Lecture 20: Chromatin

Question 1

Winding of DNA

Answer 1

Forms nucleosomes by winding the DNA around H1-H4 histone proteins
(DNA->Chromatin->Nucleosomes->Extended Chromosome->Condensed Chromosome-> Chromosome)

Question 2

Chromatin undergoing Transcription

Answer 2

Euchromatin (delicate and thread like because it is unwound to allow for transcription)

Question 3

Heterochromatin

Answer 3

Condensed chromatin localized at nuclear envelope

Question 4

Heterochromatin Model

Answer 4

HMLalpha and HMRa (only select one)
-Near telomere so likely a silencer sequence

Question 5

Telomere Heterochromatin

Answer 5

RAP1
- Binds to DNA in silencer region
- Also binds to repeated telomere sequence
SIR 1
- Cooperates with RAP1 and binds silencer region in solent mating type loci
SIR2,3,4
- Bind hypoacetylated histone tails (H3 or H4) or to Rap1 and recruits SIR2

All this forms bulky sequences at the telomeres causing heterochromatin

Question 6

Rap1 and Sir

Answer 6

Rap1 binds telomeric repeats and recruits Sir3/4 thus recruiting Sir2 which deacetylates tails in nucleosomes near telomeres

Question 7

Sir and Nonacetylated H3/4

Answer 7

Sir3 and 4 bind non acetylated H3/4, recruits Sir2 to deacetylate neighbouring nucleosomes

Question 8

HDACs

Answer 8

Acetylation allows the complex fromation of RNA polymerases

Question 9

Rpd3p

Answer 9

Histone deacetylase
Ume6p binds URS, Sin3 binds Ume6p

Question 10

Wild type vs rpd3 and sin3 mutants

Answer 10

No deacetylation possible from their complex and so the adjacent histones are hyperacetylated

Question 11

HATs

Answer 11

Activators recruit HATS like Gcn5->Gcn4 that will acetylate and thus overcome pressed chromatin state

Question 12

Epigenetic Traits

Answer 12

Transmitted independently of DNA sequence
DNA marks are read by proteins and modify histones by mSin3
Histone marks nucleate DNA and recruits HMT to repress gene activity (heritable)

Question 13

Methylation of Histones Inheritance

Answer 13

In replication, methylated histones are evenly distributed to daughter cells and then the HMT recognizes H3K9me3 to methylate neighbouring histones

Question 14

Polycomb Complexes

Answer 14

PRC1 and PRC2 repress chromatin for long periods.
PRC2 recognize repressed regions and deacetylate histone tails while its E subunit methylates H3K27. PrC1 binds to H3K27me and condenses chromatin

Question 15

Dosage compensation

Answer 15

Females inactivate one X chromosome (barrier Bodies)

Calico cat fur colour

Question 16

XIST locus

Answer 16

Mediates repression of X chromosome and acts in cis

Question 17

TSIX

Answer 17

coded in complementary strand of XIST. Ensures the X chromosome stays active in the early stages of development thus TSIX expression stops in one of the copies

Question 18

Stages of X Repression

Answer 18

XIST spreads the chromosome binding gene-rich regions near coding gene in folded chromosome then spreads to other gene rich and finally gene poor

Question 19

XIST postulation

Answer 19

is used as a scaffold for binding to a complex with SMRT and HDAC3