I think pookies answer is sufficient - however, the details are complex and prob not going to be tested on the MCAT... but I think there could be some emphasis on the process just incase.
The two strands are separated by Helicase and only DNA polymerase III is responsible for adding base pairs to complete the strands. The leading strand can be done smoothly due to the fact that the polymerase can add pairs in the 5 to 3 prime direction (the parent leading strand is 3 to 5 prime, and thus the strands are joined antiparallel)... okay that makes sense. But, the reason the lagging strand doesnt work out the same ways is that the polymerase complex is going to have to still add pairs in the 5 to 3 prime direction... and the lagging strand is 5 to 3 prime.
The complex has to basically feed the lagging strand through itself in order for the complex to go back and do 1000-2000 base pair insertions before it stops and goes back to do another one. There are small gaps between the fragments (Okazaki) which are completed by ligase.
Just know that there are both steric reasons (the complex is responsible for how big the okazaki fragments are) and directionality (the polymerase can only add pairs in a 5 to 3 prime way).
You're thinking about replication, not transcription. There is a huge difference. You can know the most intricate details of both processes, but if you mix up their very names, that's not a good thing.
Hmm, there is NO lagging strand in transcription. It's just 1 mRNA strand from the 1 DNA template. Only replication has leading and lagging strands. But I think that's what Tiger meant: why the lagging strand in REPLICATION.