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Medicine Year 1 - Semester 2 - Part 2 > Epigenetics and Abnormal Gene Expression > Flashcards

Epigenetics and Abnormal Gene Expression Flashcards

1
Q

Describe the central dogma of information flow

A

DNA
(undergoes transcription to become)
RNA
(undergoes translation to become)
protein

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2
Q

How can chromosomes be distinguished ?

A

By size and G-banding

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3
Q

What is higher DNA structure in the form of ?

A

Chromosomes

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4
Q

What overlays chromosome structure ?

A

Epigenetic tags

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5
Q

What is epigenetic information ?

A

An extra layer of contextual information above just the DNA sequence.

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6
Q

What happens to epigenetic features over time ?

A

They diverge and change

(seen in identical twin study - older twins have more divergent patterns of epigenetics)

Still identical DNA

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7
Q

What does epigenetic gene splicing involve ?

mechanisms

A

Multiple mechanisms including:

  • DNA methylation
  • Nucleosome remodelling
  • Histone modifications
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8
Q

What is epigenetics ?

A

Heritable modifications of DNA that do not alter the primary sequence (e.g. me-C at CpG)

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9
Q

What is the result of epigenetics ?

A

Altered gene expression

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10
Q

Describe DNA methylation

A

Covalent modification of a methyl group to cytosine at position C5 to make 5-methylcytosine.

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11
Q

Where does DNA methylation occur ?

A

CpG islands
- regions of genes that control gene expression

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12
Q

What are CpG islands ?

A

Most genes have GC rich areas of DNA in their promoter regions.

These areas are called CpG islands.

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13
Q

What causes gene silencing ?

A

Methylation of the C residues within the CpG islands leads to gene slicing.

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14
Q

DNA methylation in yeast and Drosophila

A

Little or NO detectable DNA methylation

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15
Q

Key feature of DNA methylation

A

DNA methylation is heritable

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16
Q

Mechanism of action of DNA methylation

A

Inhibits gene transcription

Prevents the binding of transcription factors to the promoter

AND

Inhibits transcription by converting chromatin from an open to closed formation.

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17
Q

How does DNA methylation inhibit gene transcription ?

A

Converts chromatin from an open to a closed formation.

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18
Q

Function of Methyl CPG binding proteins

A

Methyl CPG binding proteins contain a methyl binding domain that specifically recognises methylated CpGs.

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19
Q

What does DNA methylation recruit ?

A

Recruits other proteins such as histone deacetylases which remove acetyl groups, favouring compact chromatin.

20
Q

Causes of disease in epigenetics

A

Defects in DNA methylation machinery

  • X linked syndromes with variable phenotypes
21
Q

Rett Syndrome

A

Dominant X linked, neurodegenerative disorder

Caused by a mutation in gene encoding MeCP2, which in turn leads to a loss of gene silencing at many loci.

22
Q

Who does rett syndrome affect ?

A

Females only
- males do not survive

23
Q

Prader Willi syndrome signs

A

PWS
- mental retardation
- obesity

24
Q

Angelman Syndrome signs

A
  • mental retardation
  • ‘happy puppet’ syndrome
  • jerky movements + inappropriate laughter
25
Q

Causes of Angelman syndrome and Prada Willi syndrome

A

Chromosome 15 abnormality

AS - maternal deficiency
PWS - paternal deficiency

Occurs due to defects in imprinted genes: defective expression in brain tissue

26
Q

Heterochromatin

A

Highly condensed in interphase

Transcriptionally inactive (contains few genes)

Replicates late in S phase

27
Q

Euchromatin

A

Organised in 30nm fibre during interphase

Transcriptionally active

Replicates early in S phase

28
Q

Position effect

A

Spreading of heterochromatin into euchromatic regions causes cell to cell variability in gene expression.

29
Q

X chromosome inactivation

A

LIONISATION

Females need to silence one X chromosome.

Mechanism of silencing is initiated by Xist

30
Q

Why does X chromosome inactivation occur ?

A

Discrepancy of 1 X-chromosome in males (XY) but 2 X-chromosomes in females (XX)

31
Q

Mechanism of X chromosome inactivation

A

X chromosome is inactivated by transcripts that are made from that X chromosome.

Function as RNA. No protein product and RNA remains in the nucleus.

This is followed by DNA methylation.

32
Q

Feature of X chromosome inactivation

A

The X chromosome that gets inactivated is eventually inactivated by DNA methylation.

33
Q

When does X chromosome inactivation occur ?

A

Early in embryonic development
Around the 64 cell stage embryo

34
Q

Klinefelter’s syndrome

A

Male
Genotype (47XXY)

Number of Barr bodies (inactivated X chromosomes) would also be 1

35
Q

Barr body

A

Inactivated X chromosome

36
Q

Genomic imprinting

A

Silencing (around 200) DNA genes (individual genes) on autosomes.

Imprinted genes only expressed from 1 allele

Dependent on parental origin

Imprinting resets on passage through germline.

37
Q

Genomic Imprinting - fertilisation

A

There is a wave of de-methylation in the fertilised egg, but imprinted genes escape this.

During the production of haploid sperm and egg, the patterns of DNA methylation are re-set for imprinted genes

In sperm, imprint is paternal
In eggs, imprint is maternal

38
Q

Uniparental disomy

A

Both copies of a chromosome are inherited from the same parent.

Individual is missing the chromosome from one of the parents

Expression altered of imprinted genes on affected chromosome (non-imprinted genes are not affected)

39
Q

Chromosome 11 - uniparental disomy

A

Wilm’s tumour

40
Q

Chromosome 15 - uniparental disomy

A

PWS
AS

41
Q

What is angelman syndrome ?

A

Genetic disorder that affects the nervous system and causes severe physical and intellectual disability.

Caused when the Angelman gene is either absent or malfunctions.

  • paternal uniparental disomy
42
Q

What is Prader Willi syndrome ?

A

Rare genetic condition that causes a wide range of physical symptoms, learning difficulties and behavioural problems.

Fault on Chr15 leads to a number of problems and is thought to affect the hypothalamus, which produces hormones and regulates growth and appetite.

  • maternal uniparental disomy
43
Q

AS

A

Maternal deficiency

44
Q

PWS

A

Paternal deficiency

45
Q

Non-disjunction in Meiosis 2

A

Uniparental isodisomy

46
Q

Non-disjunction in Meiosis 1

A

Uniparental Heterodisomy

47
Q

Diseases associated with genetic imprinting

A

Beckwith-Wiedemann Syndrome
Wilms’ tumour
Fragile X syndrome

Myotonic dystrophy
PWS
AS

Medicine Year 1 - Semester 2 - Part 2 (53 decks)

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