  Recent progress has been made in the understanding of the physical properties
of chromatin -- the dense complex of DNA and histone proteins that occupies the
nuclei of plant and animal cells. Here I will focus on the two lowest levels of
the hierarchy of DNA folding into the chromatin complex: (i) the nucleosome,
the chromatin repeating unit consisting of a globular aggregate of eight
histone proteins with the DNA wrapped around: its overcharging, the DNA
unwrapping transition, the ''sliding'' of the octamer along the DNA. (ii) The
30nm chromatin fiber, the necklace-like structure of nucleosomes connected via
linker DNA: its geometry, its mechanical properties under stretching and its
response to changing ionic conditions. I will stress that chromatin combines
two seemingly contradictory features: (1) high compaction of DNA within the
nuclear envelope and at the same time (2) accessibility to genes, promoter
regions and gene regulatory sequences.
