Lecture 3- Meiosis

Question 1

Describe meiosis I: crossing over and segregation

Answer 1

1. Centrioles and chromosomes are replicated
2. Maternal and paternal homologs pair up
3. Genetic diversity is generated by recombination between homologous chromosomes
4. One complete chromosome (2 chromatids) get pulled to separate poles

Question 2

When does crossing over take place and how does this leave the cells?

Answer 2

Crossing over takes place when homologues pair up. These cells will be haploid with each homologue represented by two sister chromatids.

Question 3

What is the difference between meiosis II and mitosis?

Answer 3

Cells in meiosis II are haploid instead of diploid.

Question 4

What facilitates homologues pairing during meiotic prophase I?

Answer 4

Pairing is facilitated by the synptonemal complex and DNA base pairing between homologues.

Question 5

What is the purpose of the homologues pairing up during meiotic prophase I?

Answer 5

• Aligns the chromosomes up ready for anaphase

* Allows for genetic recombination between paternal and maternal DNA on the same chromosome

Question 6

How is the synaptonemal complex formed?

Answer 6

1. Paired homologs are brought to 400nm apart
2. A recombination complex helps bind the homologs together
3. The axial core are cross linked by transverse filaments cells to form the synaptonemal complex

Question 7

What is the function of the synaptomeal complex?

Answer 7

1. Aligns the two chromosomes

2. Helps in homologous recombination

Question 8

What is the fundamental difference between mitosis and meiosis?

Answer 8

Mitosis: sister chromatids separate

Meiosis: homologs separate

Question 9

What allows the differences between mitosis and meiosis?

Answer 9

1. Both kinetochores (on one chromosomes) attatch to the same spindle pole in meiosis. This is done by a protein complex that is removed after meiosis I
2. Crossing over = physical linkage between homologs in meiosis
3. Cohesion is only removed from the arms in meiosis

Question 10

What is ensured by meiosis regulation?

Answer 10

1. Ensures at least 1 crossover forms

2. Crossover interference- once one forms it inhibits others close by, thus the number per arm are limited

Question 11

What are the 2 categories of chromosome abnormalities?

Answer 11

1. Abnormalities in chromosome number

2. Chromosome structural rearrangement

Question 12

What is aneuploidy?

Answer 12

Aneuploidy is the presence of an abnormal number of chromosomes in a cell

Question 13

What are the 2 types of aneuploidy?

Answer 13

1. Monosomy- 1 copy of a chromosome

2. Trisomy- 3 copies of chromosomes

Question 14

What is polyploidy?

Answer 14

A whole set of extra chromosomes

Question 15

What causes aneuploidy?

Answer 15

Caused by chromosome non-disjunction.

Either homologous chromosomes or sister chromatids fail to separate in meiosis I/II or mitosis. Resulting in an abnormal number of chromosomes

Question 16

What are the consequences of numerical chromosomal abnormalities?

Answer 16

1. Polyploidy (Triploid)- embryonic lethal
2. Aneuploidy (Nullsomy: missing a pair of chromosomes)- pre-implantation lethal

Aneuploidy (Monosomy: missing one chromosome)- embryonic lethal

Aneuploidy (Trisomy: one extra chromosome)- usually lethal

Aneuploidy (additional sex chromosomes)- minor problems

Aneuploidy (lacking sex chromosomes)- Turner’s syndrome

Question 17

List 3 symptoms of autosomal trisomy- Trisomy 22

Answer 17

1. Underdeveloped midface
2. Malformed ears
3. Congenital heart disease
4. Microcephaly

Question 18

List 3 symptoms of autosomal trisomy- Trisomy 18 (Edwards syndrome)

Answer 18

1. Severe intellectual disability
2. Low birth weight
3. Small/abnormal head
4. Congenital heart disease
5. Small jaw and mouth
6. Clenched fists with overlapping fingers

Question 19

List 3 symptoms of monosomy- Turner’s syndrome

Answer 19

1. Poor growth
2. Short
3. Delayed pubertal development
4. Congenital heart defects
5. Skeletal abnormalities

Question 20

Why do we see lethality?

Answer 20

1. Haploinsuficiency- pseudoautosomal genes are expressed from both alleles and dose matters
2. Imprinted genes on X- i.e monoallelic expression which is lost