What is a Nucleosome and its Structure?
A nucleosome can be defined as the basic unit of eukaryotic chromatin that keeps on repeating. In the cells of human beings, a total of about 6 feet of the DNA has to be packed into the nucleus that has a diameter that is less than a single strand of human hair. There are about 150 different base pairs of DNA sequences present in one nucleosome.
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These DNA base pairs are wrapped properly around the histone protein cores which are present in the nucleosome. The arrangement of the nucleosomes resembles that of beads that are arranged on a string. These nucleosomes are repeatedly folded in order to create the chromosome which provides us with genetic characteristics. In this article, students will learn about DNA packaging nucleosomes and chromatin 310 along with some other details about nucleosomes.
What is Nucleosome?
The nucleosomes can be defined as the building blocks which are responsible for the structuring and packing of the DNA that is held within the chromosome. The entire idea of fitting a lengthy stretch of DNA material that resembles the size of an entire DNA yard into a smaller nucleosome is pretty fascinating to most scientists. Everyone knows that the size of the cell is pretty small when compared to the size of the DNA. In fact, the cell has a hundredth of a millimetre of diameter.
So, how does the long stretch of DNA fit into the smaller cell? Well, what the cell does is it coils the DNA stretch again and again. There are so many different and complex ways to do that. These fundamental blocks of cell-building can be called nucleosomes. These are the blocks that contain a core of histone proteins. The DNA wraps itself around these histone proteins in the nucleosome and hence the formation of the chromosome happens one small step at a time. The structure of the nucleosome resembles that of beads being put into the string. However, the DNA wraps around the strings instead of going through them.
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In the present times, the researchers have figured out the structure of nucleosome to be as follows: Two histones each from H2A, H3, H2B, and H4 are grouped together in order to create the histone octamer. This octamer will then bind and wrap about 1.7 turns of the DNA which consists of about 146 base pairs. With the addition of another histone known as H1, about 20 base pairs of the DNA will be wrapped properly. This means that the DNA will be wrapped in two full rounds. This leads to the creation of the structure which is called chromatosome. The 166 pairs in total don’t seem much long since a single chromosome consists of about 100 million DNA base pairs in total. Therefore, the chromosomes are made up of thousands of nucleosomes which are combined together with the help of the DNA coiling around them. The DNA that is used in the process of joining is known as linker DNA. Every single one of the chromosomes contains a lengthy chain of nucleosomes. Studying the diagram of nucleosome will help students understand the topic in a better way.
In order to find out the total base pairs of DNA in the nucleosome, scientists treated chromatin with a particular type of enzyme that can cut the DNA. These enzymes are known as DNases. The enzyme micrococcal nuclease or MNase has a very special property. It cuts the DNA which acts as a linker for the nucleosome right before it begins the process of cutting the DNA wrapped around the histone octamers.
With the proper regulation of the cutting amount after applying the enzyme, the reaction can be stopped before every single linker DNA is cut. In this case, the chromatin will just have some mononucleosomes as well as dinucleosomes and trinucleosomes along with some others. In case the DNA is separated and kept on a gel, certain bands tend to appear on the structure. There is a very simple explanation for this incident and that is the fundamentally repeating structure of chromatin.
Nucleosome and its Functions
Everyone knows that nucleosome consists of the histone proteins around which the DNA will wrap itself around. These nucleosomes are then repeatedly folded in order to create the chromosomes that provide genetic characteristics to human beings. So, the main function of the nucleosomes is to ensure that the long stretches of the DNA are able to fit properly inside the cell and create the basic unit of life that allows us to live and breathe as we are today.
Conclusion
Nucleosomes can be defined as structures that help the DNA in binding with the histone protein and creating the chromosomes. From this article mentioned above, students will be able to know about DNA packaging nucleosomes and chromatin 310.
FAQs on Nucleosome
1. What is a nucleosome as per the CBSE Class 12 syllabus for 2025-26?
A nucleosome is the fundamental structural unit of DNA packaging in eukaryotic cells. It consists of a segment of DNA wound around a core of eight histone proteins, known as a histone octamer. This structure resembles 'beads on a string', where the nucleosomes are the beads and the linker DNA between them is the string. This compact arrangement is the first and most basic level of chromosome condensation, allowing a large amount of DNA to fit within the cell's nucleus.
2. What is the primary function of a nucleosome in a cell?
The primary function of a nucleosome is to compact DNA. Eukaryotic DNA is extremely long; for example, a human cell's DNA can be about 2 metres long. Nucleosomes wrap this DNA into a much smaller, more manageable volume to fit inside the nucleus. Beyond compaction, the nucleosome structure also plays a crucial role in regulating gene expression by controlling which sections of DNA are accessible to transcription enzymes. For more details on DNA packaging, you can refer to the notes on the Molecular Basis of Inheritance.
3. What are the key chemical and structural components of a nucleosome?
A nucleosome has two main components:
Histone Octamer: This is the protein core around which the DNA wraps. It is composed of eight histone proteins: two copies each of H2A, H2B, H3, and H4. These histone proteins are rich in positively-charged amino acids, which helps them bind to the negatively-charged DNA.
DNA Segment: Approximately 147 base pairs of negatively-charged DNA helix make nearly 1.65 left-handed turns around the histone octamer.
Additionally, a short stretch of linker DNA connects one nucleosome to the next.
4. How does a nucleosome differ from chromatin and a chromosome?
The key difference lies in their level of organisation and compaction:
Nucleosome: The most basic unit. It is DNA wrapped around a single histone octamer.
Chromatin: The next level of organisation. It is a long fibre composed of a series of nucleosomes linked together, resembling 'beads on a string'. This fibre is then further coiled and folded.
Chromosome: The highest level of DNA compaction. Chromatin undergoes extensive coiling and condensation to form the dense, X-shaped structures called chromosomes, which are visible during cell division.
In essence, nucleosomes build chromatin, and chromatin builds chromosomes.
5. Why is negatively charged DNA able to wrap so tightly around histone proteins?
The tight wrapping is due to a fundamental electrostatic attraction. DNA has a net negative charge because of the phosphate groups in its sugar-phosphate backbone. In contrast, histone proteins are rich in basic amino acids like arginine and lysine. These amino acids have positively charged side chains at physiological pH. The strong attraction between the negatively charged DNA and the positively charged histones allows the DNA to wrap tightly and stably around the protein core.
6. How does the nucleosome structure influence gene expression and regulation?
The nucleosome structure is a key factor in epigenetic regulation. The degree of DNA compaction determines whether a gene can be 'read' or not.
Gene Silencing: When nucleosomes are packed tightly together (forming heterochromatin), the DNA is not accessible to RNA polymerase and other transcription factors, effectively silencing the genes in that region.
Gene Activation: When nucleosomes are spaced farther apart (forming euchromatin), the DNA is exposed and accessible, allowing for active gene transcription.
Chemical modifications to histone tails can alter this packing, making the nucleosome a dynamic regulator of gene activity.
7. What is the specific role of the H1 histone, and is it part of the nucleosome core?
The H1 histone is often called the linker histone and is not part of the core nucleosome particle (the octamer). Its role is to bind to the linker DNA region where the DNA strand enters and exits the nucleosome core. The binding of H1 helps to lock the DNA into place and further compacts the chromatin by pulling adjacent nucleosomes closer together, facilitating the formation of the more condensed 30-nanometer fibre.
