Final answer:
DNA polymerase extends nucleotide chains exclusively in the 5' to 3' direction, utilizing a free 3'-OH group to form a new strand of DNA, either continuously on the leading strand or discontinuously on the lagging strand as Okazaki fragments.
Step-by-step explanation:
DNA polymerase can only extend a nucleotide chain in the 5' to 3' direction. This enzymatic specificity means that the synthesis of a new DNA strand can only proceed if there is a free 3'-OH group available to which the enzyme can add nucleotides through the formation of a phosphodiester bond between the 3'-OH end and the 5' phosphate of the next nucleotide.
At the start of replication, an RNA primer synthesized by RNA primase provides the necessary 3'-OH end. On the leading strand, the DNA is synthesized continuously, whereas on the lagging strand, synthesis occurs discontinuously in short segments known as Okazaki fragments, each requiring a primer.