Chromatin Structure + Histone Code

Question 1

What is chromatin?

Answer 1

Chromosomes + associated proteins

Question 2

What is nucleosome?

Answer 2

DNA + histones

Question 3

Histones are responsible for …….. of packaging

Answer 3

1st level

Question 4

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

Answer 4

7 fold

Question 5

2nd level of packaging

Answer 5

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

Question 6

3rd level of packaging

Answer 6

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

Question 7

4th level of packaging

Answer 7

Is represented by mitotic chromosome

Represents 10000 fold packaging

Question 8

Chromosomes consist predominantly of

Answer 8

DNA
Histone proteins
Non-histone proteins
Non-coding RNA

Question 9

Histone assemble to form an octamer core

Answer 9

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

Question 10

Compaction of nucleosomes to form higher order structures involves

Answer 10

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

Question 11

What are histone remodelling factors?

Answer 11

Enzymes that remove + replace nucleosomes

Question 12

What is euchromatin?

Answer 12

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

Question 13

What is heterochromatin?

Answer 13

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

Question 14

What is facultative heterochromatin?

Answer 14

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

Question 15

What determines whether nucleosomes are packed as euchromatin or heterochromatin?

Answer 15

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

Question 16

What are DNAase I sensitive sites?

Answer 16

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

Question 17

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

Answer 17

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

Question 18

Give examples of chromatin modifying enzymes

Answer 18

HAT (histone acetyltransferase)

HMT (histone methyltransferase)

Question 19

Histone modification- acetylation

Answer 19

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

Question 20

What does HAT do?

Answer 20

Acetylate lysine residues on histones, leads to unpacking of chromatin

Question 21

What does HDAC do?

Answer 21

De-acetylate histones, lead to compaction of chromatin

Question 22

What does HMT do?

Answer 22

It methylates histone tails

Question 23

What does histone demethylases (HMD) do?

Answer 23

Demethylate histone tails

Question 24

What does methylation of lysine residues cause?

Answer 24

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

Question 25

Histone marks are read by…..

Answer 25

Binding proteins

Question 26

Marks for promoters and enhancers

Answer 26

Promoters strongly enriched for H3K4me3

Active enhancers are enriched for H3K4me1