3.3 Meiosis

Question 1

Meiosis

Answer 1

A reductive cell division process which produces four haploid gametes from one diploid somatic cell. (Meiosis I is reductive: the number of allele varieties per gene is reduced from 2 to 1).

Question 2

What leads to genetic variations in meiosis and sexual reproduction in general?

Answer 2

Random orientation in metaphase I & II
Crossing over in prophase I

In general:
Random mutations
Random sperm fertilizing the egg (random combination of gametes)

Question 3

Prophase I

Answer 3

Nucleus membrane dissolves
Supercoiling (condense) of chromosomes
Replicated homologous chromosomes form bivalents by synapsis
Crossing overs –> reshuffle the alleles between homologous pairs
Centrioles migrate to the poles of the cell

Question 4

Metaphase I

Answer 4

Bivalents line up at the equator
Spindle fibres attach to the centromeres from the centrioles at the poles
Random orientation: the centriole to which each sister chromatid pair faces, and eventually is pulled towards is random and independent for each bivalent

Question 5

Anaphase I

Answer 5

Segregation of alleles
Disjunction of chromosomes
Spindle fibres contract
Sister chromatid pairs for each chromosome are pulled to opposite poles

Question 6

Where crossing over takes place

Answer 6

Chiasmata

Question 7

Define segregation

Answer 7

The separation of alleles into different nuclei.

Question 8

Telophase I

Answer 8

Decondense of chromosomes
Nuclear membrane and nucleolus reform
Cytokinesis takes place: the cytoplasm begins to divide

Question 9

Non-disjuction

Answer 9

When homologous chromosomes/sister chromatids fail to separate during anaphase I or anaphase II, causing the gametes produced to have one more (trisomy)/one fewer (monosomy) chromosome.

Ex) Down syndrome, caused by disjunction in 21st pair

Question 10

Name the two sampling methods for karyotyping

Answer 10

1. Amniocentesis sampling

2. Chorionic villus sampling

Question 11

Amniocentesis

Answer 11

Sampling 20 ml amniotic fluid
16th week
Using a syring, which is directed by ultrasound
The amniotic fluid contains cells from the fetus, which can be used for karyotyping
1% chance of miscarriage

Question 12

Chorionic villus sampling

Answer 12

Sampling villus cells from the chorion membrane of the placenta
Using a needle guided by ultrasound
The cells sampled are then cultured until there is sufficient amount for karyotyping
11th week
2% chance of miscarriage

Question 13

What test is taken before amniocentesis and chorionic villus sampling? And why?

Answer 13

Nuchal translucency scan: which involves examining the amount of nuchal fluid behind the neck of the fetus.
Large Q of nuchal fluid is correlated with Down syndrome
11~13th week

Not 100% accurate, but is non-invasive: mothers with a large amount of nuchal fluid will then take the invasive amniocentesis/chorionic villus samples.

Question 14

Who are mostly likely to be suggested to take a nuchal translucency scan? Why?

Answer 14

Mothers of older age

Because the risk of having a child with trisomy increases with mother’s age.