Question

A scientist treats cells with a chemical that prevents DNA synthesis . In which stage of the cell cycle will these cells remain ?

Answer 1

Stage G1.
I'm not one-hundred percent sure, so wait around and see if someone else says something.

Answer 2

Technically it depends on what chemical is used to block cell cycle progression. Generally I agree with Opcode - if the chemical blocks the cell from entering the synthesis phase, things will stop in G1. But if it lets the cell enter the synthesis phase but not finish duplicating its DNA, then it will be trapped in S.

So bottom line: probably G1, maybe S.