Histones and DNA Packaging

FLS Lecture 6

Why is DNA packaging important?

DNA is very long, each diploid cell has 2m of DNA.

How many bases and length is involved in 1 complete turn of a double helix of DNA?

* 10 bases
* 3.4nm

What is the composition of chromatin?

* DNA
* RNA
* Proteins

What are histones?

* Most common nuclear protein
* Small and positively charged proteins

What are the 4 core proteins?

* H2A
* H2B
* H3
* H4

What is the linker histone?

H1

What is special about the core histones?

They show a high degree of conservation

What comprises chromatin?

The nucleosome

What is a nucleosome?

Histones + DNA

In what way does DNA wrap the histone?

Left handed superhelix

Name the first level of DNA packaging.

Formation of the nucleosome

Name the core histones and the process in the formation of the histone handshake motif.

* H3 AND H4
* Heterodimerization

What shape does the tetramer form after the initial motif?

Horseshoe

Name the core histones used in the 2nd dimer formation.

* H2A
* H2B

How does the H2A/H2B dimer attach to the H3/H4 tetramer?

Above and below the tetramer

What happens if a histone is altered?

* DNA path changes
* May result in a change is packaging

Name the 4 H2A varients and their location?

* H2A.Z- found in nearly all eukaryotes
* H2A.X- phosphorylated at double strand breaks
* H2A.Bbd- vertebrate specific depleted on Xi
* macroH2A- vertebrate specific, enriched XI

What does H2A.Z do?

* Alters stability of the H2A and H2B interactions
* Alters interactions with the tetramer
* Alters the canonical nucleosome
* Often associated with transcriptionally active chromatin

Name the 5 variants of H3.

* H3.1
* H3.2
* H3.3
* H3.It
* CenpA

Where do varients of H3 and H2A differentiate?

* Chromatin centromeres
* Active genes
* Heterochromatin

What core protein is abundant in epigenetically silenced chromatin?

H2A

State the number of turns of DNA in level 1 packaging.

1\.8 turns

What is level 2 of packaging?

Formation of the 10nm fiber

What is packing ration for level 2?

6-7

What is level 3 and how does it form?

* 30nm solenoid
* The 10nm further coils to form the 30nm solenoid

What core histone is essential for higher order packaging?

H1

What is the packing ratio for level 3?

\~40

What is level 4 of packaging and how does it form?

* 300nm solenoid
* The 30nm solenoid is further coiled and employs a protein scaffold to condense further

What is level 5 of packaging? How is it formed?

* 700nm fiber
* 300nm solenoid coils again, forming a coiled coil
* The packing ratio is 10^4

What is level 6 of packaging?

* Metaphase chromosome
* During metaphase, chromosomes condense further and now the DNA is a 1400nm fibre

Name the 2 types of chromatin.

* Heterochromatin
* Euchromatin

State the function of the centromere.

* Primary point of restriction
* Mediates chromosome cohesion
* Spindle attachment
* Chromosome segregation

Where is heterochromatic located?

* Telomeres
* Centromere

What are the 2 types of heterochromatin?

* Constitutive
* Facultative

Which type of heterochromatin means that the genes contained are poorly expressed?

Constitutive

How can poorly expressed genes be expressed elsewhere?

Be present within euchromatin

What is the main difference between heterochromatin and euchromatin?

Heterochromatin:

* Replicated in late S phase
* Restricted regions on the chromosome
* Repetitive DNA sequences
* Very few genes present

Euchromatin:

* Replicated in early S phase
* More open configuration
* Contains both transcriptionally active and inactive genes
* Stains light