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Cell Biology > Meiosis > Flashcards

Meiosis Flashcards

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1
Q

What does meiosis produce?

A

Haploid cells w/ genetic material for sexual reproduction

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2
Q

Briefly, what is meiosis?

A

Specialised cell division w/ 2 rounds of chromosome segregation

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3
Q

Briefly what happens in meiosis?

A

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

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4
Q

Why is meiosis advantageous?

A

Increases genetic diversity

  • mix up maternal & paternal DNA in homologous recombination
  • shuffle maternal & paternal sets of chromosomes
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5
Q

Which meiosis is most different to mitosis?

A

Meiosis 1

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6
Q

How is meiosis 1 so different to mitosis?

A

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

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7
Q

What are the 5 stages of prophase 1?

A
  1. Leptotene (‘thin’)
  2. Zygotene (‘paired’)
  3. Pachyene (‘thick’)
  4. Diplotene (‘two’)
  5. Diakinesis (‘moving through’)
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8
Q

What happens in leptotene?

What about zygotene?

A
  1. Leptotene (‘thin’)
    - condensation of sister chromatids
  2. Zygotene (‘paired’)
    - homologous chromosomes pair up
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9
Q

What happens in pachytene?

What about in diplotene?

A
  1. Pachyene (‘thick’)
    - homologous chromosomes begin to exchange genetic material
  2. Diplotene (‘two’)
    - pairing is looser & sister chromatid pairs are visible
    - crossing over
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10
Q

What happens in diakinesis?

A
  1. Diakinesis (‘moving through’)

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

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11
Q

How do homologous chromosomes pair up?

A

Via complementary DNA sequences

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

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12
Q

What are synaptonemal complexes?

A

Protein structure formed between pairs of homologous chromosomes

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13
Q

What are the 3 elements of synaptonemal complexes?

A

Cohesins
Axial core
Transverse fibres

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14
Q

What are heteroduplexes?

A

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

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15
Q

How do homologous strands find each other?

A

Rare event

- unless double stranded break occurs

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16
Q

What is a double stranded break?

Why do cells want to prevent these?

A

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

Can lead to translocations that go wrong

17
Q

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

A

Recombination

18
Q

What is used to repair broken DNA?

When is this?

A

Homologous chromosomes = a template

Occurs after DNA replication in mitosis

19
Q

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

A

Nucleases induce DSBs

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

20
Q

Describe the process of DSB repair

A
  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
21
Q

What are holiday junctions?

A

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

22
Q

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

A

Cut & reseal along vertical

Cut & reseal along horizontal

23
Q

What is the chiasmata?

A

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

24
Q

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

A

Would destabilise chromosome

= increases chance of deletion, duplication or translocation

25
Q

What is gene conversion?

A

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

26
Q

What can gene conversions lead to?

What then happens later on?

A

Some mis-matched bases due to diffs in DNA sequences

Mis-match repair
- 50% chance of choosing either strand

27
Q

Describe the kinetochores in meiosis 1

A

Sister chromatid kinetochores are fused & mono-oriented

28
Q

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

A
  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
29
Q

Describe the kinetochores in meiosis 2

A

Sister kinetochores

bi-oriented in metaphase 2

30
Q

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

A

> Cohesion depletion

> Pulling force from microtubules in metaphase 2

31
Q

How is cohesin protected in meiosis 1?

A

Protected near centromere by binding to Sgo1

32
Q

How is cohesin cleaved in meiosis 2?

A

Sgo1 is a target of the APC

Sgo1 is also involved in sensing tension at kinetochores

33
Q

What are the 3 forms of genetic diversification in meiosis?

A

Independent assortment
Crossing over
Gene conversion

34
Q

What is non-disjunction?

Give examples

A

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
35
Q

How is sperm produced in spermatogenesis?

A

Spermatogonium

  • > primary spermatocyte
  • > secondary spermatocyte -> spermatids (via meiosis I)
  • > 4 sperm cells (via meiosis II)
36
Q

Describe the transition from oocyte to zygote

A

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

Cell Biology (23 decks)

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