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

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

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

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

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

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

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

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

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

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

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. 2.12 © Scion Publishing Ltd

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. 2.17 ©Scion Publishing Ltd

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. 2.21 ©Scion Publishing Ltd

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

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

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

Parental origin of meiotic error leading to aneuploidy

New mutations: increase with paternal age 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

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

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)