Simply use a stop watch. Begin the stop watch when the necessary elements have been added and stop it immediately as soon as it turns purple. Repeat the process a few times to make sure the timing is accurate.
Answer:
D. 3' hydroxyl group
Explanation:
The polymerization activity of DNA polymerases works in 5' to 3' direction only. This means that the DNA polymerase can add new deoxyribonucleotides to the free 3' end of the primer or existing DNA strands only. The 3' end of DNA has a free OH group that exhibits the nucleophile attack on the incoming nucleotide. The free phosphate group is present on the 5' end of a DNA strand. Therefore, DNA polymerase always adds new nucleotides to the 3' OH ends (OH= hydroxyl group).