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Nucleic acid

Nucleic acid is polynucleotide. Nucleotide is the building block of nucleic acid. There are two types of nucleic acids: DNA and RNA (ribonucleic acid).


Nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base. Pentose sugars are deoxyribose and ribose for DNA and RNA respectively. Nitrogenous bases are adenine (A), guanine (G), cytosine (C), thymine (T) and urasil (U). DNA contains only adenine, guanine, cytosine and thymine. While, RNA contains adenine, guanine, cytosine and urasil.


Nucleotides are joined by phosphodiester bond to become polynucleotide. Phosphodiester bonds join the 3′-hydroxyl group of deoxypentose from one nucleotide to the 5′-hydroxyl group of the dexoxypentose of neighbor nucleotide via phosphate group. Thus, it results the 5′-end and 3′-end that are not attached to nucleotides.Phosphate and sugar make up the backbone of DNA. Two strands of nucleotides held together at the bases by hydrogen bond. Two strands run in the opposing directions and twist together to form double helix. The two strands are paired anti parallel manner where the 5′-end of one strand is paired with the 3′-end of the other strand.

Base Pairing

The base of one strand is always paired with the base of another strand. Adenine is always paired with thymine by two hydrogen bonds. Guanine is always paired with cytosine with three hydrogen bonds. Therefore, one polynucleotide chain of the DNA double helix is always the complement of the other.

The two complementary strands of DNA can be separated by heating and alteration of pH.



DNA Structure
Image credit: Wikimedia Commons


double helix dna

Double Helix DNA
Image credit: Wikimedia Commons










Recent research found that the DNA structure is not only double helix. It has quadruplex helix DNA found in human cell. Read more.