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 for the different life process including the development, reproduction, growth, and functioning of all living organisms.
In all eukaryotes, the DNA present within the nucleus is distributed between a pair of each chromosome. Every pair of a chromosome consists of a single, long linear DNA molecule associated with proteins that are involved in folding and packing of the DNA threads into a more compact structure.
In the year 1953, the DNA structure was suggested by Watson and Crick. According to their discovery, the DNA is a double helix, the long molecule made up of a double-helical structure of two polynucleotide chains or strands running anti-parallel to each other.
The two strands of DNA are connected by Hydrogen bonds between the complementary nitrogenous bases. The Adenine pairs up with Thymidine with the help of two H-bonds, and Guanine pairs up with Cytosine with the help of three H-bonds.
DNA is a polymer of nucleotides, namely the deoxyribonucleotides. The term deoxyribonucleotides can be divided into two words i.e., deoxyribose and nucleotides, where deoxyribose is sugar and nucleotide are made up of a nitrogenous base, a sugar molecule, and phosphate group.
Also Read: Molecular Basis of Inheritance
Have you ever wondered how a DNA is present in a nucleus smaller than it?
DNA packaging is the process of tightly packing up the DNA molecule to fit into the nucleus of a cell.
As mentioned above:
- 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 are distinguished from the eukaryotic cells for lacking a well-defined nucleus. However, their DNA which is negatively charged are arranged properly in a region called nucleoid. They appear as a loop wrapped around a protein molecule having positive charges.
All eukaryotes have a well-structured nucleus and other cell organelles. Hence, their DNAs are arranged systematically. DNA is a negatively charged polymer which is compactly packed inside the chromatin, around a ball of positively charged proteins known as histone proteins.
The octamer of histone proteins is wrapped with DNA helix giving rise to a structure called nucleosome. The nucleosomes makeup to a repeating structure which results in the formation of chromatin. Chromatin is a stained thread-like structure whereas nucleosomes are beads on that. These chromatin fibres later, during cell division, condense to form chromosomes.
Histones are the proteins that facilitate the packaging of DNA into chromatin fibres. Histone proteins are positively charged and have many arginine and lysine amino acids that bind to the negatively charged DNA. Histones are of two types:
- 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 how much packing can a DNA do. The addition of methyl group makes the histones more hydrophobic. Increasing histone methylation causes the tight packaging of histones. Acetylation and phosphorylation make the DNA more negatively charged and loosens the DNA packaging.
Enzymes that add methyl groups to histones are called histone acetyltransferases. The enzymes that add acetyl groups to the histones are called histone acetyltransferases while the ones that remove the histones are called histone deacetylases.
Why is DNA Packaging required?
The length of the DNA which is around 3 meters long and it has to be packed within the nucleus which is few micrometres in diameter. Therefore, in order to fix the huge DNA molecules into the minute nucleus, it needs to be packed into an extremely compressed or compacted structure called chromatin.
During the initial stage of the DNA packaging, the DNA is reduced into 11 nm fibre which denotes an approximate 5-6 –folds of compaction. This is achieved through a nucleosome order of packaging.
There are three orders of packaging
- The first order of packaging – Nucleosome.
- The Second order of packaging – Solenoid fibre.
- The third order of packaging – Scaffold loop Chromatids Chromosome.
Also Read: DNA Replication
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