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ESA1 Callum's cards > MGD S10 - Chromosomal abnormalities > Flashcards

MGD S10 - Chromosomal abnormalities Flashcards

1
Q

What are chromosomes made out of?

A

Chromatin

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2
Q

What is Chromatin made out of?

A

DNA
Histones
Non-histone proteins
RNA

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3
Q

What are the different varieties of Histones called?

A 
H1
H2a
H2b
H3
H4
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4
Q

Describe the structure of the nucleosome core

Hint: What histones? How many?

A

H2a/b
H3
H4

Contains 8 polypeptides/histones molecules - Heterooctomeric protein

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5
Q

How many base pairs are contained within each nucleosome?

A

166bp

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6
Q

What is the function of H1?

A

Stabilises the nucleosome

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7
Q

What is Euchromatin?

A

Lightly packed DNA often under active transcription

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8
Q

What is Heterochromatin?

A

Tightly packed DNA that is not being expressed

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9
Q

What are the levels of packing present in chromosome?

A

DNA packed into nucleosomes

Which are folded into solenoid fibres

Which are in turn folded tightly and then coiled to form the chromosome fibres

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10
Q

How many pairs of chromosomes are present in a human cell?

A

23 pairs

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11
Q

What are the two categories of numerical chromosomal abnormality?

A

Polyploid

Aneuplodiy

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12
Q

What is an numerical chromosomal abnormality?

A

Having a number of chromosomes that isn’t 46

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13
Q

Give examples of types of polyploidy

A

Trisomy

Tetraploidy

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14
Q

What are the two types of Aneuploidy?

A

Monosomy

Trisomy

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15
Q

What is an Aneuploidy?

A

An abnormal number of chromosomes that is not divisible by haploid number (23 in humans)

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16
Q

What is a monosomy?

A

Loss of one chromosome i.e. One chromosome pair exists as a single chromosome

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17
Q

What is a triploid?

A

Trisomy is the gain of one chromosome i.e. One chromosome pair exists as a triplet

18
Q

Describe how Aneuploidy comes about

A

Non-disjunction during meiosis in the parent organism/s

This leads to trisomy or monosomy when gametes fuse

19
Q

Which Autosomal Aneuploidies might result in a viable pregnancy to term what do these Aneuploidies result in?

A

Trisomy 21: Down Syndrome

Trisomy 18: Edwards Syndrome

Trisomy 13: Patau syndrome

No monosomy is viable (Except Turner syndrome)

20
Q

What are the sex chromosome aneuploidies and what conditions result?

A

Turner syndrome: 45,X

Triple X syndrome : 47,XXX

Kleinfelter syndrome: 47 XXY

47,XYY

21
Q

What is X chromosome Inactivation?

A

Only one X chromosome is ever active in a human cell

The others are inactivated and form condensed structures around the periphery of the cell nucleus known as ‘Barr bodies’

22
Q

What is the most common cause of Polyploidy?

A

Fertilisation of an Ovum by more than one Sperm

23
Q

What is the prevalence of triploidy and tetraploidy in humans and what is the fate of any foetus with polyploidy?

A

Triploidy occurs in 2-3% of pregnancies

Tetraploidy occurs in 1-2% of all pregnancies

All pregnancies with polyploidy result in miscarriage or in the case of triploidy, death shortly after delivery

24
Q

How might mosaicism come about?

A

Non-disjunction in mitosis can lead to two separate populations of cells with different karyotypes

Mosaicism can result, especially if this occurs early in development

25
Q

What is structural chromosomal abnormality?

What are the two major categories of Structural chromosomal abnormality?

A

Physical changes to one or more chromosomes

Balanced
Unbalanced

26
Q

What is the difference between balanced and unbalanced structural chromosomal abnormalities

A

Unbalanced changes cause missing or extra genetic information to be deleted/introduced

Balanced abnormalities do not result in missing or extra genetic info

27
Q

What are the types of structural chromosomal abnormality that affect only one chromosome?

Give a breif description of each

A

Deletion - Loss of genetic info

Duplication - Some genetic info is doubled

Inversion - Rearrangement of genetic info without loss or addition

Ring Chromosome - Loss of telomeres or ends of both arms resulting in a ring being formed

Isochromosome - Creation of two non-identical chromosomes, one is a combination of two short arms, one is the combination of two long arms

28
Q

What are the p and q arms of a chromosome?

A

p arm = short arm

q arm = long arm

29
Q

What are the types of structural chromosomal abnormality that can affect two chromosomes?

Give a brief description of each

A

Inversion - No loss of genetic material, genetic material is rearranged to a non-homologous chromosome

Reciprocal translocation - Exchange of genetic material from non-homologous chromosomes, no loss of genetic material occurs

Robertsonian translocation - The q arms of two acrocentric chromosomes combine to form one super-chromosome, the p arms are lost

30
Q

What are the consequences of a translocation?

A

Breakpoints will often be between genes, so often have a normal phenotype

Derivative chromosomes may be passed onto offspring who will probably be unbalanced/ have disease phenotype

The phenotype will vary depending on where the offspring is monosomic and trisomic

31
Q

What are the two types of deletion?

Give a brief description of each

A

Interstitial - Region of chromosome deleted is internal to chromosome

Terminal - Region of chromosome deleted at the end of a chromosome

32
Q

What are the consequences of a deletion?

A

Deletions will be unbalanced and there will have a disease phenotype

Deletions may be large enough to see via light microscopy

33
Q

What is Karyotyping?

A

Cut and pasting pictures of metaphase chromosomes into homologous pairs

34
Q

How are chromosome pairs organised in a karyotype?

A

From Largest (chromosome pair 1) to smallest (chromosome pair 22)

23rd pair is the sex chromosomes

35
Q

How does position of the centromere vary in humans?

Hint: Give names and short descriptions for each position

A

Metacentric - centromere in the middle of the chromosome

Sub-metacentric - Centromere found between telomeres and middle of chromosome

Acrocentric - Centromere found very close to the telomeres, leaving only very short p arms

Telocentric chromosomes with no p arms and centromere joined at the telomeres are not seen in humans

36
Q

How is a normal karyotype formula expressed?

Give examples of a normal male and female.

A

Number of chromosomes
Followed by a comma
Then X chromosomes
Then Y chromosomes

Eg.
Normal Male = 46,XY
Normal Female = 46,XX

37
Q

How are chromosomal abnormalities expressed in a karyotype formula?

Give an example of the karyotype formula for an otherwise normal female with a deletion on the q arm of chromosome 4

A

A plus or minus sign denotes extra or missing sections from a chromosome

A chromosome number followed by a p/q and a +/- indicate which chromosome arm is affected

Eg.
46,XX 4p-

38
Q

Why might we refer a patient for karyotyping?

A

Congenital abnormality:

Prenatal screening

  • Down’s Syndrome, especially if high maternal age (>35)
  • Family history of chromosomal defects
  • Abnormal foetal ultrasound scan

Birth defects

  • Malformations
  • Mental retardation

Abnormal sexual development
- E.g. Klienfelter Syndrome

Infertility

Recurrent foetal loss

Acquired abnormalities:

Leukaemia and related disorders

39
Q

Describe how Fluorescent In-situ Hybridisation (FISH) is performed

A

Fluorescently labelled single stranded nucleic acids (Normally DNA, sometimes RNA) are permitted to interact with DNA In-situ.

Labels can be monochrome or multicoloured (multicoloured labels can be used to band a chromosome)

They form complexes or hybrids with sufficiently complementary sequences of the patient’s DNA

40
Q

What is useful about Fluorescent In-situ Hybridisation?

A

Excellent for spotting gross alterations easily

Multicolour FISH can be used to band chromosomes to identify abberant sequences

ESA1 Callum's cards (34 decks)

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