Question

A nucleic acids is a polymer of monomer 'A'. In the structure of 'A', phosphate is always attached to which carbon of the sugar?

• A. $5^{th}$
• B. $3^{rd}$
• C. $2^{nd}$
• D. 1${}^{st}$

Answer 1

The correct option is A $5^{th}$

Nucleic acids are polymers of individual nucleotide (A) monomers. Each nucleotide is composed of three parts - a 5-carbon pentose sugar, a phosphate group and a nitrogenous base. The carbon atoms of nitrogenous bases are numbered as 1, 2, 3 and so on while that of the sugar are numbered as 1', 2', 3' and so on.

In a nucleotide the 5${}^{th}$ or 5' carbon of the pentose sugar is always attached to the phosphate group and the 1${}^{st}$ or 1' carbon is attached to the nitrogenous base.

When nucleotides polymerise to form nucleic acids, they join with the help of phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' carbon of the next nucleotide.