Chromatin structure and histone code

Question 1

DNA packaging

Answer 1

• Chromosomes + associated proteins=chromatin
• Histones are proteins responsible for the 1st level of packaging
○ DNA + histones=nucleosomes
○ Nucleosome structure increases DNA packaging 7-fold
• Nucleosomes pack themselves in fibers of 30nm constituting the 2nd level of packaging
§ Increases packaging 6-fold
• 30nm fibers pack themselves into 80-100nm fibers constituting 3rd level of packaging
○ Increases packaging 3-fold
• 4th level of packaging is represented by the mitotic chromosome
○ Represents 10000-fold packaging

Question 2

Chromosomes consist predominantly of

Answer 2

• DNA
• Histone proteins
• Non-histone proteins
• Non-coding RNA

Question 3

Interphase chromosomes

Answer 3

• Almost all DNA packaged at 1st level
○ Nuclosomes
• Much DNA is tightly packed in high order structures

Question 4

Nucleosomes

Answer 4

• Histones assemble to form an octamer core
○ 2 molecules of histones H2A,H2B,H3 and H4
• N-terminal tails outside the octamer core

Question 5

Higher order structure involves

Answer 5

• Linker histones (e.g. H1)
• Interaction of histone tails with adjacent nucleosomes

• Binding of packing proteins to histone tails

Question 6

Chromatin

Answer 6

• Structure is not static
• During DNA transcription or DNA replication, nucleosomes must be removed from the front of the polymerase and replaced behind the polymerase
• Histone remodelling factors are enzymes that remove and replace nucleosomes

Question 7

Facultative heterochromatin

Answer 7

• Contains genes not expressed in that cell type
• DNA tightly packaged as heterochromatin
• But may be packaged as euchromatin in other cell types

Question 8

What determines whether nucleosomes are packed as euchromatin or heterochromatin?

Answer 8

Modification of Lysine residues in histone tails by acetylation and methylation