Post on 06-Jan-2017
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Mitosis and MeiosisThis 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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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 Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Fig. 2.6 ©Scion Publishing Ltd
The stages of mitosisSee next slides for individual stages
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Meiosis• Function
Reduction division (23 chromosomes per gamete) reassortment of genes by:
• crossing-over• independent segregation of chromosomes
• MechanismEach 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 Healthcarewww.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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Each spermatogonium in the testis at age 15 is the result of 30 previous cell divisions
This spermatogoniummaintains the stock ofspermatogonia andcontinues to divide
Four spermatozoa
Every 16 daysfrom 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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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 spermatocy
te
SG
SG
SG
SC SC
secondary spermatocyte
sMEIOSIS IISC
4 spermatids
differentiation
MITOSIS
SG
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
22 mitotic cell divisions by 5 months gestation to make a stock of2,600,000 oocytes
Each month one is ovulated
MEIOSIS I completed at ovulation
Polar body
Meiosis II completed atfertilisation
2nd polar body Zygote
The number of cell divisions required to produce a human egg cell
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
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 untilfertilisation
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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
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.
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Normal monosomic gametes
Normal meiosisReduction division
MEIOSIS I
MEIOSIS II
Results of crossing-over not shown
Replicate DNA
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
MEIOSIS I
MEIOSIS II
Results of crossing-over not shown
Replicate DNA Nondisjunction during meiosis I
Non-disjunction
Disomic gametes Nullisomic gametes
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
New mutations: increase with paternal age
0
1
2
3
4
5
24 29 34 39 44 47
Paternal age
Rela
tive
freq
uenc
y
MarfanAchondroplasia
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 Healthcarewww.geneticseducation.nhs.uk
Meiosis
Animation from Tokyo Medical UniversityGenetics Study Group Hironao NUMABE, M.D
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
Non-disjunction in meiosis I resulting in trisomy 21 Down syndrome
Animation from Tokyo Medical UniversityGenetics Study Group Hironao NUMABE, M.D
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.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
© 2009 NHS National Genetics Education and Development Centre
Genetics and Genomics for Healthcarewww.geneticseducation.nhs.uk
MeioticNon-disjunction(Trisomy 21: 75% meiosis 1)
Trisomy Monosomy (lethal)