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