Chromatin Modification

Question 1

In interphase, chromosomes are spread out in the nucleus. The spread out bits are called

Answer 1

Euchromatin

Question 2

Some parts of chromosomes are condensed in interphase. The condensed bits are called

Answer 2

Heterochromatin

Question 3

DNA in heterochromatin

Answer 3

is inaccessible for transcription

Question 4

What is X-chromosome inactivation?

Answer 4

In female mammals, only one X chromosome is active the other is always condensed and inactive.

Question 5

Expressed genes have a more open structure than others, shown by …

Answer 5

greater sensitivity to DNase

In the expressed state the only packaging is into nucleosomes

Question 6

B-Globin genes are transcribed only in..

Answer 6

erythroid cells.

Question 7

Where would the B-Globin gene be most sensitive to DNase?

Answer 7

Erythroid cells.

Question 8

Opening up of chromatin involves what?

Answer 8

1) Histone modification, especially histone acetylases.

2) Chromatin remodelling complexes

Question 9

What is histone acetylation?

Answer 9

Addition of acetyl groups to lysine residues. Acetylation removes a positive charge from the histone.
Acetylation reduces charge, changing interactions of histones with DNA and each other - LOOSENS NUCLEOSOMES.

Question 10

What is the chromatin remodelling complex?

Answer 10

INCREASES ACCESSIBILITY OF DNA.
Active process, requires energy from ATP hydrolysis.
Enzymatically allows access of DBP by partly unwrapping or removing nucleosomes.

Question 11

What are the three types of RNA polymerase:

Answer 11

RNA polymerase I: transcribes ribosomal RNA genes
RNA polymerase II: transcribes genes coding for proteins
RNA polymerase III: transcribes genes coding for small RNAS

Question 12

All forms of eukaryotic RNA polymerase have subunits that correspond to the

Answer 12

a, beta and beta prime subunits of bacterial RNA polymerase. The structures of bacterial and eukaryotic RNA polymerases overlap.

Question 13

Purification of eukaryotic RNA polymerases gives…

Answer 13

core enzymes which do not initiate transcription specifically. A transcription complex of at least 50 polypeptides is required for transcription in vivo.

Question 14

What does specific initiation of transcription require?

Answer 14

• Enzymes that alter chromatin structure
• Specific activator proteins
• Mediator
• Basal or general transcription factors
• RNA polymerase core enzyme

Question 15

What are some examples of basal factors for transcription by RNA polymerase II

Answer 15

TFIIA, TFIIB

Question 16

What does the TATA box do?

Answer 16

directs RNA polymerase to the correct starting point.

The TATA box is about 25 bp upstream of the transcription start point.

Question 17

To initiate transcription, TATA box binding protein binds to TATA box. What does this binding cause?

Answer 17

Put a wedge into the minor groove of the DNA molecule, twisting the DNA so it bends at an angle of about 70.

Question 18

TBP forms part of what complex?

Answer 18

TFIID.

TFIID contains 8 other polypeptide subunits (TAFS) for TATA box associated factors.

Question 19

Many genes don’t have a TATA box, some have one or more other sequences that define where transcription starts.
TRUE OR FALSE?

Answer 19

TRUE.
These sequences bind to TFIBB or other subunits of TFIID. However, TBP is still required for transcription and bending of DNA.

Question 20

What are housekeeping proteins

Answer 20

Proteins that are required in all or most cells in relatively small amounts.

Question 21

What is unusual about transcription of housekeeping genes?

Answer 21

Transcription of these genes does not start at a specific base: DISPERSED initiation.

Question 22

What happens in preparation of transcription?

Answer 22

Transcription activator proteins bind to specific sequences in DNA, displacing nucleosomes.
To increase accessibility, histones are acetylated and chromatin is remodelled.
When gene is about to be transcribed, additional activator proteins bind to additional specific sequences in its DNA.

Question 23

What happens in assembly of transcription complex?

Answer 23

a) TFIID binds to promoter
b) Binding of TFIIA and TFIIB stabilises the complex
c) RNA polymerase 11 and TFIIF bind, forming core pre-initiation complex
d) Binding of TFIIE and TFIIH completes the complex.

Question 24

What happens to release RNA polymerase?

Answer 24

a)DNA is opened forming a transcription bubble (requires ATP hydrolysis by TFIIH)
B)Transcription starts, commonly pauses
C) Elongation factors join with RNA polymerase and initiation factors dissociate, gives elongation complex.

Question 25

What is required for release of RNA polymerase in the last step?

Answer 25

Phosphorylation of elongated carboxy terminal domian of Polymerase II by TFIHH and positive transcription elongation factor-b.