Mitosis & Meiosis

Question 1

What is mitosis?

Answer 1

Somatic cell division resulting in 2 identical daughter cells.

Question 2

What are the stages of mitosis?

Answer 2

1. Prophase: chromosomes condense and nuclear membrane disappears.
2. Prometaphase: MT spindle forms and connects to centromeres via kinetochores.
3. Metaphase: 46 (replicated) chromosomes line up at the metaphase plate.
4. Anaphase: centromeres divide, spindle fibres pull sister chromatids towards opposite poles.
5. Telophase: spindle fibres break down, chromosomes decondense, nuclear membrane forms and cleavage furrow starts.

Question 3

Why can the nucleolus not be seen during mitosis?

Answer 3

Chromosome condense in mitosis. In mitosis, the 5 nuceolus chromosomes mediating ribosome production are condensed and so not expressed - rRNA and proteins of the nucleolus disappear.

Question 4

Which stage follows mitosis?

Answer 4

Cytokinesis: cytoplasm divides and parent cell becomes 2 daughter cells.

Question 5

How does the chromosome number change during mitosis?

Answer 5

• It doesn’t: always 46 chromosomes, 1 of each parental copies.
• Chromatid number does double to 92 after replication so that each daughter cell gets the right number.

Question 6

What is meiosis?

Answer 6

Germ line cell division giving rise to 4 non-identical cells with 1/2 the chromosome content of parental cells (diploid (2n) reduced to haploid (n)). Involves:

• 1 round of replication
• 2 rounds of cell division

Question 7

When is meiosis used?

Answer 7

Production of gametes (eggs and sperm)

Question 8

What are the stages of meiosis I?

Answer 8

1. Prophase 1: chromosomes condense, nuclear membrane breaks down, homologous chromosomes pair and cross over (recombination), MT spindle forms.
2. Metaphase 1: recombined homologous pairs (tetrads) line up at metaphase plate.
3. Anaphase 1: separation of homologous chromosomes, movement to opposite poles via spindle contraction.
4. Telophase 1: spindle breaks down, chromosomes decondense, nuclear membrane forms.

Followed by cytokinesis - 2 daughter cells.

Question 9

How do homologous chromosomes recognise each other and pair up?

Answer 9

• For autosomal chromosomes: DNA sequence identity - complementary DNA base-pair interactions at numerous and widely dispersed sites along the chromosomes, in particular at telomeres.
• For sex chromosomes (may not be identical, X & Y): can associate via identical PAR1 and PAR2 regions at telomeres.

Question 10

What is the result of crossing over not in PAR1/2 regions in male meiosis?

Answer 10

• Sex-reversed individuals, esp. if crossing over involves SRY1.

Question 11

What are the stages of meiosis II?

Answer 11

1. Prophase 2: chromosomes condense, nuclear membrane disappears and MTs attach to chromosomes.
2. Metaphase 2: unpaired chromosomes line up at the metaphase plate via the spindle.
3. Anaphase 2: centromeres separate, sister chromatids pull apart.
4. Telophase 2: chromosomes decondense, nuclear envelope re-forms.

Followed by cytokinesis - 4 haploid (23 chromosomes) non-identical daughter cells.

Question 12

How does meiosis generate genetic diversity?

Answer 12

1. Random assortment of chromosomes: during metaphase I, lining up of maternal and paternal members of each pair is random.
2. Crossing over of genetic material (recombination): exchange of chromosome segments between homologues at several chiasmata during metaphase I.