Histones and Chromatin

What is the mass division of Eukaryotic DNA?

50% of the mass of eukaryotic DNA is in the chromatin, and the other 50% is in the proteins such as histones that make DNA structured.

What is chromatin?

The complex of DNA and proteins found in nuclei of eukaryotic cells

What are the types of proteins found in eukaryotic DNA

Histones: the major proteins DNA coils around

Nonhistones

In what phases is DNA most compact and least compact

DNA is the most compact in the M phase and the least compact in interphase (DNA is only transcriptable during interphase)

What is the charge on histones, and how does this impact their interaction with DNA?

Histones have arginine and lysine residues, which are attracted to the negative charges on the oxygen atoms on DNA

What is the histone made out of?

The histone is made out of 4 histone proteins, that form a disk DNA wraps around— these proteins are H2A, H2B, H3, and H4. This histone formation is nearly identical in all eukaryotes, and H2 shows the most variability

What is the function of H1?

H1 is a “linker protein” between core histones

How many times does the DNA wrap around the histone, and how much DNA is it?

It is 11nm of DNA, and it wraps around the protein 1.67 times. This is about 146 bp of DNA plus 20-60 linker DNA (H1)

In the protein mass portion of the nucleosome, what percentage of that is the amino acid terminal tails of the histone?

25-30%

How much DNA is in a mitotic chromosome

1,400 nm

Where is DNAse able to cut on wrapped DNA?

DNAse can only cut around the histone— it if goes to completion, it can cut anywhere but histone territory, which gives about a 200 base pair different on gel electrophoresis maps

Why do histones seem larger than they actually are?

Their positive charge counteracts the effects of sodium dodecyl sulfate, a negatively charged chemical meant to denature proteins, which makes it seem larger.

Histone folding?

That is when DNA folds around the histone. Due to this, the interactions on the phosphates of this DNA and the histone happen in the minor groove, because these are nonsequence specific.

What are NTDs?

N-terminus tails that stick out of histones. NTDs stabilize histones into tightly wound 30 nm fibers. These tight formations form in the first place from the H1 linker proteins, that cause the DNA to wrap more tightly. These tight 30 nm structures can become higher order structures from scaffolding proteins, and even higher order structures from its compaction into chromatids.

At what point is DNA too tightly wound for transcription?

Anything tighter than 10 nm

Types of chromatin

Heterochromatin:

• Little to no transcription

• Stains heavily with dye

• Packed densely

Euchromatin:

• active Transcription

• Doesn’t stain

• Highly expressed

• Loosely packed DNA

How can the type of chromatin be altered?

• Modification of the n-terminus tails (NTDs)

• Movement of individual histones to free up a binding site for a regulatory protein

Histone Code

The idea that the modification of histones can control chromosome compaction, and therefore protein interaction, and therefore gene expression.

In histone modification, there are readers, writers, and erasers

Writers: enzymes that make histone modifications by attaching to NTDs

Erasers: molecules that remove enzymes

Readers: substrates that carry out actions that are prompted by the modification made by the writer

Are eukaryotic chromosomes linear or do they have regions of supercoiling?

They have both, but the regions of supercoiling are so far from each other that they aren’t impacted by one another.