Numerical Chromosomal Abnormalities

Question 1

What are the 3 main stages of cell cycle?

Answer 1

Interphase - cell growth and DNA replication
Mitosis - duplicated genetic information is separated
Cytokinesis - division into 2 cells

Question 2

What are the stages within interphase? and what happens in each

Answer 2

G1 - growth - cellular content not genetic info is duplication
S - synthesis - duplication of dna
G2 - checks duplicated chromosomes and repairs

G0 before g1 - cell cycle arrest

Question 3

How do the sister chromatids and homologues interact?

Answer 3

They don’t. Sister chromatids (one duplicated and one original) attached to eachother until separation in mitosis whereas homologous (original maternal and paternal) do not interact.

Question 4

What is a bivalent and when does it happen?

Answer 4

Bivalent structure is when homologous chromosomes align (after duplication of chromatids) to exchange genetic material in genetic recombination.

Question 5

What happens in genetic recomination? and when does it happen?

Answer 5

Chromosomes crossover forming chiasma and reciprocal breaking and re-joining of homologous chromosomes in meiosis forms one set of recombinant chromosomes and a set of non-recombinant chromosomes.

Question 6

What is the result/leftover after genetic recombination?

Answer 6

New allele combinations. On maternal set of sister chromatids, one chromosome is a recombinant chromosome, one if original and same goes for paternal set of sister chromatids

Question 7

What are the phases in meiosis?

Answer 7

Meiosis 1 and 2

Question 8

What happens in meiosis 1?

Answer 8

Bivalent alignment !!!
Chiasma form for recombination.
Homologues are pulled apart
2 daughter cells with 23 chromosomes in each cell but as sister chromatids

Question 9

What happens in meiosis 2?

Answer 9

Independent chromosomes align!!
Sister chromatids are pulled apart
Daughter cell has 23 chromsomes but now 1 chromatid in each cell

Question 10

How is natural variation introduced?

Answer 10

Indpependent assortment of chromosomes
Recombination
= unique daughter cells

Question 11

What is independent assortment?

Answer 11

23 pairs of chromosome in beginning (1 of each pair from a parent, only 1 of each of the 23 chromosomes goes onto gametes, this random selection of maternal and paternal homologs is independent assortment) - some chromosomes will be maternal and some paternal AFTER MEISOSIS 1 DIVISION

Question 12

Compare the cell cycle in mitosis vs meisosis

Answer 12

Both replicate DNA during interphase

Separation of HOMOLOGS only happens in meiosis 1
Separation of chromatids happen in meiosis 2 and mitosis

Question 13

What is segregation?

Answer 13

Process of chromatid separation (in mitosis and meiosis 2)

Question 14

Give examples of error in segregation

Answer 14

NON DISJUNCTION - Both pairs of sister chromatids go to one pole so daughter cells have too many or too little chromosomes

Question 15

What happens if non disjunction occurs in gametes?

Answer 15

Chromosome imbalance so foetus ends up with too many or too little chromosomes

Question 16

What is aneuploidy?

Answer 16

Having extra chromosomes or missing chromsomes e.g. trisomy

Question 17

How many sets of chromosomes in haploid, diploid, triploid and tetraploid cels?

Answer 17

Haploid - 1 sister chromatid (N)
Diploid - 2 chromosomes (2N)
Triploid - 3 chromosomes (3N)
Tetraploid - 4 chromosomes (4N)

Question 18

What increases the chance of non disjunction caused aneuploidy?

Answer 18

Increased maternal age

Question 19

What are the 3 types of chromosome structures depending on p and q arms viewed on karyotypes?

Answer 19

Metacentric
Submetacentric
Acrocentric

Question 20

What determines whether a chromosome is metacentric, submetacentric or acrocentric

Answer 20

Where the centromere is

Metacentric - p and q arms are even length
Sub metacentric - p arms are shorter than q
Acrocentric - long q arms and small p arms (no unique DNA on p arms)

Question 21

How are numerical chromosomal changed identified?

Answer 21

Karyotyping, FISH, QF-PCR, NGS

Question 22

How are structural chromosomal changed identified?

Answer 22

Karyotyping
FISH
arrayCGH

Question 23

What is haploid and where is it found?

Answer 23

One set of chromosomes - in gametes

Question 24

What is diploid and where is it found?

Answer 24

Cell contains 2 sets of chromosomes - in somatic cells (body cells apart from sperm and egg)

Question 25

What is polyploid?

Answer 25

Multiple of haploid number e.g. triploid(3N) or tetraploid (4N)

Question 26

What is aneuploid different to polyploid?

Answer 26

Chromosome is not exact multiple of haploid number(N) due to extra or missing chromosome
e.g. 2N + 1 examples are trisomy, monosomy

Question 27

What causes aneuploidy?

Answer 27

Meiotic non disjunction - error in segregation of either chromosomes are segregation of chromatids - visualise this

Question 28

WHEN does monosomy occur?

Answer 28

Fertilisation - Empty gamete from non disjunction is fertilised with normal gamete (N) so only one set of chromosomes (N) in zygote = monosomy

Question 29

WHEN does trisomy occur?

Answer 29

After fertilisation. One gamete has 2 chromosomes and fertilisation adds another - 3 pairs in zygote = trisomy

Question 30

Do all cells from one non disjunction end up in either trisomy or monosomy?

Answer 30

No, during division, though 1/2 of the 4 gametes will have too many or too little chromosomes, some gametes end up having one set of chromosomes (N) and when joined by another gametes (N) becomes 2N and is normal

Question 31

Does errors in chromosomes segregation or chromatid segregation have more impact on the 4 games formed?

Answer 31

Chromosome segregation since all gametes end up having either 1 extra or no chromosomes at all since 2 daughter cells after meiosis 1 already cannot become normal after meiosis 2 but in chromatid segregation, mistake isn’t made until meiosis 2

Question 32

What is an autosome?

Answer 32

Chromosomes that are sex chromosomes

Question 33

Give examples of autosomal aneuploidy conditions which chromosome is affected?

Answer 33

Trisomy 21 = Down’s syndrome
Trisomy 13 = Patau’s syndrome
Trisomy 18 = Edward’s syndrome

Question 34

Give examples of sex chromosome aneuploidies conditions? + number of chromosomes in end

Answer 34

Turners in females (one X is missing) - only 45 chromosomes in total
Triple X syndrome in females - 47 chromosomes in total
Klinefelters in males - an extra X or an extra Y (XXY or XYY) - 47 chromosomes in total

Question 35

Where does aneuploidy happen in mitotic non-disjunction?

Answer 35

Post zygotic - only affects a proportion of cells unlike in meiotic non disjunction where all cells have anueploidy

Question 36

Define mosaicism

Answer 36

Presence of 2 or more genetically different cell lines derived from a single zygote e.g. a proportion of cells may be trisomic

Question 37

What are the 2 causes of mosaicism?

Answer 37

• Cell starts of with normal amount of some changes into aneuploidy after mitotic non disjunction
• Cell starts of aneuploidic from meiotic non disjunction then mitotic non disjunction also happens returning a cell line back into normal cells

Question 38

What causes mitotic non disjunction to reverse meiotic non disjunction? + an example of this

Answer 38

Cells recongise wrong number of chromosomes and randomly gets rid of one (random) to become disomic e.g. trisomy rescue. Some cells don’t recognise and continue as usual - mosaicism

Question 39

What is the clinical relevance of mosaicism?

Answer 39

Cause of varying severity e.g in down’s syndrome varies, mosaicism = less severe
- but despite mosaicism being less severe, more difficult to asses not knowing which cells/tissues/organs are affected e.g. which cells are disomic vs trisomic

Question 40

What causes full trisomy/monosomy?

Answer 40

Non disjunction (meiotic and mitotic)