Numerical chromosomal abnormalities

Question 1

What is mitosis ?

Answer 1

The cell division process which produces somatic cells

Question 2

What is meiosis?

Answer 2

The cell division process which produces gametes

Question 3

What are the 3 stages of the cell cycle ?

Answer 3

• Interphase (G1,S,G2) - Normal cell growth, metabolic activity, DNA replication (to give 2 identical copies of each chromosome)
• Mitosis - Replicated DNA (in nucleus) separates (all body cells except sperm + egg). forms 2 identical daughter cells. each daughter cell has 46 chromosomes (like parent cell)
• Cytokinesis - Cell separates into two cells

Question 4

What are the stages of Mitosis ?

Answer 4

Prophase:
Chromosomes condense + become visible.
Chromosome = 2 identical parts (sister chromatids) joined at centromere.
Spindle fibres start to form at opposite poles of cell.

Metaphase:
Mitotic Spindle pulls chromosomes to line up at cell equator
Anaphase:
Spindle fibres contract, pulling apart the sister chromatids to opposite poles of cell.

Telophase:
A nuclear membrane forms around each set of chromosomes. The chromosomes decondense + become less visible.

Cytokinesis:
Final stage of cell division
The cytoplasm divides, forming 2 genetically identical daughter cells.
Each daughter cell contains 46 chromosomes

Question 5

Describe Meiosis

Answer 5

Meiosis = 2-step process which produces gametes.

Meiosis 1 and Meiosis 2
The reduction from 46 chromosomes (diploid) to 23 chromosomes (haploid number) occurs in Meiosis 1.

The chromosomes will first replicate, then recombination takes place.

• Interphase - chromosomes replicate.
• then Meiosis - homologous chromosome pair align along the spindle + swap genetic material (recombination). then spindle pulls chromosome pairs to opposite poles of cell + forms daughter cells. meiosis II separates the sister chromatids.

-meiosis forms 4 haploid daughter cells (23 chromosomes each, genetically non-identical) from 1 single diploid progenitor

Question 6

Which errors occur in segregation?

Answer 6

Segregation:

• Separation of sister chromatids takes place in Mitosis and Meiosis II
• Separation of homologous chromosomes takes place in Meiosis 1

The process is called segregation. When segregation goes wrong e.g. both chromosomes/chromatids go to one pole (and none to the other) = non-disjunction.

In Meiosis nondisjunction, the resulting gametes are chromosomally unbalanced which can lead to errors of ploidy when gametes fertilise = foetus has too many/too few chromosomes

Triploidy = 69 chromosomes in zygote

Question 7

What are aneuploidies and give examples

Answer 7

Aneuploidy = Abnormal number of chromosomes in a cell - 1 extra/missing chromosome

-Turner’s syndrome - XO (Missing X chromosome)
-Patau’s syndrome - Extra chromosome 13 = Trisomy 13
-Edwards syndrome = Trisomy 18
-Down’s syndrome = Trisomy 21
(congenital heart problems, digestive tract disorders, vision/hearing disorders, leukaemia)

Question 8

Describe a typical karyotype

Answer 8

-Humans have 23 pairs of chromosomes
(22 pairs autosomes, 1 pair sex chromosomes XX/XY)

-Metacentric
p and q arms even length
1-3, 16-18

-Submetacentric
p arm shorter than q
4-12 ,19-20 X

-Acrocentric
Long q ,small p
p contains no unique DNA
13-15,21-22, Y

Question 9

What are some numerical abnormalities of chromosomes?

Answer 9

Haploid - Cell contains 1 set of chromosomes (n=23) like in a normal gamete

Diploid - Cell contains 2 sets of chromosomes (2n=46, normal in human somatic cell)

Polyploid - A Multiple of the haploid number (4n=92)

Aneuploid - Chromosome number which is not exact multiple of haploid number - due to extra/missing chromosomes (2n+1=47) -trisomy, monosomy

Question 10

How does Aneuploidy arise?

Answer 10

Aneuploidy arises through Meiotic non-disjunction which is where an error occurs in the chromosome segregation.

This can occur in the segregation of homologous chromosomes - Meiosis 1 Metaphase

Non-disjunction in Meiosis 2 is when homologous chromosomes initially separate in meiosis I, but then both sister chromatids go into 1 single cell.

Question 11

Describe the zygotes in meiotic non-disjunction (Meiosis 1)

Answer 11

Zygote = 2 Trisomy + 2 Monosomy

Question 12

Describe the zygotes in meiotic non-disjunction

Meiosis 2

Answer 12

Trisomy
Monosomy
Normal -Diploid
Normal -Diploid

Question 13

What is Recombination ?

Answer 13

Recombination = Process by which homologous chromosomes align in Meiosis I Metaphase and exchange genetic material. Generates genetic variation.

Question 14

Describe Recombination

Answer 14

Recombination - Meiosis I Metaphase:

Homologous chromosomes align, form a bivalent structure and exchange genetic material (recombine)

Question 15

What is a bivalent structure?

Answer 15

Bivalent structure = Synaptonemal complex binds 2 replicated homologous chromosomes together, allowing them to physically exchange genetic material.

This forms a Chiasma (point at which paired chromosomes remain in contact during the Meiosis I Metaphase)

Product = recombinant chromosomes

Question 16

Describe Meiosis 1

homologous chromosomes

Answer 16

• Homologous chromosomes align as 23 bivalents
• Allows for chiasma formation
• Pulls apart homologues from one another
• Daughter cells have 23 chromosomes

Question 17

Describe Meiosis II

Answer 17

• Align as independent chromosomes
• Sister chromatids are pulled apart
• Daughter cells have 23 chromosomes (1 sister chromatid each)
• Daughter cells are genetically unique
• No Recombination

Question 18

What is independent assortment ?

Answer 18

Independent assortment = Maternal and Paternal chromosomes are sorted independently from one another during Meiosis 1.
= Further genetic variation

Question 19

What are the autosomal aneuploidies?

Answer 19

Patau’s - Trisomy 13
Edward’s - Trisomy 18
Down’s - Trisomy 21

Question 20

What are the sex chromosome aneuploidies?

Answer 20

Turners (45,XO)
Triple X Syndrome (47,XXX)
Klinefelter’s (47,XXY)

Question 21

What is Mosaicism ?

Answer 21

The presence of 2 or more genetically different cell lines derived from a single zygote.

Mitosis (post-zygotic) nondisjunction – affects certain cells of individual = mosaicism = individual has a mixture of cell types

slide 30

Question 22

How does Mosaicism occour ?

Answer 22

Mosaicism occurs when non-disjunction takes place in mitosis.
The cell does not split evenly.
1 of the daughter cells becomes trisomic and the other monsomic.
The monosomic cell will die.

Question 23

Describe the difference b/w Full Monosomy and Partial Monosomy/ Trisomy

Answer 23

Full monosomy - Arises by Nondisjunction

Partial monosomy/trisomy - Microdeletion/Duplication syndromes - different mechanism - individual has deleted/additional parts of a specific chromosome - more common.

Question 24

Example of an aneuploidy

Answer 24

Down’s syndrome (trisomy of chromosome 21)

Question 25

numerical chromosomal abnormalities

Answer 25

numerical chromosomal abnormalities = having abnormal number of chromosomes

Question 26

structural chromosomal abnormalities

Answer 26

part of chromosome is deleted/duplicated/translocated

Question 27

mitosis =

meiosis =

Answer 27

mitosis = cell division that produces somatic cells
meiosis = cell division that produces gamete cells (sperm, egg)

Question 28

Turner’s Syndrome Symptoms

Answer 28

Fail to mestruate
Short stature
Puffy feet
Webbed neck

Question 29

Most trisomy occurs due to ……….

Answer 29

Meiosis I Nondisjunction in the mother
-this stage is v long in women - starts before birth + ends at Ovulation
= aneuploidy risk increases w maternal age
= miscarriage risk increases in older women

Question 30

nondisjunction is an error of ………. and can lead to ………………………………..

Answer 30

segregation

numerical chromosomal abnormalities