3LP. Transmission of genetic information from generation to generation

Question 1

Significance and source of genetic variability. Phases and events and of first prophase (leptotene, zygotene, pachytene, diplotene, diakinesis; synapsis, crossing over, chiazma).

Answer 1

Genetic variability

• Homologous recombination
• Independent assortment (homologous chromosomes in meiosis I: 2^23 variations)

Question 2

Significance of meiosis

Answer 2

1) Genetic variability

2) Reduction of chromosome number (halved)

Question 3

Stages of meiosis: meiosis I. and II

Answer 3

Meiosis I (homologous chromosomes separated in 2 cells)
- Prophase (leptotene, sygotene etc.) - crossing over
- Metaphase (chiasma visible)
- Anaphase
- Telophase
Meiosis II (sister chromatids separated in 4 cells)
- Same phases as above, but not the special prophases

Question 4

Homologous chromosomes definition

Answer 4

Chromosomes that contain DNA that codes for the same genes - but different alleles
- one maternal and one paternal

Question 5

Phases of first prophase

Answer 5

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Question 6

Leptotene phase

Answer 6

Prophase I

• Chromosome condensation
• Telomeres bind to nuclear envelope

Question 7

Zygotene phase

Answer 7

Prophase I

• Synapsis of homologous chromosomes
• Bivalent forming ->Synaptonemal complex forming (Tetrad: 2 paternal sister chromatids + 2 maternal sister chromatids + between: Central element, transverse filament, lateral elements)
• X & Y synapsis: PAR1 (pseudoautosomal region)

Question 8

Pachytene phase

Answer 8

Prophase I

• Crossing over = homologous recombination
• Recombination nodule in central region of synapsis

Question 9

Diplotene phase

Answer 9

Prophase I

- Chiazma (pl: chiasmata) = site of crossing over, now visible (!) due to separation of homologous chromosomes

Question 10

Diakinesis

Answer 10

Prophase I

• Further chromosome condensation
• Nuclear envelope fragmenting
• Bivalent ready for metaphase

Question 11

Prophase I: synapsis

Answer 11

Begins in zygotene phase, complete in pachytene phase

Question 12

Prophase I: crossing over

Answer 12

Aka homologous recombination

- Occurs in pachytene phase

Question 13

Prophase I: Chiazma

Answer 13

Site of crossing over

- Visible from diplotene phase until end of metaphase

Question 14

Random alignment and assortment of homologous chromosomes in meiosis I anaphase

Answer 14

Generates variability

2^23 different possibilities

Question 15

Role of cohesin in separation in chromosome number reduction

Answer 15

Cohesin: one out of two protein complexes influencing DNA functions (the other being condensin)

• Composed of different SMC proteins (structure maintenance of chromosomes)
• Have ATPase activity and regulatory functions
• Associate in a ring-like structure
• Hold the 2 DNA molecules (sister chromatids) together
• *Much of it detaches during prophase and at end of metaphase it’s only close to the centromere (pericentromeric cohesin) - cleaved in early anaphase => separation of sister chromatids allowed
• **Also regulates kinetochor attachment according to wiki
• *** Cornelia de Lange

Question 16

Meiosis II

Answer 16

Separation of sister chromatids -> 4 haploid cells

Question 17

Atypical meiotic process: non-disjunction

Answer 17

Meiotic nondisjunction cause aneuploid genome mutations

• May happen both in oogenesis and spermatogenesis
• More frequent in oogenesis
• Sex chromosome nondisjunction is more frequent in spermatogenesis

Question 18

Change of DNA amount and chromosome number during meiotic process

Answer 18

See notes

Question 19

Comparison of spermatogenesis and oogenesis

Answer 19

Oogenesis:

• Primary oocyte arrested in prophase I (present from birth)
• Completion of meiosis I => enter meiosis II (1 polar body formed)
• Secondary oocyte arrested in metaphase II (only completed if fertilized (2nd polar body)

Spermatogenesis

• Normal meiotic division
• Not present from birth, produced continously
• Spermatids (haploid) formed after meiosis II => differentiate to sperm cells

Question 20

Regulation of special meiotic events of oogenesis

Answer 20

1) Retinoic acid
- Metabolized (degraded) by CYP26B1 - higher amount in males => no retinoic acid => no STRA8 meiotic transcription factor (which is why males have delayed meiosis)
2) MPF
3) cAMP high levels inactivate MPF (decreased in puberty) - finishing diplotene arrest stage
4) CFS - inactivate APC via MAPK (in fertilization Ca levels increase and APC is reactivated) - finishing metaphase II arrest

Question 21

Sex specific differences in meiosis I prophase (synapsis, synaptonemal complex, chiasmata)

Answer 21

Synapsis start

• M: at end of chr
• F: inside chr

Synaptonemal complex

• M: compact, shorter
• F: less compact, longer

Sites and number of chiasmata

• M: at end of chromosomes, less
• F: inside chromosomes, more

Question 22

Role of kinetochores in chromosome number reduction

Answer 22

Kinetochores: protein structure found at the centromere of a chromatid to which microtubules attach during cell division

• Three-layered plate of protein - contains both dynein and kinesin-type motor proteins
• Associates to centromeres of chr. in prophase and prometaphase
• Help holding sister chromatids together ad plays a role in chromosome editing (wiki)
• Role: bind kinetochore microtubules(ca. 30-40 / sister chromatid)

Question 23

Role of synapsis in chromosome number reduction

Answer 23

Synapsis/syndesis: the coming together and pairing of homologous chromosomes during prophase I

• Helps make sure the chromosomes are in the right place at right time - each daughter cell gets the correct number of chromosomes
• Possible chromosomal cross-over
• When homologous chromosomes synapse, their ends are first attached to nuclear envelope => migrate until matching ends have been paired