DNA sequence

DNA sequence is represented in the form of particular English letters such as A,G,C,T.

What these letters represent?

These letters are nothing but the initial letter of the word Adenine, Guanine, Cytosine, Thymine and these are nucleotides. Nucleotides are the monomer of DNA strand which is also called as polynucleotide chain. Nucleotide is not a single chemical structure and it is made of three chemical molecules. They are

• Nitrogenous bases such as Purines (Adenine & Guanine) and Pyrimidines (Cytosine & Thymine)
• Pentose sugars such as De-oxyribose sugar in DNA and Ribose sugar in RNA.
• Phosphoric acid which is meant for bridges between nucleotides.

Complementary nature of DNA sequence:

There is some complementarity maintained between opposite strands of DNA. This complementary nature of DNA sequence is maintained by some rules decided by the nature between DNA strands. They are:

• Adenine always pairs with Thymine.
• Guanine always pairs with Cytosine.

  Example:	Template	Complementary
  	A	T
  	G	C
  	C	G
  	A	T
  	T	A
  	T	A

In this manner, complementary sequence is maintained between DNA strands.

Alternative use of letters:

 While we studying about the sequence of DNA strand, the sequence is very lengthier and very complex in nature. Just to simplify the study of DNA sequence, the International Union of Pure and Applied Chemistry (IUPAC) has created a criteria that we can use some other letters for the combination of nucleotides. These letters are introduced with some meaning and the letter derived from initial of the meaning word or it derived by some idea about the word.

Example: meaning given in brackets

R → GA ( Purine )

Y  → TC (Pyrimidine )

K  → GT ( Keto)

M → AC ( Amino )

S → GC ( Strong bond )

W → AT ( Week bond )

B → GTC ( All but A )

D → GAT ( All but C )

H → ACT ( All but G )

V → GCA ( All but T )

N → AGCT (Any )