Explanation slides Mitosis and Meiosis

© 2009 NHS National Genetics Education and Development Centre Genetics and Genomics for Healthcare
www.geneticseducation.nhs.uk
Mitosis and Meiosis
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This presentation contains diagrams of:
• Mitosis
• Meiosis
• Meiotic non-disjunction

What is the purpose of mitosis?
Cell division
Products genetically identical
Growth of organism

Fig. 2.6 ©Scion Publishing Ltd
The stages of mitosis
See next slides for
individual stages

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

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

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

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
spermatocyte
SG
SG
SG
SC SC
secondary
spermatocytes
MEIOSIS II
SC
4 spermatids
differentiation
MITOSIS
SG

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
2nd
polar body Zygote
The number of cell divisions required to produce a human egg cell

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
ovulationPolar body
Meiosis II not completed until
fertilisation
2nd
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.

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. 2.8 © 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.

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.

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.

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

Normal monosomic gametes
Normal meiosis
Reduction division
MEIOSIS I
MEIOSIS II
Results of crossing-over
not shown
Replicate DNA

MEIOSIS I
MEIOSIS II
not shown
Replicate DNA Nondisjunction
during meiosis I
Non-disjunction
Disomic gametes Nullisomic gametes

MEIOSIS I
MEIOSIS II
not shown
Replicate DNA Nondisjunction
during meiosis II
Non-disjunction
Disomic Nullisomic Monosomic Monosomic gametes

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

New mutations: increase with paternal age
0
1
2
3
4
5
24 29 34 39 44 47
Paternal age
Relativefrequency
Marfan
Achondroplasia
Higher mutation rates in males are likely to be related to the greater number of germ cell
divisions

Meiosis
Animation from Tokyo Medical University
Genetics Study Group Hironao NUMABE, M.D

Non-disjunction in meiosis I resulting in trisomy 21
Down syndrome
Animation from Tokyo Medical University
Genetics Study Group Hironao NUMABE, M.D

Normal disomy
Mitosis
Non-disjunction
Normal disomy Trisomy Monosomy (lethal to cell)
Somatic mosaicism (eg trisomy 21) as a result of mitotic
non-disjunction

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

Explanation slides Mitosis and Meiosis

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