Biology - Dna and Chromosomes

What changes in nucleosomes allow access to DNA?

1. ATP-dependent chromatin-remodelling structures (change position of nucleosomes on the DNA)

2. Reversible chemical modifications of histones (addition or removal on the N-tails of acetyl, phosphate or methyl groups)

What does the acetylation of lysine do ?

It reduces the affinity of the tails for adjacent nucleosomes, thereby loosening chromatin structure and allowing access to particular nuclear proteins

What does heterochromatin contain?

Contains genes who have been silenced, including genes that were active in early embryogenesis. Heterochromatin is usually around centromere and telomere areas

How can heterochromatin spread out?

Histone modifying complexes recognise heterochromatin specific modifications (like trymethylation of Lysine 9 in the tail of H3), and then add these modifications to adjacent nucleosomes. Then additional histone-modifying protein complexes can spread a wave of condensed chromatin which propagates along the chromosome. This continues until heterochromatin encounters a barrier DNA sequence that stops the propagation.

What do conserved sequences not coding for proteins produce?

They are known to produce untranslated RNA molecules (long noncoding RNAs) which are thought to have an important function in regulating gene transcription

What are the different levels of chromatin organisation?

1. Nucleosome: formed of an histone octamer and 147 nucleotide sequences wrapped around it + an H1 linker histone (Interphase chromosomes)

2. Cohesin: Next level of chromatin organisation into loops. The extrusion of the rings is made possible by a ring complex trough which a chromatin fibre can pass. At the core of this complex are the SMC proteins that form the ring and together with other proteins through ATP hydrolysis motor along DNA pushing out a loop of DNA. The SMC complex that organises the structure of interphase chromosomes is called Cohesin. Multiple cohesins load onto each interphase chromosome and produce extended loops. They travel along DNA until they encounter a sequence-specific clamp protein. Cohesins localised near the centromere play a crucial role in holding sister chromatids together (cessation for chromosome segregation in mitosi).

3. Condensin: Second set of ring proteins, build around SMC proteins, contain different SMC's and assessory proteins than do the Cohesins. Final level of organisation that produces mitotic chromosome structure. They replace, as cells prepare to divide, cohesins. Then they use ATP hydrolysis to form loops within loops, which is a tighter mass of coils.

What is the most extreme example of the use of Heterochromatin?

The X inactivation in female cells