What is the function of the nucleosome in DNA? Let's find out! (2023)

We all know that a single diploid human cell can contain about 6.6 x 10⁹Base pairs of DNA that make up approximately 2.2 meters in length.

How does such long DNA fit into the cell nucleus?

This is due to the DNA packaging. The DNA is supercoiled with the help of various proteins to get a compressed shape so that it can fit and settle into the nucleus of the cell.

The DNA of all eukaryotes wraps around the positively charged histone protein octamer to form a nucleosome. Histones are rich in the positively charged amino acids lysine and arginine.

Chromatin is a dynamic, highly organized structure that plays a role in all aspects of chromatin biology and controls access to DNA. The basic repeating unit of chromatin is the nucleosome, an octamer of histone proteins wrapped twice in DNA.

Let's learn more about the nucleosome and its functions...

What is the function of the nucleosome in DNA?

1.A nucleosome is the basic structural unit of DNA packaging in eukaryotes. It helps package the DNA to fit in the cell nucleus.

2.The nucleosome forms the basic repeat unit of chromatin as it consists of approximately 146 base pairs of DNA wrapped around a core of 8 histone proteins.

3.The packaging of DNA into nucleosomes shortens the length of the DNA fiber by about 7-fold. In other words, a 1 meter long piece of DNA becomes a chromatin fiber only 14 centimeters long.

4.In the nucleus, the nucleosome's histone proteins provide the electrostatic energy to fold DNA. As a result, chromatin can be compacted into a much smaller volume than DNA alone.

5.Nucleosomal histone proteins are basic proteins, and their positive charge allows them to associate with DNA that is negatively charged. Some histones act as spools around which thread-like DNA winds.

6.Nucleosomes help control DNA replication and transcription. Because the entire length of DNA is condensed by nucleosomes, only a small section of DNA can be replicated and transcribed, creating a barrier to enzymes that unwind and copy the DNA. As a result, not all DNA is involved in replication and transcription, which is very important to maintain gene expression in the biological body.

7.The nucleosome gives DNA the ability to coil and uncoil as needed. It is important to remember that these processes are reversible, allowing modified or remodeled chromatin to return to its compact state after transcription and/or replication is complete.

8.Nucleosomes in chromatin fibers look like a "beads on a string" structure under the electron microscope. These chromatin fibers can further condense with the help of nucleosomes in the metaphase of cell division, resulting in the formation of chromosomes.

9.Nucleosome positions in the genome are a very important factor in determining DNA accessibility during gene expression. Nucleosomes regulate the accessibility of the transcription machinery to DNA and act as the backbone in the complex regulation of gene expression.

10Nucleosomes carry epigenetically inherited information in the form of chemical modifications to their nuclear histones. The transcriptional processing of DNA during gene expression is largely controlled by the epigenetic regulation of nucleosomes.

What are nucleosomes? What are they made of?

Nucleosomes are the basic packaging unit of DNA, made up of 8 histone proteins (histone octamer) around which the helical strand of DNA is wound.

Histone octamer is made from the amino acids lysine and arginine, which give it a positive charge.

Nucleosomes are needed to fit DNA into the cell nucleus. This nucleosome helps package the long strand of DNA into a tightly packed structure known as chromatin.

A nucleosome is simply a structure formed by interactions between DNA and histone proteins. DNA is negatively charged and histone proteins impart a positive charge to the core of the nucleosome, causing the DNA to surround the nucleosome.

A nucleosome is formed by both the DNA strand and the histone octamer. Here a histone octamer is formed by the presence of a histone pair H2A, H2B, H3 and H4, although in some cases other histone variants can also be found in the nucleus (e.g. H2A.Z , MacroH2A, H2a.Bbd , H2A .lap1, H2A .X, H3.3, CenH3 and others).

A chain of nucleosomes can be assembled into a 30 nm fiber, a compact structure whose formation depends on the presence of histone H1.

A 147 bp segment of DNA then wraps around the histone octamer about 1.75 times, completing the formation of a single nucleosome.

The DNA double helix in each cell is tightly bound to an equal mass of histone proteins that form nucleosomes. Nucleosomes are compacted (twisted) to form a chromatin fiber of many nucleosomes.

Several of these chromatin fibers continue to coil to form a chromatid strand of a chromosome. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46.

What is the function of histones in the DNA nucleosome?

1.The histone provides structural support for the DNA to wrap around its octamer (8 histone proteins), resulting in the folding of the DNA to fit into the nucleus.

2.Histones impart a positive charge to the nucleosome and hence the negatively charged strand of DNA surrounds the histone octamer. Electrostatic interactions between the histone octamer and the coiled DNA can be seen.

3.Some histone variants are associated with the regulation of gene expression. And that's why they are very important for gene regulation.

4.The histones act as a tight latch to hold the DNA and unroll it, reducing the length of the DNA. And when that lock is opened or removed, a gene comes out. Thus, they ultimately have very important functions, not only structurally, but also in regulating gene function upon expression.

5.The histone modifications are called the histone code. Although not much is known about these modifications, gene regulation has emerged as one of their purposes.

6.The addition of methyl (-CH₃) attached to a histone can turn gene expression on or off, depending on which histone is modified by a methyl group. For example, adding 3 methyl groups to H3 will repress gene expression for any gene close to methylated histone H3.

7.There are five histone families (H1 to H5). H2A, H2B, H3 and H4 are the core histones and H1 and H5 are the binding histones. The nuclear histones make up the nucleus of the nucleosome, hence the term "nucleus". Linker histones are found at the entry and exit points of the nucleosome and fix the DNA in place, hence the term "linker".

8.Histones are alkaline (basic) in nature. The highly basic nature of histones not only supports histone-DNA interactions, but also contributes to their aqueous solubility.

9.In addition to organizing DNA and making it more compact, histones also play an important role in determining which genes are active.

Why is DNA packaging so important?

A very important function of the nucleosome is DNA packaging. The histones that make up the nucleosome help in a variety of mechanisms, both physical and chemical, to coil and supercoil DNA to package it.

Why is DNA packaging so important?

In the nucleus of every living human cell is DNA that is 2.2 meters long. DNA packaging allows the 7 billion DNA base pairs in each diploid cell to fit into a space just 6 microns wide.

DNA packaging is very necessary as this process saves a lot of space for other biological activities in the cell and carries a lot of genetic information.

In addition, the packaging of DNA does not allow for rapid replication and transcription until required, since it is very important to preserve the genetic information stored in DNA and also to make room for other cellular activities.

When needed, a small sequence of DNA is unpacked using little biochemical energy and enzymatic reactions, and only that part of the DNA is replicated and transcribed without affecting the full length of the DNA.

The DNA packaging is also very important as the two DNA strands cannot fully unwind due to the high energy requirement. This is very important to store the body's biochemical energy for other metabolic activities as well.