Question

What are the complementary bases in DNA?
(A) uracil and adenine : cytosine and guanine
(B) adenine and thymine : guanine and cytosine
(C) adenine and thymine : guanine and uracil
(D) adenine and guanine : thymine and cytosine

Answer 1

Hint: DNA is known as Deoxyribonucleic acid is a type of nucleic acid. The nitrogen bases present in it are cytosine, adenine, thymine and guanine.

Complete step by step answer:
First of all let us understand about nucleic acid. Nucleic acids are the biological polymer found in the nucleus of all living cells. Nucleic acids are polymers of nucleotides, so they are also called polynucleotides. Nucleotides are the monomers of nucleic acids. A nucleotide is composed of three components i.e., nitrogen base (heterocyclic base), pentose sugar and phosphate group. The nucleotides are joined together by phosphodiester linkage between and carbon atoms of pentose sugar.
i.e. \[Base - Sugar - Phosphate(Nucleotide)\]
If a nitrogen base is bonded to pentose sugar together, it is called a nucleoside.
There are two types of nucleic acid i.e. DNA and RNA.
(a) RNA: RNA or ribonucleic acid is a polymeric molecule essential in various biological roles in coding, regulation and expression of genes. The sugar present in RNA is \[D - ribose\] . Nitrogen bases in RNA are adenine, guanine, cytosine and uracil. RNA has a single helix structure and has low molecular mass. RNA does not replicate but it guides the synthesis of protein.
(b) DNA: DNA or Deoxyribonucleic acid is a molecule having two polynucleotide chains that coil around each other to form double helix structure. Thus it has double \[\alpha - Helix{\text{ }}structure\] . The sugar present in DNA is \[D - 2 - deoxyribose\]. DNA has high molecular mass and is present in the nucleus.DNA has the property of replication and controls the transmission of hereditary information. Nitrogen bases in DNA are adenine, cytosine, guanine and thymine.
In DNA structure, adenine(A) forms two \[H - bonds\] with thymine(T) while cytosine(C) forms three \[H - bonds\] with guanine(G). These bonds which form bonds with another nitrogen base are known as complementary bases.

So, the correct answer is Option B.

Note: The two strands of DNA are complementary because the sequence of nitrogen bases in one strand automatically determines the sequence in the other strand. The nitrogen bases generated on the other strand are complementary to the nitrogen bases on the first strand.