ChroChromosome Oragnisation mosome Oragnisation

What chromosome is made of?What chromosome is made of?

- chromosome is made of chromatin (nucleic acid and protein).

- when in mitotic and meiotic stages of cell cycle, chromatin (euchromatin & heterochromatin) stains readily.

Chromosome PackagingChromosome Packaging

Chromatin is organised on three basic levels:

- primary (nucloesome)

- secondary (solenoid)

- tertiary/quaternary (final folding into chromosome shape)

A typical eukaryotic chromosome contains 1 to 20 cm of DNA. During metaphase of meiosis/mitosis, this DNA is package into a chromosome with a length of only 1 to 10 m (a condensation of almost 104-fold in length from the naked DNA molecule).

Chromosome PackagingChromosome Packaging

Base pairing

Chromosome PackagingChromosome Packaging

double strand

Chromosome PackagingChromosome Packaging

double helix

Chromosome PackagingChromosome Packaging

beaded string (beads-on-a-string) – nucleosome structure of chromatin

Chromosome PackagingChromosome Packaging

nucleosome

Chromosome PackagingChromosome Packaging

- nucleosome contains histones (2 of each H2A, H2B, H3 and H4, and one H1)

- histones (102 amino acid residues; containing up to 20% lysine and arginine) are highly positively charged – to counteract the negative charges of the phosphate groups on the double helix backbone.

- modification (acetylation, methylation or phosphorylation) of histone tail will affect the level of positive charge to regulate DNA compaction.

Chromosome PackagingChromosome Packaging

- the diameter of nucleosome (bead) is 11nm.

- 200 bp of DNA associated with one bead, 145 bp are associated with the histone core, 23 bp protected by H1 and 8 to 114bp (depends on species and type of cells) form a linker between beads.

- the rate of DNA winding is 1.7 turns per core particle, producing a six- to sevenfold reduction in the length of DNA.

Chromosome PackagingChromosome Packaging

Second level of chromatin organization: solenoid

- the nucleosome is supercoiled and organised into a solenoid structure, with 6-7 nucleosomes per turn.

- H1 stabilizes the structure of solenoid.

- the supercoiling produces a fibre of approximately 30nm in diameter.

Chromosome PackagingChromosome Packaging

Higher order folding of chromatin into chromosome

What are the roles of chromosome What are the roles of chromosome packaging? packaging?

Roles of Chromosome PackagingRoles of Chromosome Packaging

Protection

Compaction

DNA metabolism