0121Explanation_slides_Mitosis_and_Meiosis.ppt

cell theory

1. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Mitosis and Meiosis
This PowerPoint file contains a number of slides that may be useful for
teaching of genetics concepts.
You may use these slides and their contents for non-commercial educational
purposes.
This presentation contains diagrams of:
• Mitosis
• Meiosis
• Meiotic non-disjunction

2. © 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

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Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

4. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Fig. 2.6 ©Scion Publishing Ltd
The stages of
mitosis
See next slides for
individual stages

5. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

6. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

7. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

8. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

9. © 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

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Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
A homologous pair of
parental chromosomes (e.g.
chromosome 7)
In meiosis I each chromosome duplicates producing
two sister chromatids
Crossing-over
(Recombination)
Gene re-assortment
by crossing-over
meiosis II

11. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
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Each spermatogonium in the testis at
age 15 is the result of 30 previous cell
divisions
This spermatogonium
maintains the stock of
spermatogonia and
continues to divide
Four spermatozoa
Every 16 days
from puberty
At the age of 25:
310 cell divisions have had
to occur to produce a
particular sperm.
The number of cell divisions required to produce a human
sperm
Four spermatozoa

12. © 2009 NHS National Genetics Education and Development
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www.geneticseducation.nhs.uk
MEIOSIS I
Each spermatogonium in testis at age 15 is result of 30 previous mitotic cell divisions
Pool of spermatogonia
maintained and
continues to divide
4 spermatozoa
(Every 16 days from puberty)
At the age of 25:
310 cell divisions have had to occur to
produce a particular sperm.
The number of cell divisions required to produce a human
sperm
primary
spermatocyt
e
SG
SG
SG
SC SC
secondary
spermatocytes
MEIOSIS II
SC
4 spermatids
differentiation
MITOSIS
SG

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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
Polar body
Meiosis II completed at
fertilisation
2
nd
polar body Zygote
The number of cell divisions required to produce a human egg cell

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Genetics and Genomics for Healthcare
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The stock of oocytes is ready by 5 months gestation.
Each remains in maturation arrest at the
crossing-over stage until ovulation
Each month one is ovulated
Meiosis I not
completed until
ovulation
Polar body
Meiosis II not completed until
fertilisation
2
nd
polar body
Zygote
Oocytes, time and the completion of meiosis
There may be a lengthy
interval between onset
and completion of meiosis
(up to 50 years later)
Accumulating effects on
the primary oocyte during
this phase may damage
the cell’s spindle formation
and repair mechanisms
predisposing to non-
disjunction.

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Genetics and Genomics for Healthcare
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16. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Fig. 2.7 ©Scion Publishing Ltd
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.

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Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk

19. © 2009 NHS National Genetics Education and Development
Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Fig. 2.8 © Scion Publishing Ltd
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 four-
stranded structure clear.

20. © 2009 NHS National Genetics Education and Development
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Fig. 2.12 © Scion Publishing Ltd
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.

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Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Fig. 2.17 ©Scion Publishing Ltd
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.

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Fig. 2.21 ©Scion Publishing Ltd
During meiosis I
matching chromosome
segments pair. If one
chromosome has an
inversion compared to
its homolog, they
usually form a looped
structure.

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Genetics and Genomics for Healthcare
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Normal monosomic gametes
Normal meiosis
Reduction division
MEIOSIS I
MEIOSIS II
Results of crossing-
over not shown
Replicate DNA

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Genetics and Genomics for Healthcare
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MEIOSIS I
MEIOSIS II
Results of crossing-
over not shown
Replicate DNA Nondisjunction
during meiosis I
Non-disjunction
Disomic gametes
Nullisomic gametes

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Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
MEIOSIS I
MEIOSIS II
Results of crossing-
over not shown
Replicate DNA Nondisjunction
during meiosis II
Non-disjunction
Disomic Nullisomic Monosomic Monosomic gametes

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Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
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

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Genetics and Genomics for Healthcare
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New mutations: increase with paternal age
0
1
2
3
4
5
24 29 34 39 44 47
Paternal age
R
e
l
a
t
i
v
e

f
r
e
q
u
e
n
c
y
Marfan
Achondroplasia
Higher mutation rates in males are likely to be related to the greater number of
germ cell divisions

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Centre
Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Meiosis
Animation from Tokyo Medical University
Genetics Study Group Hironao NUMABE, M.D

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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

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Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Normal disomy
Mitosis
Non-disjunction
Normal disomy Trisomy
Monosomy (lethal to cell)
Somatic mosaicism (eg trisomy 21) as a result of
mitotic non-disjunction

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Meiotic
Non-disjunction
(Trisomy 21:
75% meiosis 1)
Trisomy Monosomy (lethal)