Histones and Packaging Flashcards

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1
Q

What are histones?

A
  • Most common nuclear protein
  • Small
  • Positively charged
  • Highly conserved
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2
Q

How are DNA are histones related?

A
  • DNA is wound around histones, level of packaging determines level of transcription
  • Highly packed means less replication because harder to be transcribed
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3
Q

What are the units in chromatin?

A
  • Nucleosome
  • Histones
  • DNA
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4
Q

What are the 4 core histones?

A

H2A, H2B, H3 and H4

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5
Q

What is the linker histone?

A

H1

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6
Q

What are H3 and H4?

A
  • Highly conserved
  • Combine in a histone fold
  • Form a tetromer in the centre of the octamer
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7
Q

What are H2A and H2B?

A
  • 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
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8
Q

How does DNA associate with the octomer

A

DNA forms major and minor groove pockets around the octomer

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9
Q

What happens if a histone is altered within the octamer?

A
  • Change in the path of the DNA around the octamer
  • Change of packing of the DNA
  • Affects if gene can be transcribed
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10
Q

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

A
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
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11
Q

What is heterochromatin?

A
  • 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
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12
Q

What is euchromatin?

A
  • 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
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13
Q

What is constituative heterochromatin?

A
  • All cells of a given species will package the same regions of DNA into constituative heterochromatin
  • Poorly expressed genes
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14
Q

Facultative heterochromatin

A
  • 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
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