3.3 / 10.1 Meiosis

Question 1

Non-Disjunction leading to downsyndrome

Answer 1

Non-Disjunction:
- Failure of homologous chromosomes to seperate at anaphase

If this occurs on the 21st chromosome, it leads to a Downie (3 on 21st set)

Question 2

Meiosis Vs Mitosis

Answer 2

DNA is replicated before meiosis so so that all chromosomes consist of two sister chromatids.

Question 3

Methods to gain cells for a Karytype Graph

Answer 3

Chronic Villus Sampling:
- Uses an ultra sound to take cells from the placenta as they are the same as the babys

Amniocentesis:
- Uses Ultra sound to take a sample of the anmiotic fluid

Both have very low chance, but can lead to miscarriage 1-2%
- If needle goes into head of baby, infection, etc.

Question 4

Process of Crossing-over in prophase 1

Answer 4

Chiasma = where the crossover occurs
Tetrad = All 4 of the sister chromatids during crossing-over event

Question 5

Meiosis 1

Answer 5

PMAT

Prophase 1:
- chromosomes dense
- Nuclear membrane dissolves
- Homologous pairs swap DNA during cross-over

Metaphase 1:
- Centrioles are at the poles and grow spindle fibres attaching to the homo. pairs at the equator

Anaphase 1:
- Spindle fibres contract, seperating the homo. pairs

Telophase/ cytokinesis:
- Nuclear membranes DO NOT form
- chromosomes are invisible
- Cell pinches off forming 2 haploid cells

Question 6

Meiosis 2

Answer 6

Prophase 2:
- Nuclear membrane still dissolved
- There are 2 haploid cells with visible chromosomes

Metaphase 2:
- Chromosomes line up on the equator with centrioles at the poles, spindle fibres grow and attach to the sister chromatids at the centromere

Anaphase 2:
- Spindle fibres contract pulling the sister chromatids apart

Telophase/ Cytokinesis:
- Nuclear membrane reforms
- Chromosomes loosen
- Each cell pinches into 2 haploid cells, leaving 4 genetically unique