Chapter 23, Problem 10TE

In what way is the chemical structure of DNA different from that of RNA?

To determine

The ways in which the chemical structure of DNA is different from that of RNA.

Answer to Problem 10TE

The structure of RNA and DNA is different because in RNA the pentose sugar is $\text{D-}(-)\text{-}$ ribose whereas in DNA, it is $\text{2-}$ deoxy$\text{D-}(-)\text{-}$ ribose. RNA exists as a single stranded helix whereas in DNA, there are two strands of double helix.

Explanation of Solution

The structure of DNA molecule is postulated by Watson and Crick. In a DNA molecule, the sugar is a $\text{2-}$ deoxy$\text{D-}(-)\text{-}$ ribose sugar and heterocyclic bases are adenine, guanine, thymine and cytosine. The nucleotide chain in DNA has alternate sugar and phosphate ester residues. The phosphoric acid on one side forms ester linkage with $3\text{'}-\text{OH}$ of one sugar and on other side it forms ester linkage with $5\text{'}-\text{OH}$ of the another sugar residue. This results in giving a double helix structure to DNA. The two strands of the double helix are complementary and not identical.

The ribonucleic acid that is RNA exists as a single stranded helix. The structure of RNA is similar to that of DNA except that the sugar in it is $\text{D-}(-)\text{-}$ ribose sugar and uracil is present instead of thymine as a heterocyclic base.

The size of the DNA molecules is very large and may vary from $\text{6}\times {\text{10}}^{\text{6}}-\text{16}\times {\text{10}}^{\text{6}}\text{u}$ whereas that of RNA molecules is much smaller with molecular mass ranging from $\text{20000}$ to $\text{40000}\text{u}$.

Conclusion:

The pentose sugar in the structures of RNA and DNA is different. In RNA the pentose sugar is $\text{D-}(-)\text{-}$ ribose whereas in DNA, the sugar present is $\text{2-}$ deoxy$\text{D-}(-)\text{-}$ ribose. The one of the nitrogenous base is different in RNA is uracil instead of thymine in DNA molecule.