Chromatin is composed of DNA wound around histone proteins. It exists in two forms: euchromatin, which is loosely coiled and transcriptionally active, and heterochromatin, which is tightly packed and generally inactive. DNA wraps around histone proteins to form nucleosomes, which further coil to form a 30nm fiber. The 30nm fiber then loops and folds at increasing levels of compaction to ultimately form condensed metaphase chromosomes.

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.