Table of Contents
Introduction
DNA structure was proposed by Watson and Crick. According to them, DNA is a double-helical structure consisting of two polynucleotide fibers that work against each other. This double helix is badly charged due to the presence of phosphate groups in the DNA sequence. The cell makes histone proteins that bind to DNA to balance negative charging. These histone proteins are involved in DNA synthesis.
What is DNA Packaging?
Have you ever wondered how DNA exists in the smallest nucleus? This can be explained by the process of DNA packaging.
DNA is a living, complex, cellular organization, found in both prokaryotic and eukaryotic cells and in many viruses. It is a genetic material found in the cell nucleus and is deeply involved in the management of genetic information.
The structure of DNA has the following characteristics:
- The strands of DNA are badly damaged, each strand making the right coil.
- The pitch of each helix is 3.32 nm and about 10 nucleotides form a single curve.
- The distance between the next two basic pairs is 0.34 nm
- The total length of DNA is the distance between the next two primary pairs and the product of the total number of base pairs.
- Normal DNA has a diameter of 2.2 meters, which is much longer than the nucleus.
Prokaryotic cells can be separated from eukaryotic cells by the presence of a well-defined nucleus. However, their faulty DNA is arranged in a space called a nucleoid. They look like a loop wrapped in a protein molecule with a direct charge.
All eukaryotes have a well-defined nucleus containing DNA. DNA is a polymer with a negative charge, packed together inside a chromatin that combines histone proteins, a ball of energized proteins.
The octamer of histone proteins is wrapped in a DNA helix, creating a structure called the nucleosome. Nucleosomes are recombined leading to the formation of chromatin fibers. Chromatin fibers are a thread-like structure while nucleosomes are beads on top of it. These chromatin fibers thicken to form chromosomes during mitosis.
Histones
Histones are proteins that promote DNA packing in chromatin fibers. Histone proteins are well charged that they contain a small amount of arginine and lysine amino acids that bind to the badly charged DNA. There are two types of Histones:
- Core Histones
- Linker Histones
H2A, H2B, H3, and H4 are primary histones. Two dimers for H3, H4, and two H2A, H2B make an octamer.
Linker histones lock DNA in place in the nucleosome and can be removed for transcription.
Histones can be modified to change the amount of DNA packaging. The addition of a methyl group increases the hydrophobicity of histones. This results in strong DNA packaging.
Acetylation and phosphorylation make DNA very poorly charged and loosen DNA packaging.
Enzymes that add methyl groups to histones are called histone methyltransferases. Enzymes that add acetyl groups to histones are called histone acetyltransferases while those that remove histones are called histone deacetylases.
Why is DNA Packaging needed?
DNA length is around 3 meters that need to be inserted into a nucleus that is a few micrometers wide. In order to enter the DNA molecules in the nucleus, they need to be packed in a highly compressed and compact structure called chromatin.
During the early stages of DNA packing, DNA is reduced to 11 nm fiber which means approximately 5-6 folds of bonding. This is achieved by ordering the packing nucleosome.
There are three instructions for DNA packing
- The first order of DNA packing – Nucleosome.
- The second order of DNA packing – Solenoid fiber.
- Third-order of DNA packing – Scaffold loop Chromatids Chromosome.
One of the benefits of DNA packing is that it can be divided into the things we use the most and the things we do not use at all. There are certain parts of DNA that are only occasionally needed. The regions that are essential for protein synthesis are loosely packed and are known as euchromatin. This helps the DNA to enter easily and form RNA. Heterochromatin contains the strongest DNA that is not needed.
FAQs
What do you understand about DNA packaging?
DNA packing is the wrapping of the DNA of an organism into a compact structure that can fit into the nucleus of a cell.
How is DNA compiled into a cell?
Cells wrap their DNA around scaffold proteins to form a complex structure called chromatin. Chromatin is also synthesized to form different structures that eventually form chromosomes.
What is the significance of DNA packing?
DNA packing is important because DNA is too long. For DNA to enter the nucleus, it needs to be packed properly.
What is the role of histones in DNA synthesis?
Histones are proteins responsible for DNA packaging. DNA binds histones. Histones are positively charged proteins and therefore can easily bind to badly charged DNA. Histones are also involved in regulating gene expression.
