Cell Division

Question 1

Describe difference between mitosis & meiosis

Answer 1

Mitosis = produces new body cells as an organism grows and develops; retains full diploid number of chromosomes
Meiosis = produces gametes with half number (haploid number) of chromosomes

Question 2

Describe meiosis

Answer 2

1) DNA replicates so there are two identical copies of each chromosome
2) DNA condensed to form double armed chromosomes, made from 2 sister chromatids
3) Chromosomes arrange into homologous pairs (of matching chromosomes)
4) First division - homologous pairs are separated, halving chromosome number
5) Second division - Pairs of sister chromatids are separated
6) Four new daughter cells (the gametes) genetically different from each other produced

Question 3

How does chromosome number change during stages of meiosis?

Answer 3

• Initially 46 pairs/ 92 chromosomes (2 copies of each chromosome)
• After first division, 46 chromosomes
• After 2nd division, 23 chromosomes

Question 4

Name the phases of the cell cycle?

Answer 4

• Interphase
• Nuclear Division (mitosis)
• Cell Division (Cytokinesis)

Question 5

How does the cell move between phases in the cell cycle?

Answer 5

Triggered by chemical signals called cyclins

Question 6

Describe the steps involved in interphase

Answer 6

1 G1 (Gap 1)
- cell makes more organelles and proteins for cell division
2 S-phase (Synthesis)
- DNA is replicated; chromosome becomes two sister chromatids joined together by a centromere
3 G2 (Gap 2)
- Cell keeps growing; new DNA that has been synthesised is checked and errors are repaired

Question 7

Describe the first stage in mitosis

Answer 7

Prophase - cell prepares for division
- chromosomes condense
- two centrosomes (replicated in G2) move towards opposites of poles of nucleus
- spindle fibres (protein microtubules)
begin to emerge from centrosomes
- nuclear envelope breaks down into small vesicles

Question 8

Describe the second stage in mitosis

Answer 8

Metaphase - chromosomes align along middle
- centrosomes reach opposite poles
- spindle fibres continue to extend from centrosomes
- chromosomes line up at equator of spindle
- each sister chromatid is attached to a spindle fibre originating from opposite poles

Question 9

Describe the third stage of mitosis

Answer 9

Anaphase - chromosomes pulled apart by microtubules
- sister chromatids separate at centromere
- spindle fibres begin to shorten
- separated sister chromatids (now chromosomes) are pulled to opposite poles by spindle fibres

Question 10

Describe the final stage in mitosis

Answer 10

Telophase - two nuclei reform
- chromosomes arrive at opposite poles + begin to decondense
- nuclear envelopes begin to reform around each set of chromosomes
- spindle fibres break down

Question 11

What is independent assortment?

Answer 11

Describes how in meiosis only one chromosome from each pair ends up in each gamete
- ensures genetic variation; random process

Question 12

Describe crossing over

Answer 12

• In the first meiotic division, homologous pairs come together + alf four chromatids come into contact
• at these points, chromatids break at the chiasma and rejoin, exchanging sections of DNA between non sister chromatids
• results in genetic variation