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fundamentals of living cells > Chromatin Structure And Histones Code > Flashcards

Chromatin Structure And Histones Code Flashcards

1
Q

What does chromatin consist of?

A

DNA + Histones + RNA

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2
Q

Give the definition of chromatin

A

Nuclear complex of DNA and associated proteins that forms chromosomes within the nucleus of eukaryotic cells

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3
Q

Give the general function of chromatin

A

Packaging long DNA molecules into more compact and smaller volume to fit into the nucleus and to protect the DNA structure and sequence

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4
Q

What are the four levels of chromatin packaging, how much do they increase packaging and what is the rough size of them

A

1- nucleosomes- 7fold - 10nm
2- nucleosomes packed into fibres- 6fold- 30nm
3-fibres pack into loops and TADs to form chromatin - 3fold- 100-250nm
4- mitotic chromosome - 700-1000nm

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5
Q

When does chromatin condensation happen?

A

Prophase

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6
Q

Give the structure and function of euchromatin

A

Low compactivity (10nm fibre) - beads on a string
Gene rich
Enzyme accessible
Involved in DNA transcription, replication or repair

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7
Q

Give the structure and function of heterochromatin

A

Highly condensed (30nm) fibre
Darkly staining areas of chromatin often associated with nuclear envelope
Gene poor
Constitutive or facultative

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8
Q

What is constitutive heterochromatin?

A

Contains highly repetitive sequences of DNA which are genetically inactive and serve as a structural element (in the telomeres and centromeres)- always switched off

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9
Q

What is facultative heterochromatin?

A

Regions on chromosomes which become heterochromatin in certain cells and tissues- can be turned on/off at any given point in the organisms development or in a certain tissue

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10
Q

How does chromatin show plasticity?

A

A choice of histone variants, modifications of DNA bases and reversible posttranslational modifications of histone tails

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11
Q

Give the definition of a nucleosome

A

A little DNA wrapped around histones

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12
Q

What does an octamer core consist of?

A

H2A, H2B, H3 AND H4 (two of each)

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13
Q

Where are the N-terminal tails in relation to the octamer core?

A

Outside

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14
Q

How much DNA wraps around the histone cores?

A

146 base pairs or 1.7 turns of DNA

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15
Q

Where is the H1 protein found?

A

Wrapped around another 20 base pairs of linking DNA

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16
Q

What is The purpose of a H1 protein?

A

Stabilises the zigzagged 30nm chromatin fibre?

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17
Q

What happens to the histones each time a polymerase reaches it?

A

They have to be removed and replaced behind it (having been recycled and resynthesised)

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18
Q

When does a chromatin loop happen?

A

When stretch’s of genomic sequences that lie on the same chromosome are in close physical proximity to each other

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19
Q

What is cohesion in relation to chromatin loops?

A

A protein ring that binds to DNA and facilitates loops

20
Q

What do chromatin loops exist?

A

They provide a favourable environment for process such as DNA replication, transcription and repair

21
Q

What is a topologically associated domain (TAD)?

A

Highly conserved chromatin domain that shape functional chromosomal organisation.

22
Q

What do TADs do?

A

We don’t know, but their disruption leads to disease so they must do something

23
Q

What is a chromosome territory?

A

A non overlapping domain of the nucleus occupied by uncondensed chromosomes- composed of TADs

24
Q

What is epigenetics?

A

Heritable and reversible changes in gene expression which don’t involve a change in the DNA sequence. Results from external or environmental factors or as a part of a development programme

25
Q

Give some examples of covalent histone modifications

A

Methylation, acetylation, phosphorylation, ubiquitylation

26
Q

What happens in histone methylation?

A

Histone tails are methylated by histone methyltransferases (HMTs) and demethylated by histone demethylases (HDMs)

27
Q

What are the most common methylations?

A

Mono, Di, tri of lysines and argenines

28
Q

What does methylation of lysine residues cause (2 options)?

A

Chromatin condensation or decondensation

29
Q

What happens when lysine residues are methylated?

A

They are more susceptible to transcription factors

30
Q

What does H3K9me3 stand for?

A

Histone 3, 9th lysine on that tail, 3 methylations

31
Q

What is histone acetylation?

A

Histone tails are acetylated by histone acetyl transferases (HATs) and deacetylase by histone deacetylase (HDACs)

32
Q

What is most commonly acetylated?

A

Lysines

33
Q

What are acylated histones normally associated with?

A

Relaxed chromatin and gene expression

34
Q

What are deacylated histones normally associated with?

A

Closed chromatin and the silencing of genes

35
Q

What is aberrant acetylation associated with?

A

Solid tumours and haematological malignancies

36
Q

What are histone tails phosphorylated and dephosphorylated by?

A

Histone tails are phosphorylated by protein kinases and dephosphorylated by proteases

37
Q

Where can histones be phosphorylated?

A

Serine, threonine and tyrosine

38
Q

What is the phosphorylation of H3s10 and H3s28 involved in?

A

Chromatin condensation during mitosis and meiosis as well as chromatin relaxation linked to transcription activation

39
Q

Where (and using what) does histone ubiquitylation take place?

A

Ubiquitin ligases and deubiquitinating enzymes. Takes place primarily on lysines of histones H2A and H2B

40
Q

What is H2Aub more commonly correlated with?

A

Gene silencing

41
Q

What is H2Bub more frequently associated with?

A

Transcription activation

42
Q

What is a histone code?

A

Combinations of post-translational modifications on the same or different histone tails

43
Q

What are histone code readers?

A

Protein complexes that read combinations of marks

44
Q

What does a chromodomain specifically recognise? (Histone code readers)

A

Methylated residues

45
Q

What does a bromodomain specifically recognise? (Histone code readers)

A

Bind to acylated residues

fundamentals of living cells (41 decks)

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