Meiosis

Question 1

What is the outcome of mitosis ? Meiosis ?

Answer 1

Mitosis: 2 diploid cells
Meiosis: 4 haploid cells (gametes)

Question 2

How many rounds of replication and division take place in mitosis ? In meiosis ?

Answer 2

Mitosis: 1 round of replication 1 round of division
Meiosis: 1 round of replication 2 rounds of division

Question 3

What are the main differences between meiosis and mitosis ?

Answer 3

• 2 diploid daughter cells vs 4 halpoid daughter cells
• 1 round of replication 1 round of division vs 1 round of replication 2 rounds of division
• Duplicated chromosomes pair early on in meiosis, unlike in mitosis
• Recombination in meiosis, not mitosis

Question 4

Describe the main steps of meiosis.

Answer 4

P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents (by process of synapsis, come towards each other), crossing over occurs

M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell

A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

Question 5

How do homologous chromosomes line up in mitosis ? in meiosis ?

Answer 5

Mitosis: Homologous chromosomes lined up at the metaphase plate independently

Meiosis 1: Homologous chromosomes are paired at the metaphase plate

Question 6

What is a bivalent ?

Answer 6

A pair of synapsed (aligned) homologous chromosomes

Question 7

What is a chiasma ? How many chiasma may there be per chromosome ?

Answer 7

“The points at which members of a chromosome pair are in contact during prophase 1 of meiosis and because of which recombination, or crossing over, occurs on separation”

2 or 3 chiasma along chromosome pair

Question 8

What are the main features of recombination ?

Answer 8

• Process that creates new combinations of genes by shuffling the linear order of the DNA.
• If genes close, more likely to be inherited together
• Occurs through Holliday Junction

Question 9

What is a Holliday Junction ?

Answer 9

the cross-strand structure formed when two DNA duplexes cross in a recombination event.

Question 10

May the outcomes of a recombination event be predicted ?

Answer 10

No

Question 11

How does pairing and recombination of sex chromosomes in males occur ?

Answer 11

• X and Y cannot pair perfectly (not the same size) but still pair up
• Pseudo-autosomal regions (on tip of X and on tip of Y) allow X and Y to pair up and undergo recombination.
• Rest of Y does not undergo recombination (or very little)

Question 12

How may the Y chromosome be used to trace paternal ancestry ?

Answer 12

Because most of Y chromosome (except for pseudo-autosomal region) does not undergo much recombination so stable region of genome

Question 13

What are the events in meiosis which help create genetic diversity ?

Answer 13

1. Independent assortment of maternal and paternal homologs during meiosis 1: “Pairs of homologous chromosomes (tetrads) are lined up independently of each other (maternal and paternal chromosomes are also lined up on the metaphase plate at random)”
2. Recombination: in prophase 1

Question 14

What is the pause between meiosis 1 and 2 called ?

Answer 14

Interkinesis

Question 15

What is MPF ?

Answer 15

Maturation Promoting Factor (a CDK-cyclin complex)

Question 16

Describe the levels of MPF throughout meiosis.

Answer 16

During stages of meiosis (both 1 and 2) high activity of MPF, but dips at resting stage between meiosis 1 and meiosis 2.

Question 17

What is the consequence of an inhibition of cyclin degradation on meiosis ?

Answer 17

Inability to exit meiosis

Question 18

Which protein is key is both meiotic divisions ?

How so ?

Answer 18

Cohesins
-In meiosis 1, degradation of cohesin complexes holding the homologous chromosomes together
whilst leaving those holding the sister chromatids together alone (cohesins lost at the arms but not at the center)
-In meiosis 2, degradation of cohesin complexes holding sister chromatids together

Question 19

What are possible meiotic errors ?

Answer 19

Non-disjunction in meiosis 1: one of potential gametes inherits both maternal and paternal copies of a chromosome. When that precursor undergoes meiosis 2, 2 copies of said chromosome in 2 gametes (aneuploid gametes), and 2 gametes lacking a copy of chromosome. May lead to trisomy (3 copies of one chromosome).

Non-disjunction in meiosis 2: sister chromatids both inherited in the same gamete, leading to a aneuploid gamete with 2 IDENTICAL copies of a chromosome (from same parent, unlike non-disjunction in meiosis 1, wherein aneuploid gametes have one paternal and one maternal copy of a chromosome), and a gamete with no copies of that chromosome.

Question 20

What are the consequences of errors in meiosis 1, and 2 ? (following fertilisation of the gametes)

Answer 20

Outcomes of error in meiosis 1 = Uniparental heterodisomy
-2 non-identical chromosomes
-If fertilised, may end up with trisomic zygote
with three different chromosomes
(eg one paternal, two from maternal grandparents)

Outcomes of error in meiosis 2 = Uniparental isodisomy
-2 identical chromosomes
-If fertilised, trisomic zygote with 2 identical chromosomes
but the third will not be different

Question 21

Define uniparental disomy. Give the two types of it.

Answer 21

Both members of a chromosome pair are inherited from one parent, and the other parent’s chromosome
for that pair is missing. Happens because sometimes, extra copy of chromosome can be removed very early in development (i.e. third copy of chromosome lost). Depending which chromosome lost, may end up with individuals inheriting two chromosomes from same parent
or from different parents.
Unidisomy may lead to Angelman Syndrome, Prada-Willi Syndrome, Beckwith-Wiedemann Syndrome.

Uniparental Heterodisomy = Two from mother (one from grandpa one from grandma so different) one from father or vice versa (and the one ‘normal’ chromosome coming from has been removed)

Uniparental Isodisomy = Two from mother (both from grandpa or both from grandma) one from father or vice versa. (and the one ‘normal’ chromosome coming from has been removed so same genetic information in both remaining chromosomes)

Question 22

What is the link between age, errors in meiosis 1, and errors in meiosis 2 ?

Answer 22

Overall, errors in meiosis 1 more frequent than those in meiosis 2.
With increase in age, more of both errors in meiosis 1 and meiosis 2

Question 23

Are there errors associated with male ageing ? Why or why not ?

Answer 23

No, because:

• spermatogenesis is continuous
• spermatogenesis always produces 4 gametes from 1 precursor cell

Question 24

What are main differences leading to errors in oogenesis but not spermatogensis ?

Answer 24

1. Much more STOP START then spermatogenesis
   Plenty of times when meiosis stopped
2. Oogenesis does not always form 4 gametes from 1 precursor cell (oogenesis is an unequal process with significant wastage in generation of polar bodies (that usually don’t go on to divide))

Question 25

Identify the periods of arrest in oogenesis (one of the causes of age-meiotic error correlation in women).

Answer 25

1) BIRTH TO PUBERTY: Long period of arrest in prophase 1 (stage when chromosomes present in bivalent structure, ready to recombine).
2) END OF MEIOSIS 1 TO OVULATION: Completion of meiosis 1 occurs in puberty. However, that individual egg does not proceed to meiosis 2 until ovulation
3) FERTILISATION: Meiosis 2 can only be completed after fertilisation (FERTILISATION)

Question 26

What is the reason that old age results in a higher rate of meiotic errors ?

Answer 26

Chromosome cohesion defects underlie maternal age defect:
Correlation between age and separation between centromeres (adjacent centromeres is essential for proper segregation and eventual success of meiosis)

Question 27

Which trisomies are more frequent ?

Answer 27

1) Trisomy 16 (most likely overall)
2) Trisomy 21 (most likely between age of 13 and 18)
3) Double trisomy (becomes the most likely when the mother is over 40)

Question 28

Does every chromosome have the same probability of being altered during meiosis ?

Answer 28

No, possibly due to nature of proteins encoded by genes on those chromosomes (whether or not compatible with foetal development)

Question 29

What are reasons for meiotic errors ?

Answer 29

1) Old Age (higher separation of centromeres)
2) Characteristics of oogenesis (Plenty of times when meiosis stopped + oogenesis does not always form 4 gametes from 1 precursor cell as it is an unequal process with significant wastage in generation of polar bodies)