crossorigin="anonymous">COMPLEMENTARY STRAND - Questions House



DNA has 4 nitrogen bases which are Adenine(A) , Guanine(G) , Thymine(T) , Cytosine(C). RNA also has 4 bases but in RNA Uracil (U) is present instead thymine. Adenine always pair with thymine by two hydrogen bonds in DNA and to Uracil in RNA. Guanine pair with cytosine by three hydrogen bonds. In a double stranded DNA base pairing exists. Thus, adenine is present opposite to thymine and guanine present opposite to cytosine in double strand. This is called as complementary base pairing.


If a strand has the sequence 5′ ATTGCCC 3′ then the complementary strand means other strand of double strand would have the sequence 3′ TAACGGG 5′ .

The original sequence is from 5′to 3′ sequence that pair Adenine with Thymine and cytosine with Guanine. The complementary strand read from 3′ to 5′direction. Thus, complementary strand start with 3’ and ends with 5’.

Base-Pair Rule In DNA

In Base-Pair Rule In DNA, each strand sequence of bases is a complement to another strand’s sequence. So if you know the sequence of one strand of DNA, then you can predict the sequence of other strand.

The DNA is made up of four kinds of nitrogenous bases which are :

Guanine (G) Cytosine (C) Adenine (A) Thymine (T)

Two of the bases are purines, which are adenine and guanine. Pyrimidines bases are thymine and cytosine.

These bases are held together by hydrogen bonds. They are formed by complementary pairs of nitrogen bases — Adenine pair with thymine by two hydrogen bonds. Guanine pair with Cytosine by three hydrogen bonds. This is known as the “Base-Pair Rule”. In a double stranded DNA, because of this base pairing , an adenine always pair with thymine and a guanine always pair with cytosine. This is called complementarity of DNA.

Chargaff’s rule

Chargaff’s rule based on the bases of DNA or the genetic code. According to this rule, DNA from any species must consist of a certain set of bases with a certain order.

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