11: Cell cycle and division

Question 1

Meiosis I - Prophase I

Answer 1

Chromosomes shorten and thicken and become visible under a microscope
Nuclear membrane disintegrates
Homologous chromosomes pair up
Crossing over may occur

Question 2

Crossing over

Answer 2

Occurs during prophase I in meiosis I
- Homologous chromosomes pair up
- Matching regions of sister chromatids wrap around each other to form an X-shaped structure called chiasma
- At the chiasma, sister chromatids break, exchange corresponding parts and rejoin

Question 3

Independent assortment (Metaphase I)

Answer 3

• Homologous chromosomes line up randomly in the middle of the cell
  Each pair of homologous chromosome sorts maternal and paternal chromosomes into each daughter cell independent of other pairs of homologous chromosomes

Question 4

Cytokinesis in animal cells

Answer 4

Cell membrane constricts inwards until the cell spilts into 2
Cleavage furrow is formed

Question 5

Cytokinesis in plant cells

Answer 5

Cell plate consisting of cell membranes and cell wall forms and grows
When it matures, it spilts the cell into 2 equal parts

Question 6

Significance of mitosis

Answer 6

Growth, repair, asexual reproduction

Question 7

Significance of meiosis

Answer 7

• Result in genetic variation so that the chance of survival can increase when environmental conditions change
• Form haploid gametes to restore diploid number of chromosomes after fertilisation

Question 8

Why is it difficult to categorise stages of cell division with certainty?

Answer 8

• No absolute clear-cut criteria to categorise different stages of cell division
• Categorisation of the in-between stages: subjective, based on judgement of the observer

Question 9

How are 2 genetically identical nuclei produced through mitosis?

Answer 9

During interphase, DNA replication occurs
During metaphase, chromosomes line up in the middle of the cell
The identical sister chromatids separate and divide equally into each daughter cell

Question 10

How are 4 haploid nuclei produced through meiosis?

Answer 10

In prophase I, homologous chromosomes pair up
In metaphase up, homologous chromosomes separate and each member moves to opposite poles of the cell to form 2 haploid nuclei
In meiosis II, sister chromatids of each chromosome separate to form 4 haploid nuclei