Anew nucleotide can only be added to the end of a growing dna strand. dna therefore always grows in the direction. 5′; 5′ to 3′ 3′; 3′ to 5′ 3′; 5′ to 3′ 5′; 3′ to 5′

 3′; 5′ to 3′ 

A new nucleotide can only be added to the 3' end of a growing dna strand. dna therefore always grows in the 5' to 3' direction.

3' means 3' hydroxil groups that is free before synthesis can be initiated.

5' to 3' means that DNA is always synthesized in the 5' to 3' direction.