Chromosomal abnormalities

Question 1

Homologous chromosomes

What can be different between homologous chromosomes?

Answer 1

Chromosomes that have the same sequence of genes, that have the same structure, and that pair during meiosis

While they contain the same genes on them, the allelic forms may vary

Question 2

Cell cycle phases

Answer 2

Interphase (G1, S, G2), mitosis, cytokinesis

Question 3

G1 phase

Answer 3

Cell growth

-cells increase in size and synthesise new proteins and organelles needed for DNA replication

Question 4

S phase

Answer 4

Synthesis

-chromosomes are replicated to that each chromosome now consists of two sister, identical chromatids

Question 5

G2 phase

Answer 5

Cell Growth

• synthesis of protein, especially microtubules
• cell also double-checks the duplicated chromosome for any errors, making any needed repairs

Question 6

Karyotype

Answer 6

A display of the chromosome pairs of a cell arranged by size and shape.

Question 7

Types of chromosomes

Answer 7

Metacentric
Submetacentric
Acrocentric

Question 8

Metacentric chromosome

Answer 8

• centromere is in the middle
• p and q arms even length
• chromosomes 1-3, 16-18

Question 9

Submetacentric chromosome

Answer 9

• centromere is displaced toward one end
• p arm shorter than q arm
• chromosomes 4-12, 19-20, X

Question 10

Acrocentric chromosome

Answer 10

• centromere located very close to one end
• small p arm, long q arm
• p arms contains no unique DNA
• chromosomes 13-15, 21-22, Y

Question 11

Chromosomal changes can either be…

Answer 11

Numerical
OR
Structural

Question 12

Haploid

Diploid

Answer 12

a cell that has one set of chromosomes (n=23)
-gametes

a cell that contains two sets of chromosomes (2n=46)
-autosomes

Question 13

Polyploidy (numerical abnormality)

Aneuploidy (numerical abnormality)

Answer 13

A chromosomal alteration in which the organism possesses more than two complete chromosome sets
-multiple of the haploid number (e.g. 4n=96)

A chromosome number that is not an exact multiple of the haploid set
-due to extra or missing chromosomes (e.g. 2n+1=47)

Question 14

Effect of numerical abnormalities on zygote

Answer 14

• Trisomy
• Monosomy
• Mosaicism

Question 15

Describe bivalents and recombination

Answer 15

• Homologous chromosomes align and form om a bivalent structure
• They are stuck together by a synaptonemal complex
• This enables to exchange of genetic material at the chiasma (crossing over/recombination)
• forms recombinant chromosomes (new allele combinations)

Question 16

Meiosis Summary

Answer 16

Two Meiotic Divisions

Meiosis I:

• homologous chromosomes ALIGN as bivalents
• allows for chiasma formation
• pulls apart homologues from one another (anaphase I)
• daughter cells have 23 chromosmes

Meiosis II:

• align as independent chromosomes
• sister chromatides pulled apart (anaphase II)
• 4 haploid daughter cells produced genetically different to parent cells due to genetic variation via allelic recombination

Question 17

How does mitosis differ from meiosis?

Answer 17

• only a single round of cell division
• NO ALIGNMENT from homologous chromosomes, they just line up at the equator and pulled apart at anaphase (disjunction)
• diploid daughter cells produced which are genetically identical to parent cells

Question 18

How does aneuploidy occur?

Autosomal aneuploidies

Answer 18

Non-disjunction in meiosis (I and II) and mitosis

• Trisomy 14 (Patau’s)
• Trisomy 18 (Edward’s)
• Trisomy 21 (Downs)

Question 19

Sex Chromosome aneuploidies examples

Answer 19

Turners (45,X)
Triple X syndrome (47, XXX)
Kinefelter’s (47, XXY)
47, XYY

Question 20

Meiotic non-disjunction

Answer 20

Non-disjunction occurring where there is impaired separation of either the homologous chromosomes in meiosis I or the chromatids in meiosis II

Question 21

What does non-disjunction in meiosis cause after fertilisation with a normal gamete?

Answer 21

Whenever non-disjunction occurs, there is a mixture of trisomy and monosomy zygotes.

Question 22

What is mosaicism?

Answer 22

the presence of two or more genetically different cell lines derived from a single zygote (should be identical)

Question 23

What causes mosaicism?

Answer 23

Mosaicism is caused by 2 mechanisms:

· Post-zygotic (mitotic) nondisjunction= all 2n to mixture of 2n and 2n+1

· Anaphase lag, i.e. trisomic rescue= all 2n+1 to mixture of 2n+1 and 2n

Question 24

Post-zygotic (mitotic) non-disjunction

Answer 24

Failure of sister chromatids to separate during anaphase of mitosis
-leads to mosaicism

Question 25

Anaphase lag

Answer 25

one homologous chromosome in meiosis or one chromatid in mitosis fails to be included in the reforming nucleus

Instead, the lagging chromosome forms a micronucleus and is lost from the cell, resulting in one normal daughter cell and one with monosomy.

Question 26

Trisomy rescue

Answer 26

Anaphase lag can also cause a rescue of the daughter cell if the cell was originally trisomy.

Question 27

Clinical relevance of mosaicism

Answer 27

Mosaic phenotype is thought to be less severe

Difficult to assess

Question 28

How does Turner’s syndrome occur?

Answer 28

Non-disjunction of the X chromosomes in meiosis I or II

Therefore at fertilisation, when either of the nullisomic daughter cells fertilise with a normal gamete (+X), results in Turner’s syndrome (XO).