The DNA structure was proposed by Watson and Crick. According to them, the DNA is a double-helical structure with two polynucleotide strands running anti-parallel to each other. This double helix is negatively charged due to the presence of phosphate groups in the DNA backbone. The cell makes histone proteins that bind the DNA to counterbalance the negative charge. These histone proteins are involved in DNA packaging.
Read on to explore what is DNA packaging and why it is required.
DNA Packaging Definition
“DNA packaging is the process of tightly packing up the DNA molecule to fit into the nucleus of a cell.”
What is DNA Packaging?
Have you ever wondered how a DNA is present in a nucleus smaller than it? This can be explained by the process of DNA packaging.
The DNA is an organic, complex, molecular structure, found in both prokaryotic and eukaryotic cells and also in many viruses. It is a hereditary material which is found in the nucleus of the cell and is mainly involved in carrying the genetic information.
The DNA structure has the following characteristics:
- The strands of the DNA are helically wounded, every single strand forms a right-handed coil.
- The pitch of each helix is 3.32 nm and about 10 nucleotides make up one turn.
- The distance between two succeeding base pairs is 0.34 nm
- The total length of a DNA is the distance between two succeeding base pairs and the product of a total number of base pairs.
- A typical DNA has an extent around 2.2 meters, which is much longer than a nucleus.
Prokaryotic cells can be distinguished from the eukaryotic cells by the presence of a well-defined nucleus. However, their negatively charged DNA is arranged in a region called the nucleoid. They appear as a loop wrapped around a protein molecule having a positive charge.
All eukaryotes have a well-defined nucleus that contains the DNA. DNA is a negatively charged polymer, packed compactly within the chromatin engirdling the histone proteins, a ball of positively charged proteins.
The octamer of histone proteins is wrapped with DNA helix, giving rise to a structure called nucleosome. The nucleosomes are further coiled which results in the formation of chromatin fibres. Chromatin fibres are stained thread-like structures whereas nucleosomes are beads present over it. These chromatin fibres condense to form chromosomes during mitosis.
Also Read: Difference between Prokaryotic and Eukaryotic Cells
Histones are the proteins promoting the DNA packaging into chromatin fibres. Histone proteins are positively charged possessing several arginine and lysine amino acids binding to the negatively charged DNA. There are two types of Histones:
- Core Histones
- Linker Histones
H2A, H2B, H3 and H4 are the core histones. Two H3, H4 dimers and two H2A, H2B dimers form an octamer.
Linker histones lock the DNA in place onto the nucleosome and can be removed for transcription.
Histones can be modified to change the amount of packaging a DNA does. The addition of methyl group increases the hydrophobicity of histones. This results in tight DNA packaging.
Acetylation and phosphorylation make the DNA more negatively charged and loosens the DNA packaging.
Enzymes that add methyl groups to histones are called histone methyltransferases. The enzymes that add acetyl groups to the histones are called histone acetyltransferases while the ones that remove the histones are called histone deacetylases.
Also Read: Molecular Basis of Inheritance
Why is DNA Packaging required?
The length of the DNA is around 3 meters that need to be accommodated within the nucleus which is only a few micrometres in diameter. In order to fit in the DNA molecules into the nucleus, it needs to be packed into an extremely compressed and compact structure called chromatin.
During the initial stages of DNA packaging, the DNA is reduced to an 11 nm fibre that denotes approximately 5-6 folds of compaction. This is achieved through a nucleosome order of packaging.
There are three orders of DNA packaging
- The first order DNA packaging – Nucleosome.
- The second order DNA packaging – Solenoid fibre.
- The third order DNA packaging – Scaffold loop Chromatids Chromosome.
Also Read: DNA Replication
One of the benefits of DNA packaging is that it can be separated into things we use a lot and the things we don’t use at all. There are certain parts of DNA that are required only at certain times. Regions that are essential for the synthesis of proteins are loosely packed and are known as euchromatin. This helps the DNA to easily get in and make RNA. The heterochromatin has tightly packed DNA which are rarely required.
Also Read: DNA structure