Direction that new DNA is built in during replication?

• A. 3' to 5'
• B. 5' to 3'
• C. 5' to 5'
• D. 3' to 3'

Answer: (B)

DNA synthesis proceeds in the 5' to 3' direction because nucleotides are added to the free 3' end of the growing strand. DNA polymerases catalyze the formation of a phosphodiester bond between the 3' hydroxyl of the last nucleotide and the 5' phosphate of an incoming dNTP, releasing pyrophosphate in the process. This means growth must occur from the 5' end toward the 3' end of the new strand. The template is read in the opposite direction, 3' to 5', to provide the correct bases, producing an antiparallel complementary strand. In practice, the leading strand is synthesized continuously, while the lagging strand is made in short Okazaki fragments, but all DNA synthesis happens 5' to 3'.