Molecular biologists can determine experimentally whether a region of dna contains closed chromatin or open chromatin by assessing the sensitivity of the region to

The nucleoplasm, surrounded by the nuclear envelope, contains chromatin, which can be considered as DNA (deoxyribonucleic acid) plus all molecules related to its organization, mainly histones. The DNA is formed by 4 deoxyribonucleotides (abbreviated as nucleotides). Each nucleotide contains a succession of three components: base, pentose and phosphate group. The bases are four, two puric: adenine (A) and guanine (G), and two pyrimidines: thymine (T) and cytosine (C) (Figure 1). The pentose is the deoxyribose. Each base binds to a pentose form forming a deoxyribonucleoside. Each deoxyribonucleoside is linked to a phosphate group by a carbon from the pentose forming a deoxyribonucleotide. Thus, a DNA chain is composed of a succession of nucleotides linked together by phosphate groups. This is a simple chain but the DNA is formed by two simple chains thanks to the complementarity that exists between the bases A and T and between G and C, whatever the hydrogen bridge type junctions. It is arranged as the tracks of a train, where the phosphate-ribose chains are the tracks and the hydrogen base-bridges are the crossbars. But the two strands are antiparallel, that is, at the ends we have 3 'carbon of one chain and 5' of the other. Both, in addition, are wound in the form of double helix of about 2.5 nm in width.

Nucleotides are not only in the DNA. They may be part of other molecules with totally different functions. For example, ATP (adenosine triphosphate) is the energy transfer molecule by exelence in cells, or cAMP (cyclic adenosine monophosphate) is a very important second cell messenger.

The DNA is not free in the nucleus but associated with proteins such as histones and other proteins involved in its processing, forming together the chromatin. Histones are proteins associated with DNA that determine their organization. There are two types: the nucleosomes that are four (H2A, H2B, H3 and H4) and histone H1. The four nucleosomal histones are responsible for forming together with the DNA the so-called nucleosomes, which are the basic structural unit of chromatin. To a lesser extent there are other proteins that may be associated with DNA. Among them are all proteins responsible for expression (transcription), synthesis (replication) and DNA packaging.

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