifestations of crossing-over
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
A homologous pair of parental
chromosomes (e.g.
chromosome 7)
Gene re-assortment by
crossing-over
In meiosis I each chromosome duplicates producing two
sister chromatids
Crossing-over
(Recombination)
© 2009 NHS National Genetics Education and Development Centre
meiosis II
Genetics and Genomics for Healthcare
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The number of cell divisions required to produce a human sperm
Each spermatogonium in the testis at age 15
is the result of 30 previous cell divisions
Every 16 days
from puberty
This spermatogonium
maintains the stock of
spermatogonia and
continues to divide
Four spermatozoa
Four spermatozoa
© 2009 NHS National Genetics Education and Development Centre
At the age of 25:
310 cell divisions have had to
occur to produce a particular
sperm.
Genetics and Genomics for Healthcare
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The number of cell divisions required to produce a human sperm
SG
Each spermatogonium in testis at age 15 is result of 30 previous mitotic cell divisions
MITOSIS
primary
spermatocyte
SG
SC
secondary
spermatocytes
MEIOSIS I
SC
SC
4 spermatids
MEIOSIS II
4 spermatozoa
differentiation
(Every 16 days from puberty)
SG
SG
Pool of spermatogonia
maintained and continues
to divide
© 2009 NHS National Genetics Education and Development Centre
At the age of 25:
310 cell divisions have had to occur to
produce a particular sperm.
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
The number of cell divisions required to produce a human egg cell
22 mitotic cell divisions by 5 months gestation to make a stock of
2,600,000 oocytes
Each month one is ovulated
MEIOSIS I completed at ovulation
Meiosis II completed at
fertilisation
Polar body
2nd polar body
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Zygote
Genetics and Genomics for Healthcare
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Oocytes, time and the completion of meiosis
The stock of oocytes is ready by 5 months gestation. Each
remains in maturation arrest at the crossing-over stage
until ovulation
There may be a lengthy interval
between onset and completion
of meiosis (up to 50 years later)
Each month one is ovulated
Meiosis I not
completed until
ovulation
Polar body
Accumulating effects on the
primary oocyte during this
phase may damage the cell’s
spindle formation and repair
mechanisms predisposing to
non-disjunction.
Meiosis II not completed until
fertilisation
2nd polar body
Zygote
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
The stages of meiosis.
Meiosis is used only for the
production of sperm and eggs.
It consists of two successive cell
divisions, producing four
daughter cells (although in
oogenesis only one of these
develops into a mature oocyte;
the others form the polar
bodies).
Meiosis has two main functions:
to reduce the chromosome
number in the gamete to 23, and
to ensure that every gamete is
genetically unique.
Fig. 2.7 ©Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Examples of chromosomes
during meiosis.
(a)
Two cells from a testicular biopsy
showing chromosomes during
prophase I of male meiosis. Each
of the 23 structures is a bivalent,
consisting of two homologous
chromosomes, each having two
chromatids. Note the end-to-end
pairing of the X and Y
chromosomes.
(b)
A bivalent seen in meiosis in an
amphibian, which has large
chromosomes that make the fourstranded structure clear.
Fig. 2.8 © Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
The effects of
non-disjunction in
meiosis.
The non-disjunction
involves only the single
pair of chromosomes
(meiosis I) or the single
chromosome (meiosis II)
shown; all the other
chromosomes (not
shown) disjoin and
segregate normally.
Fig. 2.12 © Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Possible ways the
chromosomes could segregate
in the first meiotic division.
During prophase 1, matching
chromosome segments pair,
resulting in a cross-shaped
tetravalent containing the normal
and translocated copies of
chromosomes 1 and 22.
At anaphase 1 they pull apart, and
the diagram shows various ways this
could happen.
The gamete that gave rise to Baby
Elliot is circled. Other more complex
segregation patterns (3:1
segregation) are also possible.
Fig. 2.17 ©Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
During meiosis I matching
chromosome segments
pair. If one chromosome
has an inversion compared
to its homolog, they
usually form a looped
structure.
Fig. 2.21 ©Scion Publishing Ltd
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
MEIOSIS I
Replicate DNA
Normal meiosis
Reduction division
Results of crossing-over
not shown
MEIOSIS II
Normal monosomic gametes
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
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MEIOSIS I
Replicate DNA
Non-disjunction
Nondisjunction
during meiosis I
Results of crossing-over
not shown
MEIOSIS II
Disomic gametes
© 2009 NHS National Genetics Education and Development Centre
Nullisomic gametes
Genetics and Genomics for Healthcare
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Nondisjunction
during meiosis II
Replicate DNA
MEIOSIS I
Results of crossing-over
not shown
MEIOSIS II
Non-disjunction
Disomic
Nullisomic
© 2009 NHS National Genetics Education and Development Centre
Monosomic
Monosomic
gametes
Genetics and Genomics for Healthcare
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Parental origin of meiotic error
leading to aneuploidy
Chromosome abnormality Paternal (%)
Maternal (%)
Trisomy 21 (Down)
15
85
Trisomy 18 (Edwards)
10
90
Trisomy 13 (Patau)
15
85
45,X (Turner)
80
20
47,XXX
5
95
47,XXY
45
55
47,XYY
100
0
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
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New mutations: increase with paternal age
Relative frequency
5
4
3
Marfan
Achondroplasia
2
1
0
24
29
34
39
44
47
Paternal age
Higher mutation rates in males are likely to be related to the greater number of germ cell
divisions
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Meiosis
Animation from Tokyo Medical University
Genetics Study Group Hironao NUMABE, M.D
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Non-disjunction in meiosis I resulting in trisomy 21
Down syndrome
Animation from Tokyo Medical University
Genetics Study Group Hironao NUMABE, M.D
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Somatic mosaicism (eg trisomy 21) as a result of mitotic
non-disjunction
Mitosis
Non-disjunction
Normal disomy
Normal disomy
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Trisomy
Monosomy (lethal to cell)
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Meiotic
Non-disjunction
(Trisomy 21:
75% meiosis 1)
Trisomy
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Monosomy (lethal)
Genetics and Genomics for Healthcare
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