Chromatin is composed of DNA wrapped around histone proteins, which allows it to be tightly packed in the cell nucleus. There are two main types of chromatin: euchromatin, which is loosely coiled and allows for transcription; and heterochromatin, which is tightly packed and generally not transcribed. DNA combines with histone proteins to form nucleosomes, which involve 146bp of DNA wrapped around an octamer of core histone proteins. Nucleosomes further fold into a 30nm fiber, which then loops and coils to allow the long DNA molecules to fit inside the cell nucleus.

1. CHROMATIN STRUCTURE
• Chromatin is composed of DNA and proteins, mostly basic proteins called histones
• That help chromatin fold so it can pack into the tiny volume of a cell’s nucleus.
• The two basic types of chromatin are
1. Euchromatin
2. Heterochromatin

2. EUCHROMATIN
• The chromatin fibres in this region are loosely coiled .
• Euchromatin undergoes the normal process of condensation and de condensation in the cell
cycle.
• Euchromatin constitutes the majority of the chromosomal material and is where most
transcription takes place.

3. HETEROCHROMATIN
• The chromatin fibres in this region are more tightly folded
• Heterochromatin remains in a highly condensed state throughout the cell cycle, even during
interphase.
• All chromosomes have heterochromatin at the centromeres and telomeres.
• In addition to remaining condensed throughout the cell cycle, heterochromatin is characterized
by a general lack of transcription.

4. HISTONES
Most eukaryotic cells contain five different kinds of histones: H1, H2A, H2B, H3,and H4.
These are extremely abundant proteins; the mass of histones in eukaryotic nuclei is equal to the
mass of DNA.

5. NUCLEOSOMES
• The total length of human DNA, if stretched out, would be
about 2 m, and this all has to fit into a nucleus only about 10
mm in diameter.
• In fact, if you laid all the DNA molecules in your body end
to end, they would reach to the sun and back hundreds of
times.
• DNA folding must occur in your body and in all other living
things. We will see that eukaryotic chromatin is indeed
folded in several ways.
• Eukaryotic DNA combines with basic protein molecules
called histones to form structures known as nucleosomes.
These structures contain four pairs of core histones (H2A,
H2B, H3, and H4) which wrapped a stretch of about 146 bp
of DNA.

6. 30-NM FIBER
• Nucleosomes fold on themselves to form a dense,
tightly packed structure that makes up a fiber with
a diameter of about 30 nm .
• Two different models have been proposed for the
30-nm fiber:
• A solenoid model, in which a linear array of
nucleosomes are coiled,
• Helix model, in which nucleosomes are arranged in
a zigzag ribbon that twists or supercoils. Recent
evidence supports the helix model

7. HIGHER-ORDER CHROMATIN FOLDING
• The next-higher level of chromatin structure is a series of loops of 30-nm fibers.
• On average, each loop encompasses some 20,000 to 100,000 bp of DNA and is about 300 nm in
length.
• The 300-nm loops are packed and folded to produce a 250-nm-wide fiber.
• Tight helical coiling of the 250-nm fiber in turn produces the structure that appears in metaphase
individual chromatids approximately 700 nm in width.