Meiosis

Question 1

Describe, generally, what happens during meiosis I

Answer 1

• Centrioles + chromosomes replicated (like mitosis)
• Homologous chromosomes pair up
• Homologous recombination occurs
• 2 Chromatids pulled to separate poles (Anaphase I)
• Cytoplasm splits

Question 2

Describe, generally, what happens during meiosis II

Answer 2

• Sister chromatids separate in Anaphase II
• Produces haploid daughter cells

Question 3

What is the main difference between meiosis and mitosis?

Answer 3

Meiosis largely resembles mitosis except there are extra steps that segregate homologous chromosomes

Question 4

What happens during meiotic prophase I?

Answer 4

Homologous chromosomes pair up
- facilitated by synaptonemal complex & DNA base pairing between homologues

Question 5

What facilitates the pairing up of homologous chromosomes?

Answer 5

the synaptonemal complex (proteins)

DNA base pairing between homologues

Question 6

What is the purpose of homologous chromosomes pairing in meiotic prophase I?

Answer 6

2 purposes:
1. Aligns chromosomes up ready for anaphase (along with the formation of synaptonemal complex)
2. Allows for genetic recombination between paternal and maternal DNA = genetic variation

Question 7

What is a chiasma?

Answer 7

Point of contact between homologous chromosomes in crossing over = chromosomes swap regions of genetic info

Question 8

Describe how the homologous chromosomes are actually brought together.

Answer 8

• Paired homologues brought to 100nm apart
• Thought that a recombination complex (which detects double stranded breaks) helps bind the homologues together
• The axial core (proteins that bind chromatin via cohesion) are cross-linked by transverse filaments to from the synaptonemal complex

Question 9

What is the fundamental difference between mitosis and meiosis?

Answer 9

Meiosis: homologous chromosomes separate = haploid daughter cells

Mitosis: sister chromatids separate = diploid daughter cells

Question 10

Describe how is crossing-over is regulated

Answer 10

There’s at least one crossing over site per bivalent (arm) but no more that 4
But, if these sites from at DS breaks - why aren’t there more?
- DS breaks tend to form in open chromatin (not all chromatin)
- Coupled with, Crossover interference - once one forms it inhibits others close by
= No. of crossing over sites limited

Question 11

What are the 2 general categories for meiosis going wrong?

Answer 11

Abnormalities in chromosome number (nondisjunction)

Chromosome structural rearrangements (caused by homologous recombination)

Question 12

Why can even small structural arrangements on chromosomes have severe effects?

Answer 12

Homologous recombination can run risk of DNA being swapped over incorrectly
= due to huge no. of genes even small rearrangements can have severe effects

Question 13

What is aneuploidy?

Answer 13

the condition of having an abnormal number of chromosomes in a haploid set

Question 14

What are the 2 types of aneuploidy?

Answer 14

Monosomy → 1 copy of a chromosome
Trisomy → 3 copies of chromosome

Question 15

What is Spectral karyotyping (SKY) and why is it useful?

Answer 15

→ uses DNA probes to show artificial colours on chromosomes during metaphase
Useful bc it is much easier to identify rearrangements due to bright colours

Question 16

What causes aneuploidy?

Answer 16

Homologous chromosomes / sister chromatids fail to separate in: Meiosis I, Meiosis II or mitosis

Question 17

Describe the effects of nondisjunction in meiosis I

Answer 17

serious effect: all gametes will either be trisomic / monosomic

Question 18

Describe the effects of nondisjunction in meiosis II

Answer 18

less serious than meiosis I: ½ gametes ‘normal’, ¼ trisomic, ¼ monosomic

Question 19

What is nullisomy?

Answer 19

missing pair of chromosomes

Question 20

What are the consequences of aneuploidy in sex chromosomes?

Answer 20

Additional sex chromosome = minor probs but normal lifespan

Lacking a sex chromosome = 45,X can be highly lethal, some normal but infertile. 45, Y not viable

Question 21

Give an example of syndromes involving autosomal trisomy

Answer 21

Trisomy 22:
Microcephaly
Congenital heart disease
Malformed ears
Rare in live born, usually found in miscarriages

Question 22

Give an example of a syndrome involving monosomy

Answer 22

45, XO Turner’s syndrome:
Complete or partial absence of 2nd sex chromosome in phenotypic females
Common symptoms: Poor growth, short, delayed/ absent puberty, congenital heart defects
Only 1% survive to term with 50% mosaic (nondisjunction in only some of the cells)

Question 23

Why do we see lethality from lacking an X chromosome e.g Turner’s syndrome?

Answer 23

Thought that X chromosome inactivation in ‘females’ would mean these conditions would have no effect, but researchers found:

Haploinsufficiency → pseudoautosomal genes (on X chromosome) expressed from both alleles, so if one X is lost the dose matters

Question 24

Where does meiosis occur?

Answer 24

Only in testes / ovaries

Question 25

What are the products of meiosis?

Answer 25

Haploid gametes

Question 26

How many cell divisions occur in meiosis?

Answer 26

2

Question 27

What is the relationship like between daughter cells like in meiosis?

Answer 27

Genetically different due to:
- Recombination
- indepndent assortment of homologues