L2U2 - Chromosome Strucutre and DNA Condensation

Bacterial Chromosome Shape

Closed Circle, with “Nucleoid” protein structures, not static

What does static mean

The different regions can apparently find any other region → no evidence that specific DNA are always associated with the same scaffold(protein structure)

Chromatin

Complex of DNA and proteins found in the nuclei

Histones

Major proteins found associated wiht eukaryotic DNA, small, very basic proteins

What are the different lengths of Eukaryotic DNA?

DNA - 2nm

Nucleosome - 11nm

30 nm filament - 30nm

Extended form of chromosome - 300 nm

Condensed section of chromosome - 700 nm

Mitosis Chromosome - 1,400 nm

“Beads on a string”

10nm fiber, DNA wrap around histones,

Core Histones on Beads on a string

Very basic small proteins → positively charged, which allows the negatively charged phosphates from DNA to wrap around them

Disk is made up of 2 copies each of H2A, H2B, H3 and H4

H1

“Linker” protein between the core histones

Are histone proteins conserved

YES they are highly conserved

Highly Conserved

Same across species; H4 from cows differed by 2 amino acids from H4 of peas

Nucleosome

Bead on a string

How many times does a dsDNA wrap around a histone core

1.67 times, 146 bp of DNA with 20-60 bp linker DNA between

How does digestion work with Nuclease on bead on a string

Can only cut at linker DNA (dna wrapped around histone can’t be cut)

How is DNA wrapped around the Histones

Not randomly wrapped - has very specific pathways on where to enter and exit

What are the amino-terminal tails of histone

Each core histone has tail that stretches outward outside of Nucleosome - helps to stabilize structure by binding to DNA around local histone, along with neighboring histone cores - also serve as sites for modification

“Histone Fold”

Interactions with phosphates and minor groove bases bind to DNA binding sites on histone cores, which allow DNA to then bind to the histones and bend the DNA

How many N-Terminal Tails are there

8; 1 per histone (4 types of histones, 2 of each type)

What 2 components with beads on a string is needed to make 30 nm fibers

H1 and Histone Tails

How does H1 work

Promotes compaction to 10nm beads on a string - binds at enter and exit points of DNA on histones and cause them to wrap more tightly - more compact

What is the most condensed form of chromosome

Mitotic Chromosome; 1,400nm

What compaction level does DNA become not accessible for transcription, replication, or repair?

After 10nm

Heterochromatin

Tightly packed, dense DNA, 30nm etc.

Euchromatin

Loosely packed DNA

Chromatin structure can determine which genes are turned off and on

• DNA must be made accessible for replication and repair

• Genes that are highly expressed are in euchromatic regions of the nucleus, while poorly expressed go into heterochromatic regions

Chromatin Structure Regulation

Needed in order to have DNA accessible while compacting everything while not in use

“Histone code”

N terminal Tails get modified for different roles to control chromosome compaction and protein interaction (both together influence gene expression)

Writer N Terminal tail modification

Carry out specific modifications

Eraser N terminal tail modificaiton

Remove modifications

Reader N Terminal Tail modificaiton

Carries out function

Are eukaryotic chromosomes supercoiled even if they are linear

Yes; they are very long so they are topologically constrained; local regions are affected and coil

What type of coiling is wrapping around the histones equivalent to?

Negative supercoiling - one net negative cupercoil