Mol. and Cell Biol: Chromatin Structure

Prep for exam 1.

Components to a Nucleosome:

An octet of histones H2A, H2B, H3, and H4

DNA wraps around a nucleosome ______ times.

1.65

“Beads on a string” models of chromatin are associated with ________ gene expression.

more

When in the cell cycle is the chromatin least compacted?

Interphase

When in the cell cycle is the chromatin most compacted?

M phase/ Mitosis

Properties of histones:

1. basic

2. positive charge at neutral pH

3. contain lots of lysine and arginine

4. well conserved

5. encoded by multiple genes

Euchromatin:

regions of more open chromatin structure and active gene expression

Heterochromatin:

regions of more closed chromatin and inhibition of gene expression

Addition of H1 histone _________.

shifts chromatin from beads on a string to more condensed forms

Histone modification typically occurs in the _____________.

N-terminus end of histone tail (bc its easily accessible and extend past the nucleosome core)

“Histone Code“:

chemical modifications to histones will form preferences to more open or closed chromatin structure, promoting or inhibiting gene expression

How to write a histone modification:

1. Name the histone which is altered

2. Name the specific amino acid which has been altered

Types of post translational histone modifications:

1. Acetylation

2. Hypoacetylation

3. Methylation

Histone acetylation involves the ________ of a _______.

addition of an acetyl group/lysine

The __________ is removed from histones after acetylation.

positive

Acetylation promotes ________________ expression.

increased gene

Histone hypoacetylation involves ___________.

removal of an acetyl group at specific locations

Hypoacetylation typically occurs where?

centromeres and telomeres

RAP1 protein recruits which proteins?

Sir2, Sir3, and Sir4

Function of Sir2:

removes acetyl groups from histones

Function of Sir3 and 4:

bind to deacetylated lysine’s, bringing Sir2 with them

Hypoacetylation promotes ______________ expression/ ___________.

less gene/heterochromatin

___________ prevents Sir2 from hypo acetylating the entire chain of chromatin.

Boundary elements

Histone methylation involves the _____ and the _______ protein.

H3K9 amino acid/ HP1

Function of HP1:

chromo domain allows it to bind to methyl group and oligomerize, or agglutinate with other HP1 proteins. It can also bind to methyl transferase and continue methylating down the chain of chromatin

TAD:

topographically associated domains in a chrmosome where regulatory loops occur and are defined by boundary elements

Cohesin function:

forms a ring which chromatin is pulled through to form a loop

CTCF function:

brackets the boundary elements

Heterodimer responsible for nucleosome removal during transcription:

FACT complex

Components of FACT Complex:

SPT16 and SSRP1

SPT16 function:

destabilizes nucleosome

SSRP1 function:

stabilizes nucleosome so it can be removed and added back to the DNA

Epigenetics:

changes in gene expression or information that is passed through cell division and does not involve changes in the DNA sequence