First level
DNA is bound to basic proteins known as histones [contain basic amino acids].The Second Level
DNA with bound histone is termed chromatin.Five classes of histones:
Histones are extremely rich in the positively charged basic amino acids lysine and arginine. The positive charge of the histones is one of the major features of the molecules, enabling them to bind to the negatively charged phosphates of the DNA.
- H1
- H2A
- H2B
- H3
- H4
The histones are highly evolutionarily conserved. Especially H3 and H4 Less so with H1.
H4 from cows differs by only 2 amino acids from H4 of peas. What you can conclude from this is that the structure/sequence of the histones (and therefore function) has not diverged in the ~ 10^9 years since plants and animals diverged.The appearance of DNA complexed to histones : beads on a string
The beadlike structures are called nucleosomes.Each nucleosome core (octameric protein disc) is found to consist of:
Two molecules each of histonesAround this core of histones a 146 base pair segment of DNA which is wrapped like a ribbon (1.75 times). DNA (linker DNA links nucleosomes one to another) 146 base pairs in association with nucleosome pretty consistent from organism to organism. The linker DNA does vary quite a bit in length.
- H2A
- H2B
- H3
- H4
H1 is then bound to the histone octamer and to linker DNA between nucleosome cores. When H1 binds, there are two full turns of the DNA (166 base pairs) around the nucleosome core particle. In this conformation the structure is referred to as a chromatosome.
The resulting chromatin fiber is approximatley 10nm in diameter. Overall length of DNA has been shortened 6X at this point. See Fig. 4.
The H1 binds and pulls the chromatosomes together by coiling into fibers which are approximately 30 nm thick. This structure is termed a 30nm chromatin fiber.Higher LevelsSee Fig. 4
H1 can be removed and the beads on a string structure stays intact but the 30 nm fiber structure is lost. Saccharomyces lacks H1 and its DNA does not form the 30nm chromatin fiber structure.
H1- is composed of an amino terminal arm, a globular central portion, and a carboxyl terminal arm.
To give you an idea of conservation of the process. Histones will even package bacterial DNA into nucleosomes in vitro! Doesn't occur naturally in bacterial cells because there is no histones.
The 30 nm chromatin fibers are further condensed by looping. Even further condensation occurs during the transistion from chromatin to the highly condensed metaphase chromosome. This transition is less well understood.Cell Cycle
As cell passes through its growth cycle, chromatin structure changes. In a non-dividing cell the chromatin is dispersed and fills the entire nucleus. The euchromatin is believed to be primarily in the 30nm chromatin fiber form. These fibers are further organized into large loops. ~ 10% of the euchromatin is in more relaxed (decondensed) state [10 nm chromatin fiber] and this is the DNA that is most transcriptionally active.
However, it is not randomly dispersed. Although interphase chromain appears to be uniformly distributed, the chromosomes are actually arranged in an organized fashion and divided into discrete functional domains that play an important role in regulating gene expression. The chromosomes are closely associated with the nuclear membrane at many sites. Individual chromosomes occupy distinct territories within the nuclei .