Cells synthesize new nucleic-acid strands by template-directed polymerization. An existing nucleic-acid strand acts as a template, and complementary base pairing determines which nucleotide is selected at each position.

Polymerases extend the new strand by adding nucleotides to its free $3'$ hydroxyl. As a result, the new strand always grows

$$\boxed{5'\rightarrow3'}.$$

Because complementary strands align antiparallel, the polymerase reads the template in the opposite direction:

$$3'\rightarrow5'\quad\text{template}$$

while synthesizing

$$5'\rightarrow3'\quad\text{new strand}.$$

Incoming nucleotide triphosphates

The substrates are nucleoside triphosphates. DNA synthesis uses deoxyribonucleoside triphosphates such as dATP, dGTP, dCTP and dTTP. RNA synthesis uses ribonucleoside triphosphates such as ATP, GTP, CTP and UTP.

The $3'$ hydroxyl at the end of the growing strand becomes joined to the phosphate closest to the sugar of the incoming nucleotide, extending the phosphodiester backbone and releasing pyrophosphate, a two-phosphate product. Subsequent hydrolysis of pyrophosphate helps make the overall polymerization thermodynamically favorable.

The template determines sequence, not direction

Suppose a DNA template segment is

3'-A C G T T A-5'

A complementary DNA strand synthesized along it is

5'-T G C A A T-3'

whereas a complementary RNA product would be

5'-U G C A A U-3'.

The same chemical logic therefore supports both DNA replication and RNA transcription.

Different polymerases have different initiation requirements, accessory proteins and fidelity mechanisms. Those distinctions matter, but they are specializations of this shared rule: template complementarity selects the sequence, and polymerization extends the new strand only in the $5'\rightarrow3'$ direction.