The chain elongation reactions catalyzed by DNA or RNA polymerases are nucleophilic addition-elimination reactions similar to the acyl substitution reactions among carboxylic acid derivatives but occurring here upon the innermost phosphate group of a deoxyribonucleoside triphosphate instead of a carbonyl group. Picture deoxyribonucleoside triphosphate in your mind. The nucloephile is the 3' -OH terminus of the primer. Thermodynamically, the synthesis is driven forward by the hydrolysis of pyrophosphate.

The other major announced change with the new 2007 MCAT, aside from the somewhat shorter, computerized format, will be an increased emphasis on genetics and molecular biology. A major part of the challenge in preparing will be to work on your vocabulary and comprehension skills for passages in this area. In the above passage, were you comfortable reading 'deoxyribonucleoside triphosphate' or even more importantly, '-OH terminus of the primer'?