Question

A hydroxyl is present at the 3' end of the growing DNA strand. What is at the 5' end?

a nitrogenous base
a phosphate group
a deoxyribose
a ribose

Answer 1

In DNA, nucleotides are joined together to form a strand through a process called polymerization. Each nucleotide has a sugar-phosphate backbone and a nitrogenous base. The sugar in DNA is deoxyribose, which has a hydroxyl (-OH) group on its 3’ carbon and a phosphate group (-PO4) on its 5’ carbon.

When a new nucleotide is added to the 3’ end of a growing DNA strand, the hydroxyl group on the 3’ carbon of the deoxyribose sugar forms a phosphodiester bond with the phosphate group on the 5’ carbon of the next nucleotide. This forms a sugar-phosphate backbone with a free phosphate group on the 3’ end and a free hydroxyl group on the 5’ end.

Therefore, at the 5’ end of the growing DNA strand, there is a phosphate group (-PO4).