MEIOSIS

Question 1

Diploid organisms have two versions of each chromosome. What are these called?

Answer 1

Homologues.

Question 2

Homologues are either ……………………… or ………………………..

Answer 2

Maternal

Paternal

Question 3

How is meiosis different from mitosis?

Answer 3

1. Meiosis results in cells with half the genetic information required. Mitosis results in 23n cells.
2. Meiosis undergoes PMAT twice where as this only occurs once in Mitosis.
3. In prophase 1 of meiosis, homologous chromosomes pair with one another. In mitosis the chromosomes continue to condense.
4. In prophase 1 of meiosis, homologous recombination occurs. This does not happen in mitosis.
5. In anaphase 1 of meiosis, homologous chromosomes are pulled to opposite poles whereas in mitosis sister chromatids are separated.

Question 4

What is the purpose of homologous recombination?

Answer 4

Serves two purposes:

1. Aligns chromosomes ready for anaphase (along with formation of synaptonemal complex)
2. It allows for genetic recombination between maternal and paternal DNA on the same chromosome.

Question 5

What facilitates pairing and recombination of homologous chromosomes?

Answer 5

The synaptonemal complex.

Question 6

What makes up the synaptonemal complex?

Answer 6

Chromatin loops bind to cohesin that are attached to the axial cores of the homologues. The axial cores of the homologues are then bound in a zip like fashion by transverse filaments to bring the homologous chromosomes to 400nm apart.

Question 7

What are the stages of formation of the synaptonemal complex?

Answer 7

1. Interphase
2. Leptotene
3. Zygotene
4. Pachytene
5. Diplotene
6. Diakinesis

Question 8

Homologous segregation (Anaphase 1) depends on sever unique features of Meiosis 1, what are they?

Answer 8

In mitosis it is sister chromatids that separate.

In meiosis 1 is is homologues that separate.

Question 9

What allows homologues to separate and not for their sister chromatids to separate?

Answer 9

1. Both kinetochores attach to the same spindle pole in meiosis 1.
2. Due to crossing over, there is a physical link between homologues.
3. In meosis 1, cohesin is only removed from the arms of the chromosomes.

Question 10

How many times can crossing over occur in a single bivalent chromosome?

Answer 10

At least one but no more than four.

Question 11

When crossing over occurs once, what happens close by?

Answer 11

Crossover interference means crossing over nearby is inhibited.

Question 12

When meiosis goes wrong, what are the two categories of chromosome abnormalities?

Answer 12

1. Abnormalities in chromosome number

2. Chromosome structural rearrangements.

Question 13

Define aneuploidy.

Answer 13

Having an abnormal number of chromosomes.

Question 14

What is monosomy?

Answer 14

Having only one copy of a chromosome.

Question 15

What is trisomy?

Answer 15

Having 3 copies of a chromosome.

Question 16

What is polyploidy?

Answer 16

Having extra sets of chromosomes.

Question 17

How does aneuploidy occur?

Answer 17

Disorders of chromosome number are caused by chromosome non-dysjunction.
This means homologous chromosomes or sister chromatids have failed to separate in either Meosis 1 or 2, or Mitosis

Question 18

Does chromosome non-dysjunction cause greater defects in Meiosis 1 or Meiosis 2?

Answer 18

Non-dysjunction occuring in Meiosis 1 is more severe as this will produce no normal gametes.
If this occurs in Meiosis 2, then only half of the gametes will be abnormal.

Question 19

What is the expected quality of life in someone with additional sex chromosomes?

Answer 19

Most will have a normal life span with only minor issues.

Question 20

What is the expected quality of life for someone lacking sex chromosomes?

Answer 20

45Y is not usually viable.

45X produces Turners Syndrome. In 99% if case of 45X, foetuses abort.

Question 21

What is the expected quality of life for a person that is triploid ie 69 chromosomes

Answer 21

Embryonic lethal.

Question 22

What is a nullsomy?

Answer 22

A missing pair of chromosomes, this is pre implantation lethal.

Question 23

Most aneuploidies are lethal especially when it comes to what type of chromosome?

Answer 23

Autosomal chromosomes.

Some autosomal trisomies survive.

Question 24

What autosomal trisomies can survive?

Answer 24

Trisomy 18 - (Edwards Syndrome)

Trisomy 22

Question 25

What are some of the features of Edwards Syndrome?

Answer 25

Severe intellectual disability
Low birth weight
A small, abnormally shaped head
A small jaw and mouth
Clenched fists
Overlapping fingers
Congenital heart defects
Abnormal organans

Question 26

What are the features of Trisomy 22?

Answer 26

Undeveloped mid face
Malformed ears
Wide spaced eyes
Congenital heart disease

Question 27

What are most cases of Trisomy 21 known as?

Answer 27

Down’s Syndrome

Question 28

What are the two potential explanations as to why we see lethality in aneuploidy?

Answer 28

1. Haploinsuffiency
2. Imprinted genes on X
   Usually, in females and X is inactivated. In Turners Syndrome, only one full X is present, if at random this X is inactivated then there is no transciption and therefore death.
   If there is no inactivation of X, the single X is haploinssuficient because pseudoautosomal genes on sex chromosomes still require two copies for suffiecient expression and in this case not enough can be transcribed.
   If a gene is imprinted and there is no other copy then monoallelic expression is lost and the gene is null.

Question 29

Which model for lethality is actually correct?

Answer 29

Haploinsufficiency model.

Question 30

Why is the haploinsuffieciency correct and how can we test for that?

Answer 30

1. out of a group of hESC in culture, some will random losing a sex chromosome
2. in screening both types, which are pseudoautosomal or escape x inactivation and what is expressed in XX that is not expressed in X0
3. 37 genes meet this criteria
4. out of these, only 3 are expressed and escape x inactivation
5. test for imprinting - look for SNPs in the genome that are heterozygous but homozygous in the gene you are looking at - none showed this therefore it is the haploinsuffiency model that is correct.