Meiosis

Question 1

What does meiosis produce?

Answer 1

Haploid cells w/ genetic material for sexual reproduction

Question 2

Briefly, what is meiosis?

Answer 2

Specialised cell division w/ 2 rounds of chromosome segregation

Question 3

Briefly what happens in meiosis?

Answer 3

After DNA replication
(2n–>4n)
–> meiosis 1 = segregate homologas chromosomes (4n–>2n)
–> meiosis 2 = segregate sister chromatids (2n–>1n)

Question 4

Why is meiosis advantageous?

Answer 4

Increases genetic diversity

• mix up maternal & paternal DNA in homologous recombination
• shuffle maternal & paternal sets of chromosomes

Question 5

Which meiosis is most different to mitosis?

Answer 5

Meiosis 1

Question 6

How is meiosis 1 so different to mitosis?

Answer 6

Prophase 1: homologous chromosomes pair & homologous recombination occurs

Metaphase 1 : sister chromatids only have 1 kinetochore

Anaphase 1: sister chromatids stay together

Telophase 1: No cytokinesis

Question 7

What are the 5 stages of prophase 1?

Answer 7

1. Leptotene (‘thin’)
2. Zygotene (‘paired’)
3. Pachyene (‘thick’)
4. Diplotene (‘two’)
5. Diakinesis (‘moving through’)

Question 8

What happens in leptotene?

What about zygotene?

Answer 8

1. Leptotene (‘thin’)
   - condensation of sister chromatids
2. Zygotene (‘paired’)
   - homologous chromosomes pair up

Question 9

What happens in pachytene?

What about in diplotene?

Answer 9

3. Pachyene (‘thick’)
   - homologous chromosomes begin to exchange genetic material
4. Diplotene (‘two’)
   - pairing is looser & sister chromatid pairs are visible
   - crossing over

Question 10

What happens in diakinesis?

Answer 10

5. Diakinesis (‘moving through’)

- chromatid pairs separate, nuclear envelope breaks down & spindle forms

Question 11

How do homologous chromosomes pair up?

Answer 11

Via complementary DNA sequences

Process called synapsis:
2 pairs of sister chromatids form a 4-chromosome bivalent
- joined by synaptonemal complex

Question 12

What are synaptonemal complexes?

Answer 12

Protein structure formed between pairs of homologous chromosomes

Question 13

What are the 3 elements of synaptonemal complexes?

Answer 13

Cohesins
Axial core
Transverse fibres

Question 14

What are heteroduplexes?

Answer 14

DNA double helix composed of strands that originate from 2 diff duplexes

Question 15

How do homologous strands find each other?

Answer 15

Rare event

- unless double stranded break occurs

Question 16

What is a double stranded break?

Why do cells want to prevent these?

Answer 16

When both strands of DNA double helix are severed
e.g. by nucleases

Can lead to translocations that go wrong

Question 17

What do programmed double strand breaks in meiosis 1 lead to?

Answer 17

Recombination

Question 18

What is used to repair broken DNA?

When is this?

Answer 18

Homologous chromosomes = a template

Occurs after DNA replication in mitosis

Question 19

What happens in the meiosis-specific double strand break repair process?

Answer 19

Nucleases induce DSBs

Protein complexes cause maternal-paternal chromosome heteroduplexes to form
favoured over sister chromatids

Question 20

Describe the process of DSB repair

Answer 20

1. broken end processing
   - nucleases degrade broken ends = leaves ssDNA overhanging
2. Strand exchange
   - protein complex binds ssDNA + a double helix (which it pulls apart)
   - if sequences match, pairing of diff strands can occur
3. DNA synthesis to fill in gaps
4. cut & ligate repaired strands

Question 21

What are holiday junctions?

Answer 21

Cross-shaped DNA in open configuration
- forms during genetic recombination
=when 2 dsDNA molecules become separated into 4 strands

Question 22

What are the 2 diff ways holiday junctions can be resealed?

Answer 22

Cut & reseal along vertical

Cut & reseal along horizontal

Question 23

What is the chiasmata?

Answer 23

Point where crossing over & exchange of genetic material occurs between strands

Question 24

Why do cells ensure that crossovers don’t occur near one another?

Answer 24

Would destabilise chromosome

= increases chance of deletion, duplication or translocation

Question 25

What is gene conversion?

Answer 25

When a bit of maternal strand is copied into the paternal strand (& vice versa)

Question 26

What can gene conversions lead to? What then happens later on?

Answer 26

Some mis-matched bases due to diffs in DNA sequences Mis-match repair - 50% chance of choosing either strand

Question 27

Describe the kinetochores in meiosis 1

Answer 27

Sister chromatid kinetochores are fused & mono-oriented

Question 28

What happens in meiosis 1 that prevents sister chromatids from being separated in anaphase 1?

Answer 28

1. Synaptonemal complex is degraded in prophase 1 2. Cohesin near kinetochores are protected from separate in meiosis 1 3. sister chromatid pairs move to poles in anaphase 1

Question 29

Describe the kinetochores in meiosis 2

Answer 29

Sister kinetochores | bi-oriented in metaphase 2

Question 30

What does the re-orientation of kinetochores from metaphase 1 to metaphase 2 involve?

Answer 30

>Cohesion depletion >Pulling force from microtubules in metaphase 2

Question 31

How is cohesin protected in meiosis 1?

Answer 31

Protected near centromere by binding to Sgo1

Question 32

How is cohesin cleaved in meiosis 2?

Answer 32

Sgo1 is a target of the APC | Sgo1 is also involved in sensing tension at kinetochores

Question 33

What are the 3 forms of genetic diversification in meiosis?

Answer 33

Independent assortment Crossing over Gene conversion

Question 34

What is non-disjunction? Give examples

Answer 34

Failure of chromosomes to segregate properly ``` Down syndrome = trisomy 21 Trisomy 18 or 13 = usually mosaic Turner syndrome = X Klinefelter syndrome = XXY XYY - no symptoms ```

Question 35

How is sperm produced in spermatogenesis?

Answer 35

Spermatogonium - > primary spermatocyte - > secondary spermatocyte -> spermatids (via meiosis I) - > 4 sperm cells (via meiosis II)

Question 36

Describe the transition from oocyte to zygote

Answer 36

Oocyte = arrested in prophase I 1st meiotic division->50% of chromosomes disposed of in polar body after metaphase 1 2nd meiotic division -> 50% of chromosomes disposed of in 2nd polar body after metaphase 2 Meiosis is only completed if fertilised - -> fertilised egg - -> zygote