Mitosis and Meiosis 03.10.22

Question 1

Where is the DNA found inside the cell?

Answer 1

Nucleus and mitochondria

Question 2

What is the structure of DNA?

Answer 2

• Long double helix
• Complementary base pairs (A=T, G=C)
• Coils around nucleosomes (protein)
• Coils again into supercoils
• and again into chromosomes

Question 3

How many chromosomes in the human genome?

Answer 3

46 chromosomes (22 pairs + sex chromosomes XY)

Question 4

How many base pairs in each human chromosome?

Answer 4

10^7 bp and contains several hundred genes

Question 5

How is a chromosome structured?

Answer 5

Long arm (q) and short arm (p) separated by the centromere. They can be stained for identification, Giemsa and Quinacrine (light and dark staining)

Question 6

What is a telomere?

Answer 6

Section at the end of chromosome that stops the chromosome unraveling. It gets shorter with every cell division until they can’t divide (normal). When someone has cancer they stay there so the cell can continue to divide. Telomerase helps telomere rebuild, this enzyme is active in cancer cells.

Question 7

What is mitosis for?

Answer 7

• producing two daughter cells which are genetically identical to the parent cell
• growth
• replace dead cells (e.g. cells in gut and red blood cells every 120 days)

Question 8

What are the phases of the cell cycle?

Answer 8

• G0 = mitotically inactive
• G1 = first growth phase
• S phase = synthesis phase (DNA replicates)
  Taking from 46 to 92 chromosomes (2n to 4n)
• G2 = interphase
• Then mitosis

Question 9

What happens in prophase?

Answer 9

• Chromatin condenses into chromosomes
• Centrosomes nucleate microtublues and then are pushed to opposite poles of nucleus as it produces tubulin leaving web of cytoskeletal filaments in between

Question 10

What happens in prometaphase?

Answer 10

• Nuclear membrane breaks down so no nucleus
• Chromatids (same as chromosomes) attach to microtubules, two sister chromatids paired together and held by centromere

Question 11

What happens in metaphase?

Answer 11

Chromosomes line up along equatorial plane

Question 12

What happens in anaphase?

Answer 12

Microtubules shorten which pulls sister chromatids separate to opposite poles of the cell for all 23 pairs

Question 13

What happens in telophase?

Answer 13

• Nuclear membrane reforms around each set and the chromosomes unfold into chromatin
• Then cytokinesis begins, which is not a part of mitosis

Question 14

What is the clinical relevance?

Answer 14

• We are able to detect chromosomal abnormalities
  (e.g. 47, XY, +21 which is downs syndrome, as extra copy of chromosome 21)
• Can categorise tumours and benign or malignant (high number of mitosis makes it more likely it is malignant)
• Can help us grade malignancies. Higher the mitosis, the more aggressive the tumour.

Question 15

How does mitosis treat cancer?

Answer 15

Lots of cancer drugs which target stages of mitosis to stop it or slow it down e.g.
- Taxol: targets the mitotic spindle
- Ispinesib: Targets the centrosomes
- Colchicine- like drugs: attacks anaphase, leads to ring mitosis so can also tell the doctor that the patient is taking this drug

Question 16

What is meiosis?

Answer 16

• Only in gametes
• Recombination of genetic material generates diversity
• Two cell divisions
• 4 haploid daughter cells
• Not a cycle

Question 17

What happens in cross over in Prophase I?

Answer 17

Child can inherit any combination of genes e.g both recessive, both dominant, one of each. Then the genes sort independently. Non sister chromatids stick together and we can have genetic crossover and this is a random process.

Question 18

What happens in sperm production?

Answer 18

• Primordial germ cells undergoes lots of mitoses to produce spermatogonia (gamete).
• Meiotic division commence at puberty.
• Cytoplasm then divides evenly so 4 gametes all the same size
• Millions of mature sperm continuously produced which takes 60-65 days

Question 19

What happens in ovum production?

Answer 19

• One primordial germ cell, undergoes 30 mitoses to produce an oogonia (gamete)
• Oogonia enters prophase of meiosis I by 8th month of intrauterine life
• Process is then suspended before meiosis II
• Cytoplasm divides unequally (1 egg and 3 polar bodies that apoptose/die. Cytoplasm goes to one cell mainly at each division
• Meiosis I completed at ovulation and meiosis II completed if fertilisation occurs.

Question 20

What is non-disjunction?

Answer 20

Failure of chromosome pairs to separate in Meiosis I or sister chromatids to separate properly in meiosis II
- can lead to Trisomy 21 (Down Syndrome)

Question 21

What is gonadal mosaicism?

Answer 21

• Occurs when precursor germline cells to ova or spermatozoa are a mixture of two or more genetically different cell lines, which happens due to random mutation.
  E.g. Two populations of spermatozoa. Some sperm will have mutation and some don’t. So the parent is healthy but the child may not be
• Incidence increases with advancing paternal age
• Can be observed with any inheritance pattern, most common autosomal dominant and X-linked
• Observed in a number of conditions e.g. osteogenesis imperfecta (brittle bone, blue sclera) and Duchenne muscular dystrophy