1. Mitosis and Meiosis
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This presentation contains diagrams of:
Mitosis
Meiosis
Meiotic non-disjunction

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

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

9. 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

10. Gene re-assortment by crossing-over
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)
meiosis II

11. 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
At the age of 25:
310 cell divisions have had to occur to produce a particular sperm.
Four spermatozoa

12. secondary spermatocytes
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
4 spermatozoa
4 spermatids
primary spermatocyte
secondary spermatocytes SC SG SC SC
differentiation
MEIOSIS I
MEIOSIS II
(Every 16 days from puberty) SG
At the age of 25:
310 cell divisions have had to occur to produce a particular sperm.
Pool of spermatogonia maintained and continues to divide SG

13. 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
Zygote

14. 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)
Accumulating effects on the primary oocyte during this phase may damage the cell’s spindle formation and repair mechanisms predisposing to non-disjunction.
Each month one is ovulated
Meiosis I not completed until ovulation
Polar body
Meiosis II not completed until fertilisation
2nd polar body
Zygote

16. 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.
Fig. 2.7 ©Scion Publishing Ltd

19. Fig. 2.8 © Scion Publishing Ltd
Examples of chromosomes during meiosis.
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.
A bivalent seen in meiosis in an amphibian, which has large chromosomes that make the four-stranded structure clear.
Fig. 2.8 © Scion Publishing Ltd

20. 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.
Fig © Scion Publishing Ltd

21. 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.
Fig ©Scion Publishing Ltd

22. 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.
Fig ©Scion Publishing Ltd

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

25. MEIOSIS I MEIOSIS II Nondisjunction during meiosis I
Replicate DNA
Nondisjunction during meiosis I
Results of crossing-over not shown
Non-disjunction
MEIOSIS II
Disomic gametes
Nullisomic gametes

26. MEIOSIS I MEIOSIS II Nondisjunction during meiosis II Replicate DNA
Results of crossing-over not shown
MEIOSIS II
Non-disjunction
Disomic
Nullisomic
Monosomic
Monosomic
gametes

27. Parental origin of meiotic error leading to aneuploidy

28. New mutations: increase with paternal age
Higher mutation rates in males are likely to be related to the greater number of germ cell divisions

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

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

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

32. Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1)
After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none.
The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21.
Trisomy
Monosomy (lethal)