Chromatin Structure + Histone Code Flashcards

1
Q

What is chromatin?

A

Chromosomes + associated proteins

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2
Q

What is nucleosome?

A

DNA + histones

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3
Q

Histones are responsible for …….. of packaging

A

1st level

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4
Q

Nucleosome structure increases DNA packaging in 1st level of packaging by…..

A

7 fold

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5
Q

2nd level of packaging

A

Nucleosomes pack themselves in fibres of 30nm constituting 2nd level of packaging
Increases packaging 6 fold

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6
Q

3rd level of packaging

A

30nm fibres pack themselves into 80-100 fibres constituting to 3rd level of packaging
Increases packaging 3- fold

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7
Q

4th level of packaging

A

Is represented by mitotic chromosome

Represents 10000 fold packaging

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8
Q

Chromosomes consist predominantly of

A

DNA
Histone proteins
Non-histone proteins
Non-coding RNA

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9
Q

Histone assemble to form an octamer core

A

2 molecules of each histone: H2A,H2B, H3& H4
N-terminal tails outside octamer core=> there is an electrostatic attraction between negatively charged DNA backbone + positive N terminal tail

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10
Q

Compaction of nucleosomes to form higher order structures involves

A

Linker histones
Interaction of histone tails with adjacent nucleosomes
Binding of packing proteins to histone tails

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11
Q

What are histone remodelling factors?

A

Enzymes that remove + replace nucleosomes

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12
Q

What is euchromatin?

A

Lightly staining areas of chromatin
Rich in genes
Made up of nucleosomes but not dense higher order packaging

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13
Q

What is heterochromatin?

A

Darkly staining areas of chromatin
Few genes
Dense higher order packaging of nucleosomes

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14
Q

What is facultative heterochromatin?

A

Contains genes not expressed in that cell type
DNA tightly packaged as heterochromatin
But may be packaged as euchromatin in other cell types

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15
Q

What determines whether nucleosomes are packed as euchromatin or heterochromatin?

A

One key of control- chemical modification of lysine residues in histone tails
Acetylation
Methylation

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16
Q

What are DNAase I sensitive sites?

A

Sequences of DNA without histones
May be naked or binding transcription factors
Cut by very brief digestion with DNAase I
Found in promoters + enhancers

17
Q

If DNA is tightly packaged in higher order chromatin structure, how can all these proteins get at DNA?

A

They can’t do until you start to unwind chromatin
First TF opens up chromatin structure
Then recruits basal TFs
Transcription occurs

18
Q

Give examples of chromatin modifying enzymes

A

HAT (histone acetyltransferase)

HMT (histone methyltransferase)

19
Q

Histone modification- acetylation

A

Amino group is positively charged
When it is acetylated, HAT takes acetyl group from acetyl coA + attaches it to amino group
This leads to loss of positive charge

20
Q

What does HAT do?

A

Acetylate lysine residues on histones, leads to unpacking of chromatin

21
Q

What does HDAC do?

A

De-acetylate histones, lead to compaction of chromatin

22
Q

What does HMT do?

A

It methylates histone tails

23
Q

What does histone demethylases (HMD) do?

A

Demethylate histone tails

24
Q

What does methylation of lysine residues cause?

A

Methylation of some lysine residues causes chromatin condensation
Methylation of other lysine residues causes chromatin decondensation

25
Q

Histone marks are read by…..

A

Binding proteins

26
Q

Marks for promoters and enhancers

A

Promoters strongly enriched for H3K4me3

Active enhancers are enriched for H3K4me1