In Escherichia coli, replication of DNA can occur throughout the cell cycle while the cell is also actively transcribing its genes. This means collisions between replication forks and RNA polymerases are inevitable. Depending on the orientation of the genes, collisions can be rear-end (when both machines are traveling in the same direction) or head-on (when they are traveling in opposite directions). In the following paragraph, match each of the letters (A to D) to one appropriate number below. Do not use a number more than once. Your answer would be a four-digit number composed of digits 1 to 5 only, e.g. 1253.
"Typically, in a rear-end collision, the (A) of RNA polymerase collides with the (B) in the replication fork. In contrast, in a head-on collision, the (C) of RNA polymerase hits the (D) in the fork."
1. front edge (of RNA polymerase)
2. rear edge (of RNA polymerase)
3. DNA helicase
4. leading-strand DNA polymerase
5. lagging-strand DNA polymerase

Question

Quizplus · Accepted Answer

The replication fork in E. coli can progress almost 20 times faster than the RNA polymerase, meaning that rear-end collisions are typically between the rear edge of the RNA polymerase and the front edge of the leading-strand DNA polymerase that is traveling (faster) in the same direction on the same template strand. In a head-on collision, however, the front edge of RNA polymerase hits the DNA helicase that is traveling on the same (lagging) strand in the opposite direction. Note that the polymerases translocate with respect to their template in the 3?-to-5? direction, while the helicase in the fork translocates in the 5?-to-3? direction.

In Escherichia Coli, Replication of DNA Can Occur Throughout the Cell