Mitosis and Meiosis

Question 1

Describe DNA and its structure

Answer 1

DNA is a double helix that coils into nucleosides and further into super coils and finally into chromosomes.
It is made up of nucleotides (bases, phosphate group and deoxyribose)
Each chromosome contains 10x7 base pairs
Each human has 46 chromosomes - 22 pairs and 2 sex chromosomes.

Question 2

How do you stain chromosomes?

Answer 2

Chromosomes can be stained with Giemsa (G banding) 400-500 bands. The bands are labelled middle out, e.g 14P1

Quinacrine - Q bonding

Question 3

P and Q arms of chromosome…

Answer 3

P = short (petite)
Q = long

Question 4

Job of mitosis?

Answer 4

Produces 2 genetically identical diploid cells

Question 5

Describe the cell cycle

Answer 5

G1 - growth 1 - increase organelles
S1 - synthesis phase - DNA replication occurs
G2 - final preparations for mitosis

23 hours and 1 hour of mitosis

Note cells can be in G0 - where they are not actively undergoing mitosis

Question 6

Interphase

Answer 6

Not part of mitosis
Made up of G1 - S1 - G2
Nucleolus visible and nuclear envelope in place
DNA replication takes place at the end of interphase

Question 7

Prometaphase:

Answer 7

Nuclear envelope breaks down
Microtubles invade nuclear space
Chromatids attach to tubules

Question 8

Metaphase:

Answer 8

Chromosomes line up along the equatorial plane

Question 9

Anaphase:

Answer 9

Sister chromatids separate and are pulled to opposite sides of the cell

Question 10

Telophase:

Answer 10

Nuclear membrane reforms
Chromosomes — chromatin
Cytokinesis (not part of mitosis)

Question 11

Meiosis:

Answer 11

Produces 4 genetically different haploid cells

Meiosis 1:
Prophase 1 - crossing over takes places
Telophase 1 — splits into 2 cells then the cycle repeats (meiosis 2), but with no crossing over

Crossing over - equivalent proportions of chromosome are exchanged at points known as chiasmata

Independent segregation - homologous chromosomes randomly move into the first 2 cells

Question 12

Egg production

Answer 12

• Primordial germ cell → 30 mitoses → oogonia
• Oogonia enter prophase of meiosis I by 8th month of intrauterine life
• Process suspended
• Cells enter ovulation 10-50 years later
• Cytoplasm divides unequally – 1 egg and 3 polar bodies (that apoptose)
• Meiosis I is completed at ovulation
• Meiosis II only completed if fertilisation occurs