Each histones, which include H1, H2A, H2B, H3,

Each of your cells has about 2 meters of linear DNA.

This has to be packed into a nucleus roughly 10 micrometers in diameter. This means your nucleus must be excellent at packaging your genetic material! This lesson tells you how your DNA is packaged.

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Nucleosomes, Chromatin, and Chromosomes

A nucleosome is a structure in your chromosomes, or bundled DNA. Each nucleosome has a core particle, DNA, and a linker protein. The proteins in the core particle and linker proteins are called histones. The DNA will wrap around the core particle about 1.65 times and is secured by the linker protein.

This figure shows a drawing of a nucleosome.

Nucleosome Structure
Image of a nucleosome

About 200 bases of DNA are involved with each nucleosome. This includes the portion that is wrapped around the core and a bit of a tail region that connects to the next nucleosome. This arrangement is said to look like beads on a thread. Several nucleosomes together are called chromatin. Chromosomes are bundles of tightly packed chromatin.

Humans have 23 pairs of chromosomes.

Histones

The protein portion of a nucleosome is made of histones. There are five major families of histones, which include H1, H2A, H2B, H3, H4, and H5. The core particle has eight total histones.

One H2A and H2B bind together to form a dimer, or two proteins bound together. An H3 and an H4 will bind to also form a dimer. Next, one H3/H4 dimer binds to another H3/H4 dimer to form a tetramer, or four proteins bound together.

Finally, an H2A/H2B dimer will bind an H3/H4 tetramer to create the core particle.After DNA wraps around the core particle either H1 or H5 will bind. The purpose of H1 and H5 is to secure the DNA strand to the core particle. Either H1 or H5 can do this function.

Nucleosomes and Types of Chromatin

DNA arranged in nucleosomes is called chromatin. The histones in nucleosomes interact to form more complex structures of chromatin. As a result, there are several types of chromatin. The type depends on the phase of the cell cycle and how much the genes in a particular region of DNA are needed for cell function.When a cell prepares to divide it must first compact its DNA into chromosomes.

This ensures that the DNA is not broken and helps ensure transfer into daughter cells. DNA in this form is called chromosomal chromatin.When a cell is not undergoing cell division, its DNA is not packaged into chromosomes. This part of the cell cycle is called interphase. Thus, DNA in this form is called interphase chromatin.

The DNA is spread out throughout the nucleus and looks like a plate of spaghetti.Interphase chromatin can be of two types. The first is a tightly bound version called heterochromatin. This is also called silent chromatin because the DNA is not accessible to the protein-making machinery of the cell. Therefore, the genes in heterochromatin are not expressed, or silent.The second type of interphase chromatin is called euchromatin.

This version of chromatin is more loosely packed than heterochromatin. This is also called active chromatin because the genes are accessible to the protein-making machinery of the cell. Thus, the genes in euchromatin are expressed, or active. This figure shows these different types of chromatin.

Different Types of Chromatin
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