© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Mitosis and Meiosis This

© 2009 NHS National Genetics Education and Development Centre

Genetics and Genomics for Healthcarewww.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



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

• 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



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 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



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

s

MEIOSIS IISC

4 spermatids

differentiation

MITOSIS

SG



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



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.






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 meiosisReduction division

MEIOSIS I

MEIOSIS II

Results of crossing-over not shown

Replicate DNA



MEIOSIS I

MEIOSIS II


Replicate DNA Nondisjunction during meiosis I

Non-disjunction

Disomic gametes Nullisomic gametes



MEIOSIS I

MEIOSIS II


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

Rela

tive

fre

quency

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 UniversityGenetics Study Group Hironao NUMABE, M.D



Non-disjunction in meiosis I resulting in trisomy 21 Down syndrome

Animation from Tokyo Medical UniversityGenetics 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



MeioticNon-disjunction

(Trisomy 21: 75% meiosis 1)

Trisomy Monosomy (lethal)

Documents

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Mitosis and Meiosis This