Meiosis

Question 1

Meiosis results in cells which are ?

Answer 1

Haploid

Question 2

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

Answer 2

1 replication
2 divisions

Question 3

Describe the chromosomes in Meiosis 1

Answer 3

Homologous chromosomes are paired at the metaphase plate.

Question 4

Meiosis

Answer 4

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

Question 5

Describe the chromosomes in Mitosis

Answer 5

Homologous chromosomes line up at the metaphase plate independently.

Question 6

Describe Meiosis 1

Answer 6

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

Question 7

Step 1 of Meiosis 1

Answer 7

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

Chromosomes condense and homologous chromosomes pair up.

Question 8

Stage 2 of Meiosis 1

Answer 8

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.

Question 9

Stage 3 of Meiosis 1

Answer 9

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.

Question 10

Stage 4 of Meiosis 1

Answer 10

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

Question 11

Stage 5 of Meiosis 1

Answer 11

Cytokinesis occurs and 2 daughter cells form

Question 12

When do chiasma develop ?

Answer 12

Prophase 1

Question 13

Meiosis 2

Answer 13

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

Question 14

Function of crossing over - recombination

Answer 14

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

Useful tool for geneticists.

Question 15

Holliday junction

Answer 15

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

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

Question 16

Describe the complex that forms at the Holliday junction

Answer 16

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

Question 17

Describe pairing and recombination of sex chromosomes in males

Answer 17

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.

Question 18

Describe the Y chromosome

Answer 18

Shorter than the X chromosome

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

Question 19

Significance of pairing and recombination of sex chromosomes in males

Answer 19

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

Question 20

Feature of the pseudoautosomal region

Answer 20

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

The pseudo-autosomal dominant region escapes X chromosome inactivation.

Question 21

Describe X chromosome inactivation

Answer 21

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.

Question 22

How is genetic diversity generated ?

Answer 22

Independent assortment of maternal and paternal homologs during Meiosis 1.

Crossing over during meitotic prophase 1.

Question 23

Bivalent

Answer 23

Set of 4 paired chromatids

Question 24

How many gametes are formed from one round of Meiosis ?

Answer 24

4

Question 25

What is key to correct meiotic division ?

Answer 25

Degradation of cohesion complexes

Question 26

What feature increases in Meiosis 1 and 2 ?

Answer 26

MPF - maturation promoting factor CDK:cyclin complex

Question 27

Key protein involved in degradation of cohesion complexes

Answer 27

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

Question 28

State a disease arising from an error from meiosis

Answer 28

Down syndrome - error in meiosis 1

Question 29

Describe the meiotic error in Down syndrome

Answer 29

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)

Question 30

What is non-disjunction ?

Answer 30

Improper separation of bivalent into intermediates between Meiosis 1 and 2 Duplicated chromosomes in meiosis 2 can separate.

Question 31

Describe an error in Meiosis 2

Answer 31

Bivalent structure is separated properly in Meiosis 1 Problem in Meiosis 2 Duplicated chromatids are not separated correctly.

Question 32

Meiosis 1

Answer 32

Separation of homologs

Question 33

Meiosis 2

Answer 33

Separation of sister chromatids

Question 34

Describe fertilisation following meiotic errors

Answer 34

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

Question 35

Uniparental disomy after MI error

Answer 35

Uniparental heterodisomy - Different chromosomes - Same parent

Question 36

Uniparental disomy after MII error

Answer 36

Uniparental isodisomy - Same chromosome - Same parent

Question 37

What is a problem with uniparental disomy ?

Answer 37

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

Question 38

State the syndromes caused by uniparental disomy

Answer 38

Angelman Prada-Willi syndrome Beckwith-Wedemann syndrome

Question 39

What is strongly associated with meiotic errors ?

Answer 39

Maternal Age Women >40 years old Errors in Meiosis 1 are more common than Meiosis 2

Question 40

Spermatogenesis

Answer 40

Occurs continuously from puberty onwards. Meiosis - 24 days long 4 sperm are generated from 1 precursor

Question 41

Oogenesis

Answer 41

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

Question 42

Differences between Meiosis in spermatogenesis and oogenesis

Answer 42

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.

Question 43

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

Answer 43

Long arrest at prophase 1 - if chromosomes are not held together properly, this may result in a meiotic error

Question 44

Do chromosome cohesion defects underlie maternal age effect ?

Answer 44

Yes Suggested that in older women chromosomes are not held together properly. Based on a study with mice.