CBSE Class 12-science Answered

The three-dimensional structure of DNA was discovered in 1953 by Watson and Crick for which work they won a Nobel Prize. The main features of the structure are:

• DNA is double-stranded, so there are two polynucleotide stands alongside each other.

• Each single strand of DNA is made of repeating units of nucleotides.

• Single nucleotide is made up of: pentose sugar (deoxyribose sugar), phosphate group and nitrogenous bases.

• The strands are complementary and antiparallel i.e. they run in opposite directions thus 5' to 3' is parallel to 3' to 5'.
• The two strands are wound round each other to form a double helix.

• The two strands are joined together by hydrogen bonds between the nitrogenous bases.

• The bases therefore form base pairs, which are like rungs of a ladder.

• The base pairs are specific. A only binds to T by a double bond of hydrogen) and C only binds to G by triple bond of hydrogen.

• These are called complementary base pairs. This means that whatever the sequence of bases along one strand, the sequence of bases on the other stand must be complementary to it.