Changes in chromatin structure and transcription

Question 1

What is constitutive heterochromatin?

Answer 1

Densely packaged chromatin maintianed in its dense form (so is never transcribed)
-Includes replicative DNA seqs (telomeres, centromeres, satellite DNA)

Question 2

What is facultative heterochromatin?

Answer 2

Densely packaged chromatin which can decondense (to become transcriptionally active euchromatin)
-More dynamic

Question 3

What post translational modifications are markers of transcriptionally active DNA?

Answer 3

H3K9Ac (acetylation of Lys9 at histone 3)
H3K4me (methylation of Lys 4 at histone 3)

Generally acetylation

Question 4

What post translational modifications are markers of heterochromatin?

Answer 4

H3K9me (methylation of Lys9 at histone 3)
H3K27me (methylation of Lys27 at histone 3)

Generally methylation

Question 5

What is the histone code hypothesis?

Answer 5

Histone modifications are correlated with chromatin structure -act as epigenetic markers to allow recognition

Question 6

What happens to convert euchromatin to heterochromatin?

Answer 6

Deacetylation and methylation
-histone deacetylases and histone methyltransferases

Question 7

What happens to convert heterochromatin to euchromatin?

Answer 7

Acetylation and
-histone acetyltransferases and H3K9 methyltransferases

Question 8

How is histone acetylation directed by activators?

eg. SAGA coactivator

Answer 8

eg. SAGA coactivator contains histone deacetyl transferase (HAT) complex so acetylates histone side chains
-acetylated histones can then be recognised by proteins containing bromodomains, such as chromatin remodelling machines (with helicase activity)

Question 9

How is histone deacetylation directed by activators?

Answer 9

eg. Corepressor histone deacetylase complex recruited to URS

Question 10

What is heterochromatin spreading?

Answer 10

When the methylation of one nucleosome induces the methylation of neighbouring nucleosomes.
-Stimulated by Suv3-9 SET domain
-Stopped by chromatin boundary elements/insulator elements (define regions of heterochromatin so that actively transcribed genes continue being transcribed)

Question 11

How can histone modifications (aka transcriptional status) of genes be shown experimentally?

Answer 11

Using Chromatin Immunoprecipitation to analyse protein/DNA interactions
-Stabilise interactions with cross-linking and fragment DNA to analyse
-Use antibiotics specific to acetylated or methylated histones

-create genome-wide histone maps using gene chips

-show nucleosome-free regions in promotor and terminator regions (regions next to these