Here's why:
* DNA Polymerase: The enzyme responsible for DNA synthesis can only add nucleotides to the 3' hydroxyl group of the existing strand. This means it can only build in one direction.
* Antiparallel Strands: DNA strands are antiparallel, meaning they run in opposite directions. One strand runs 5' to 3', and the other runs 3' to 5'.
* Leading and Lagging Strands: During DNA replication, the leading strand is synthesized continuously in the 5' to 3' direction. The lagging strand is synthesized in fragments (Okazaki fragments) that are later joined together, also in the 5' to 3' direction.
In summary, the direction of synthesis for new strands of nucleic acids is always from the 5' end to the 3' end.
