Histones and Packaging

Question 1

What are histones?

Answer 1

• Most common nuclear protein
• Small
• Positively charged
• Highly conserved

Question 2

How are DNA are histones related?

Answer 2

• DNA is wound around histones, level of packaging determines level of transcription
• Highly packed means less replication because harder to be transcribed

Question 3

What are the units in chromatin?

Answer 3

• Nucleosome
• Histones
• DNA

Question 4

What are the 4 core histones?

Answer 4

H2A, H2B, H3 and H4

Question 5

What is the linker histone?

Answer 5

H1

Question 6

What are H3 and H4?

Answer 6

• Highly conserved
• Combine in a histone fold
• Form a tetromer in the centre of the octamer

Question 7

What are H2A and H2B?

Answer 7

• Combine in a histone fold
• One H2A and H2B dimer below and one above tetromer to complete octomer
• Variants differentiate chromatin at centromeres, active genes and heterochromatin

Question 8

How does DNA associate with the octomer

Answer 8

DNA forms major and minor groove pockets around the octomer

Question 9

What happens if a histone is altered within the octamer?

Answer 9

• Change in the path of the DNA around the octamer
• Change of packing of the DNA
• Affects if gene can be transcribed

Question 10

What are the different H2A’s and what do they do?

Answer 10

macroH2A
- Vertebrate specific
H2A.Z
- All eukaryotes
- Only 60% identical to H2A
- Alters the interaction stability between H2A and H2B
- Alters the interaction between H2A:H2B dimer and the H3:H4 tetramer
- Alters the nucleosome
- H2A.Z containing nucleosomes often associated with transcriptionally active chromatin
H2A.X
- Phosphorylated at double strand breaks
- Markers for DNA breaks

Question 11

What is heterochromatin?

Answer 11

• Restricted regions of chromosome
• Highly compacted even during interphase
• Consitutive heterochromatin remains condensed most of the time in all cells
• Replicated late in S phase
• Very few genes present
• Highly condensed
• Rarely genes

Question 12

What is euchromatin?

Answer 12

• Lightly stained regions of chromosomes
• Open chromatinconfiguration during interphase
• Replicated in early S phase
• Contrains both transcriptionally active and inactive genes
• Differential histone modifications
• More loose
• Easily accessible

Question 13

What is constituative heterochromatin?

Answer 13

• All cells of a given species will package the same regions of DNA into constituative heterochromatin
• Poorly expressed genes

Question 14

Facultative heterochromatin

Answer 14

• DNA packaged in facultative heterochromatin will not be consistent within the cell types of a species
• A sequence in one cell that is packaged in facultative heterochromatin may be packaged in euchromatin in another cell
• Regulated
• Associated with differentiation