Nucleosome

Nucleosome is the basic unit of DNA packaging in eukaryotes. It consists of a segment of DNA wound twice around eight histone protein cores. The DNA is folded through a series of successively higher order structures into a chromosome, which allow the large eukaryotic genomes to be packed in the nucleus while still ensuring appropriate access to it. The degree of chromatin packaging determines whether or not genes within that segment are expressed, being very important to control gene expression.

The basic structure of a nucleosome consists of approximately 147 base pairs of DNA, wrapped in 1.67 left-handed superhelical turns around a histone octamer, which consists of a central tetramer with 2 dimers each of the core histones H2A, H2B, H3, and H4. Adjacent nucleosomes are connected by stretches of "linker DNA", about 80 bp long. This structure is often called “beads-on-a-string”. Higher levels of organization of the chromatin involves the linker histones, H1 providing more chromatin compaction, such as the 30 nm fiber. The N-terminal tails of the histone octamer interact with each other allowing condensation. Their acetylation reduces the affinity for DNA and the interaction between individual nucleosomes, being potentially relevant for the higher-order structure of nucleosomes and for DNA expression. In heterochromatin the histones are generally unacetylated while those in functional domains are acetylated, indicating that this type of modification is linked to DNA packaging. 

Biologia Aplicada grupo 5

Cecília Cristelo a66766;

Diana Silva a68847; 

Filipa Martins a68872