DNA is a long polymer of deoxyribonucleotides.
Length of DNA: The number of nucleotides present in a DNA is called the length of DNA. A pair of nucleotides is called a base pair. Number of base pairs in a DNA is also taken as the length of DNA. The length of DNA is also the characteristic of an organism. For example, Escherichia coli has 4.6 × 106 bp, and haploid content of human DNA is 3.3 × 109 bp.
A nucleotide has three components: a nitrogenous base, a pentose sugar, and a phosphate group.
There are two types of nitrogenous bases – Purines and Pyrimidines.
A nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.
When a phosphate group is linked to 5'-OH of a nucleoside through phosphoester linkage, a corresponding nucleotide is formed. Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide.
More nucleotides can be joined in such a manner to form a polynucleotide chain. A polymer thus formed has at one end a free phosphate moiety at 5'-end of ribose sugar. This end is referred to as 5’-end of polynucleotide chain. At the other end of the polymer the ribose has a free 3'-OH group. This is referred to as 3'-end of the polynucleotide chain.
Sugar and phosphates form the backbone of a polynucleotide chain. The nitrogenous bases linked to sugar moiety project from the backbone.
The salient features of the Double-helix structure of DNA are as follows:
The Central dogma in molecular biology states that the genetic information flows from DNA → RNA → Protein.
In prokaryotes, the DNA is not scattered throughout the cell. The DNA is negatively charged. It is held with some proteins (which are positively charged) in a region termed as ‘nucleoid’. In the nucleoid, the DNA is organized in a large loop held by proteins.
The organization of DNA is much more complex in eukaryotes. There is a set of positively charged, basic proteins called histones. Histones are organised to form a unit of eight molecules called as histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome.
A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin. Chromatins are thread-like stained (coloured) bodies seen in nucleus. When viewed under electron microscope, the nucleosomes appear as ‘beads on string’ structure.
Non-Histone Chromosomal (NHC) Proteins: The packaging of chromatin at higher level requires additional set of proteins. These proteins are collectively called non-histone chromosomal proteins.
Euchromatin: In a typical nucleus, some regions of chromatin are loosely packed. These stain light, and are called euchromatin. Euchromatin is transcriptionally active chromatin.
Heterochromatin: Some chromatins are densely packed. They stain dark, and are called heterochromatin. Heterohcromatin is transcriptionally inactive.
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