Chromatin structure

information about chromatin

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 Euchromatin 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.