chromatin structure and histone code

Question 1

DNA packaging

Answer 1

Chromosomes + associated proteins = chromatin

Histones are proteins responsible for 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

Nucleosomes 

Much DNA is tightly packed in higher 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 packaging proteins to histone tails

Question 6

Chromatin

Answer 6

Structure is not static

During DNA transcription or DNA replication, nucleosomes must be removed form the front of the polymerase and replaced behind the polymerase 

Histone remodeling 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