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Medicine Year 1 - Semester 2 - Part 2 > Meiosis > Flashcards

Meiosis Flashcards

1
Q

Meiosis results in cells which are ?

A

Haploid

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2
Q

In meiosis, how many rounds of DNA replication and division are there ?

A

1 replication
2 divisions

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3
Q

Describe the chromosomes in Meiosis 1

A

Homologous chromosomes are paired at the metaphase plate.

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4
Q

Meiosis

A

The division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte.

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5
Q

Describe the chromosomes in Mitosis

A

Homologous chromosomes line up at the metaphase plate independently.

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6
Q

Describe Meiosis 1

A

Replicated chromosomes align to form a bivalent

Chiasma develop in prophase 1

Recombination occurs - important source of genetic diversity

Homologous chromosomes line up at spindle and are separated and move towards opposite poles.

Cytokinesis

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7
Q

Step 1 of Meiosis 1

A

The chromosomes replicated prior to Meiosis 1, each consist of genetically identical chromatids attached at the centromere.

Chromosomes condense and homologous chromosomes pair up.

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8
Q

Stage 2 of Meiosis 1

A

Chiasmata form at points of contact between the non-sister chromatids of a homologous chromosome and sections of DNA are exchanged.

Crossing over of DNA is random and produces genetically different recombinant chromosomes.

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9
Q

Stage 3 of Meiosis 1

A

Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle.

The orientation of the pairs of homologous chromosomes at the equator is random.

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10
Q

Stage 4 of Meiosis 1

A

The chromosome of each homologous pair are separated and move towards opposite poles.

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11
Q

Stage 5 of Meiosis 1

A

Cytokinesis occurs and 2 daughter cells form

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12
Q

When do chiasma develop ?

A

Prophase 1

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13
Q

Meiosis 2

A

Each of the 2 cells produced in Meiosis 1 undergoes further division during which the sister chromatids of each chromosome are separated.

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14
Q

Function of crossing over - recombination

A

Allows exchange of genetic information and is an important source of genetic diversity.

Useful tool for geneticists.

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15
Q

Holliday junction

A

Shows what happens at the crossing over point between different strands of DNA.

Recombination enzyme has the capacity to cleave DNA in different locations.

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16
Q

Describe the complex that forms at the Holliday junction

A

The key complex that does the nicking and ligating of the strands together.

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17
Q

Describe pairing and recombination of sex chromosomes in males

A

There is a small region of the X and Y chromosome called the pseudoautosomal region.

This allows the X and Y chromosome to pair up and undergo recombination within the short region.

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18
Q

Describe the Y chromosome

A

Shorter than the X chromosome

So, lack most of the corresponding alleles to the X chromosome.

19
Q

Significance of pairing and recombination of sex chromosomes in males

A

Implications for tracing paternal ancestry, which can go back generations.

20
Q

Feature of the pseudoautosomal region

A

Shox is a gene that lies in the pseudo-autosomal region

The pseudo-autosomal dominant region escapes X chromosome inactivation.

21
Q

Describe X chromosome inactivation

A

IN homogametic females, one of the 2X chromosomes are inactivated randomly at an early stage of development.

This prevents a double dose of gene products which could be harmful to cells.

22
Q

How is genetic diversity generated ?

A

Independent assortment of maternal and paternal homologs during Meiosis 1.

Crossing over during meitotic prophase 1.

23
Q

Bivalent

A

Set of 4 paired chromatids

24
Q

How many gametes are formed from one round of Meiosis ?

A

4

25
Q

What is key to correct meiotic division ?

A

Degradation of cohesion complexes

26
Q

What feature increases in Meiosis 1 and 2 ?

A

MPF - maturation promoting factor

CDK:cyclin complex

27
Q

Key protein involved in degradation of cohesion complexes

A

Rec8 - containing mitotic divisions

In Meiosis 1 - loss of Rec8 from the arms of chromosomes NOT centromere

In Meiosis 2 - loss of Rec8 from the centromere

28
Q

State a disease arising from an error from meiosis

A

Down syndrome
- error in meiosis 1

29
Q

Describe the meiotic error in Down syndrome

A

Paternal and Maternal chromosomes
Normal duplication of chromosomes
Recombination can occur normally

Non-disjunction OCCURS

Gametes with 2 copies of chromosome 21

(occurs after DNA replication)

30
Q

What is non-disjunction ?

A

Improper separation of bivalent into intermediates between Meiosis 1 and 2

Duplicated chromosomes in meiosis 2 can separate.

31
Q

Describe an error in Meiosis 2

A

Bivalent structure is separated properly in Meiosis 1

Problem in Meiosis 2

Duplicated chromatids are not separated correctly.

32
Q

Meiosis 1

A

Separation of homologs

33
Q

Meiosis 2

A

Separation of sister chromatids

34
Q

Describe fertilisation following meiotic errors

A

Trisomic zygote formed which can try to become an embryo.

  • different chromosomes in the trisomic zygote, depending if the error was in Meiosis 1 or 2
35
Q

Uniparental disomy after MI error

A

Uniparental heterodisomy

  • Different chromosomes
  • Same parent
36
Q

Uniparental disomy after MII error

A

Uniparental isodisomy

  • Same chromosome
  • Same parent
37
Q

What is a problem with uniparental disomy ?

A

Receiving 2 copies of a chromosome from one parent and none from another can have serious consequences.

38
Q

State the syndromes caused by uniparental disomy

A

Angelman
Prada-Willi syndrome

Beckwith-Wedemann syndrome

39
Q

What is strongly associated with meiotic errors ?

A

Maternal Age
Women >40 years old

Errors in Meiosis 1 are more common than Meiosis 2

40
Q

Spermatogenesis

A

Occurs continuously from puberty onwards.

Meiosis - 24 days long

4 sperm are generated from 1 precursor

41
Q

Oogenesis

A

Meiosis 1 begins during foetal development.

Undergoing Meiosis 1 during puberty.

  • polar body generated

Secondary oocyte carries onto Meiosis 2

Meiosis 2 ends when the egg is fertilised.

Fertilisation - second polar body discarded

42
Q

Differences between Meiosis in spermatogenesis and oogenesis

A

IN spermatogenesis, continuous meiosis, generating 4 gametes from 1 precursor

In females, discontinuous, very long periods of arrest and only generating 1 gamete from 1 precursor. Last stage only happens if the egg is fertilised.

43
Q

What stage are the eggs arrested in Meiosis - in oogenesis ?

A

Long arrest at prophase 1

  • if chromosomes are not held together properly, this may result in a meiotic error
44
Q

Do chromosome cohesion defects underlie maternal age effect ?

A

Yes

Suggested that in older women chromosomes are not held together properly.

Based on a study with mice.

Medicine Year 1 - Semester 2 - Part 2 (53 decks)

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