mitosis and meiosis

Question 1

chromosomes

Answer 1

46 (22 pairs + sex chromosomes), contains continuous DNA duplex of around 107 bp and several hundred genes, has long arm (q) and a short arm (p) separated by centromere, can be characteristically stained for identification and analyses with dyes (Giemsa:G banding, Quinacrine:Q banding), G banding gives around 400-500 bands/haploid human chromosome set, each band is around 6-8 MBp

Question 2

cells not in the cell cycles (in G0 unless damaged)

Answer 2

liver cells - stable, heart muscle and neurons- permanent cells)

Question 3

steps of mitosis

Answer 3

Prophase- chromatin condenses into chromosomes
Centrosomes nucleate microtubles and move to opposite poles of nucleus
Prometaphase – nuclear membrane breaks down, microtubles (made of tubulin) invade nuclear space, chromatids attach to microtubles
Metaphase – chromosomes line up along equatorial plane (metaphase plate)
Anaphase – sister chromatids separate and are pushed to opposite poles of the cell
Telophase – nuclear membranes reform, chromosomes unfold into chromatin, cytokinesis begins

Question 4

clinical relevance

Answer 4

• detecting chromosomal abnormalities (e.g. down syndrome)
• categorising tumours as benign or malignant
• grading malignant tumours
• drugs have been developed to affect mitosis (Taxol, Vinca alkaloids used to damage mitotic spindle, Colchicine and like drugs affect anaphase, Ispinesib affects spindle poles)

Question 5

meiosis - differences to mitosis

Answer 5

Differences to mitosis – only in gametes, recombination of genetic material generates diversity, two cell divisions, 4 haploid daughter cells (1n), it is not a cycle
cross over in prophase 1, genes sort independently

Question 6

sperm production

Answer 6

• Primordial germ cells → lots of mitoses → spermatogonia
• Meiotic divisions commence at puberty
• The cytoplasm divides evenly
• After meiosis II four equal gametes
• Millions of mature sperm continuously produced
• Process takes 60-65 days
• 100-200 million sperm/ejaculate

Question 7

egg production

Answer 7

• Primordial germ cells → 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

Question 8

non-disjunction

Answer 8

failure of chromosome pairs to separate in meiosis I or sister chromatids to separate properly in meiosis II

Question 9

down syndrome

Answer 9

most cases result from regular trisomy 21 (75% maternal meiosis I, 25% maternal meiosis II, 3-5% paternal non disjunction)

Question 10

Gonadal mosaicism

Answer 10

• occurs when precursor germline cells to ova or spermatozoa are a mixture of two or more genetically different cell line
• One cell line is normal, other is mutated
• Incidence increases with advancing paternal age
• Parent is healthy, but the fetus may have a genetic disease
Can be observed with any inheritance pattern, but most commonly autosomal dominant and X-linked
Observed in a number of conditions, including osteogenesis imperfecta and Duchenne muscular dystrophy