DNA is always synthesized in the 5'-to-3' direction. This means that new nucleotides are added only to the 3' end of the growing strand. The 5'-phosphate group of the new nucleotide binds to the 3'-OH group of the last nucleotide of the growing strand.
Here's why this directionality matters:
- DNA Polymerase: The enzyme responsible for synthesizing DNA, DNA polymerase, can only add nucleotides to the 3' end of a pre-existing strand.
- Leading and Lagging Strands: During DNA replication, one strand (the leading strand) is synthesized continuously in the 5'-to-3' direction. The other strand (the lagging strand) is synthesized discontinuously in short fragments called Okazaki fragments, also in the 5'-to-3' direction.
Therefore, DNA is not made from 3 to 5, but rather from 5 to 3.
