Meiosis (3.3, 10.1)

Question 1

Aneuploidy

Answer 1

An abnormal number of chromosomes in a cell.

Question 2

Types of aneuploidy

Answer 2

• Autosomal aneuploidy (eg. trisomies)
• Sex chromosome aneuploidy
• Polyploidy

Question 3

Give two examples of autosomal aneuploidy

Answer 3

Downs syndrome (Trisomy 21)
Edwards syndrome (Trisomy 18)

Question 4

Give two examples of sex chromosome aneuploidy

Answer 4

Klinefelter syndrome (extra X chromosome in men) eg. XXY
Turner syndrome (missing X chromosome in women) eg. XO

Question 5

Non-disjunction

Answer 5

Refers to the chromosomes failing to separate correctly in meiosis, resulting in gametes with one extra, or one missing chromosome.

Question 6

Trisomy

Answer 6

A genetic disorder where an individual has an additional copy of a chromosome (i.e. three instead of two).

Question 7

Monosomy

Answer 7

A zygote with only one homologue.

Question 8

What can influence non-disjunction?

Answer 8

The age of the parents. There is a particularly strong correlation between maternal age and non-disjunction.

Question 9

What is Meiosis?

Answer 9

Cell division for the production of gametes (sex cells). It occurs in the ovaries and testes and involves the division of a diploid cell into four genetically distinct haploid nuclei.

Question 10

Stage that precedes Meiosis

Answer 10

Interphase - Here the DNA is replicated to produce chromosomes consisting of two sister chromatids.

Question 11

Meiosis 1

Answer 11

The division of homologous chromosomes into two intermediate cells (i.e. diploid -> haploid).

This stage is where mutations may occur that result in an abnormal number of chromosomes eg. Downs Syndrome.

Question 12

First stage of Meiosis 1

Answer 12

Prophase 1 (P1) - The chromosomes condense and the nuclear membrane dissolves. The homologues chromosomes pair up in synapsis to form bivalents. Crossing over can occur.

Question 13

Second stage of Meiosis 1

Answer 13

Metaphase 1 (M1) - The bivalents line up at the equator randomly. This results in independent assortment.

Question 14

Third stage of Meiosis 1

Answer 14

Anaphase 1 (A1) - The bivalents separate and the homologous chromosomes move to opposite poles of the cell.

Question 15

Last stage of Meiosis 1

Answer 15

Telophase 1 (T1) - Chromosomes decondense. The nuclear membrane may reform. Cell divides in cytokinesis to form two haploid (n) daughter cells.

Question 16

Meiosis 2

Answer 16

The separation of the sister chromatids with the number of chromosomes remaining the same.

Question 17

First stage of Meiosis 2

Answer 17

Prophase 2 (P2) - Chromosomes condense. Centrosomes move to opposite poles. Nuclear membrane dissolves.

Question 18

Second stage of Meiosis 2

Answer 18

Metaphase 2 (M2) - Spindle fibres from centrosomes attach to the chromosomes at the centromere and align them in the centre of the cell.

Question 19

Third stage of Meiosis 2

Answer 19

Anaphase 2 (A2) - Spindle fibres separate the sister chromatids which move to opposite poles.

Question 20

Last stage of Meiosis 2

Answer 20

Telophase 2 (T2) - Chromosomes decondense. Nuclear membrane reforms. Cells divide in cytokinesis to form four haploid daughter cells.

Question 21

Meiosis creates genetic variation by;

Answer 21

• Allele segregation (Mendel’s law of segregation) (Anaphase 1)
• Independent assortment (Metaphase 1)
• Crossing over (Prophase 1)

Question 22

Explain how allele segregation in meiosis creates genetic variation

Answer 22

An individual that is a diploid has a pair of alleles for a particular trait. Each allele is carried on separate homologous chromosomes. In Meiosis 1, these are separated. So pairs of alleles are sorted into different gametes which creates variation.

Question 23

Explain how independent assortment in meiosis creates genetic variation

Answer 23

Independent assortment occurs in Meiosis 1 (Metaphase). The bivalents are oriented randomly in the centre of the cell, so the chromosomes are sorted independently of each other.

Question 24

Explain how crossing over in meiosis creates genetic variation

Answer 24

Crossing over occurs in Meiosis 1 (Prophase). Homologous pairs of sister chromatids undergo synapsis, where they line up side by side. During synapsis, the chromatids can tangle (chiasmata formation) and exchange sections of DNA. So all of the chromatids will be genetically different from each other. This process is called crossing over.