The direction of DNA synthesis is always from the 5' end to the 3' end. DNA synthesis occurs through a process called DNA replication, in which a new DNA strand is synthesized complementary to each of the original DNA strands. The 5' end of a DNA molecule refers to the end that has a free phosphate group attached to the 5' carbon of the sugar molecule, while the 3' end refers to the end that has a free hydroxyl group attached to the 3' carbon of the sugar molecule. The DNA polymerase enzyme adds new nucleotides to the growing DNA strand by forming phosphodiester bonds between the 3' hydroxyl group of the existing DNA strand and the 5' phosphate group of the incoming nucleotide. As a result, the DNA strand grows in the 5' to 3' direction.
