MIMS Summary: DNA Structure

Study Notes

B-DNA is the most common form, while dsRNA adopts the A form
A-form is wider than B-form, with a more compressed major groove, wider minor groove, and tilted bases
GC-rich DNA adopts the A form

Z-DNA is a left-handed DNA structure, with a phosphate backbone that follows a zigzag trajectory

Chromatin is formed by chromosomal DNA bound to proteins like histones and other DNA-packaging factors
The basic unit of chromatin is the nucleosome
The nucleosome core particle consists of a sequence of 146nt wrapped around a barrel-shaped octamer of histone in 1.7 turns of a left-handed superhelix

Histones are positively charged
The histone octamer of the NCP particle consists of 2 H2A-H2B dimers flanking a tetramer of 2 H3 and H4
Amino-terminal tails of histones project and can be modified, leading to compaction of chromatin and control of gene transcription

10nm fibre is the most abundant chromatin form, with nucleosomes separated by linker DNA
The repeat length of nucleosomes is 200nt
10nm fibre can transition to 30nm fibre (regular/compact form), with most folds being random

DNA loops are the next stage in the organisation of DNA, anchored to a protein-chromosome scaffold
DNA loops are segregated from the genome and bring enhancers and promoters together, playing an important role in gene regulation during development and differentiation

Cohesin and condensin are SMC proteins with split ATPase domains
Two SMC proteins hinge dimerise to give a V-shape
Role in DNA loop formation
Cohesin and condensin heads are linked by a non-SMC subunit, forming a topological ring that traps DNA and forms a loop

Chromosome territory is a unique volume occupied by each chromosome in the nucleus
Chromosome territory is divided into compartments A and B
Compartment A is euchromatin, while compartment B is heterochromatin

Chromatin status is a dynamic state of chromatin (open/close etc.) and can be modified by the binding of H1, leading to compaction
Post-translational modification of histone tails can also modify chromatin status
Chromatin remodelers alter the position of nucleosomes
Lysine acetylation to histone tails reduces overall positive charge, reducing chromatin compaction
Histone tail modifications represent an epigenetic histone code, which is dynamic due to chromatin writer/eraser proteins
Nucleosome remodelling shifts nucleosome position to allow availability of DNA-binding sites