Original: DNA polymerase can add free nucleotides to only the 3 ' end of the newly-forming strand . This results in elongation of the new strand in a 5 ' -3 ' direction .
Simplified: DNA polymerase can only add deoxyribonucleotides to the 3 ' end of a new piece of DNA , so DNA is always made in the 5 ' -3 ' direction .