Packaging of DNA Helix: Histones & Importance

DNA packaging refers to the process through which DNA molecules are tightly compacted into a smaller volume so that they can fit into the nucleus of a cell. DNA packaging is important because the length of DNA molecules is much greater than the size of the cell nucleus, and therefore, if the DNA were not packaged properly, it would be impossible for it to fit inside the nucleus. The degree of DNA packaging is regulated and can have a significant impact on gene expression

What is DNA Packaging?

DNA is a complex organic molecule structure present in both prokaryotic and eukaryotic cells, as well as numerous viruses. It is a hereditary substance present in the nucleus of the cell that is primarily engaged in the transmission of genetic information.

About 3 billion base pairs of DNA make up the human genome, which must be compactly arranged to fit inside the cell nucleus. The fundamental building blocks of DNA packaging, nucleosomes, are created by first enclosing the DNA around a collection of proteins known as histones. Eight histone proteins make up the centre of each nucleosome, around which 1.65 times the amount of DNA is wrapped. The linker DNA, or DNA in between the nucleosomes, is subsequently further compacted and wound to form chromatin fibres.DNA packaging is further compressed and organized into observable chromosomes during cell division. Two identical DNA molecules linked together at a location known as the centromere make up each chromosome.

Specialised proteins, enzymes, and epigenetic alterations control the packing of DNA. For instance, alterations to the DNA or the histone proteins might influence how accessible and compact the DNA sequence is, hence regulating gene expression.

Why is the DNA package required?

Our cells’ DNA is extraordinarily lengthy, and if it were not correctly packed, it would not fit within the nucleus. Furthermore, proper DNA packaging is critical for gene expression regulation, which is essential for cell function. The wrapping of DNA around histone proteins to create nucleosomes is the first stage of DNA packaging. Nucleosomes are the fundamental units of chromatin, the substance that forms chromosomes. Histones are positively charged proteins that attract and attach to the negatively charged phosphate groups on the DNA backbone, keeping the DNA in place and forming a compact, readily organized structure. The nucleosomes are subsequently arranged into a more complicated structure called chromatin.

Histone

Histones are a class of tiny, positively charged proteins that play an important role in the packing and organisation of DNA within eukaryotic cell nuclei. They are the most important protein components of chromatin, which is the substance that forms chromosomes.

Histones are highly basic proteins present in eukaryotic cell nuclei that are rich in lysine and arginine residues. They function as spools around which DNA coils to form structural units known as nucleosomes. Nucleosomes are then bundled into 30-nanometer fibres, which create densely packed chromatin. Histones keep DNA from becoming twisted and shield it from harm. Furthermore, histones play critical roles in gene regulation and DNA replication. Unwound DNA in chromosomes would be very lengthy if histones were not there. Each human cell, for example, contains around 1.8 metres of DNA if completely stretched out; but, when twisted around histones, this length is reduced to approximately 90 micrometres (0.09 mm) of 30 nm diameter chromatin fibres.

Histones are classified into five families: H1/H5 (linker histones), H2, H3, and H4 (core histones). Two H2A-H2B dimers and an H3-H4 tetramer make up the nucleosome core. The tightly wrapped DNA around histones is mostly due to electrostatic interaction between the positively charged histones and the negatively charged phosphate backbone of DNA.

Structure of DNA

Most creatures have DNA or Deoxyribonucleic acid as their genetic material, whereas viruses have RNA or Ribonucleic acid. A DNA molecule is made up of two polynucleotide chains, which are chains that contain numerous nucleotides. Let’s take a closer look at the chain’s structure.

Structure Of Polynucleotide Chain

A nucleotide is made of the following components:

• Pentose sugar – A pentose sugar is a 5-carbon sugar. In the case of DNA, this sugar is deoxyribose whereas, in RNA, it is ribose.
• Phosphate group
• Nitrogenous bases – Purines and Pyrimidines are the two categories. Adenine and Guanine are purines, while Cytosine and Thymine are pyrimidines. Thymine is replaced by Uracil in RNA. Nucleoside = nitrogenous base + pentose sugar (through N-glycosidic bond).

The free phosphate group at the 5′ ends of a polynucleotide is referred to as the 5′ ends. Similarly, the sugar possesses a free 3′-OH group at the polynucleotide’s opposite end, known as the 3′ ends. The backbone of a polynucleotide chain is made up of pentose sugars and phosphate groups, from which nitrogenous bases extend.

FAQs on DNA Packaging

Question 1: What is Double Helix Structure?

Answer:

In molecular biology, the structure created by double-stranded molecules of nucleic acids like DNA is referred to as a “double helix.” A nucleic acid complex’s double helical helix results from its secondary structure, 

Q2: What happens if DNA is not packaged properly?

Answer:

Inadequate packaging of DNA can cause it to become tangled and damaged, which can result in mutations and perhaps lead to genetic diseases.

Q3:  Can DNA packaging be modified?

Answer:

Yes, epigenetic alteration, a process, can change the way DNA is packaged. This may have an impact on gene expression and contribute to the emergence of specific diseases.