The two most common factors that influence enzyme activity are:

They're temperature and pH.

Each enzyme has their own optimum temperature and pH, they're the conditions where the enzyme will work the fastest and best in acting as a biological catalysts. For example, enzyme Pepsin in your stomach has a optimum pH of 2, and it'll help the break down of protein the fastest if the pH value is 2.

About temperature, if the temperature in the environment is too low for an enzyme, the enzyme would tend to be inactive. This means it'll work very slow. The effect of them would return to normal if we raise the temperature again. However, if the temperature is too high, the enzyme may denature. The active site of the enzyme may be changed and can no longer bind into substrates. Which in this case, there's no going back. It cannot speed up any more reactions.

The effect on pH is very similar to the temperature factor, except both too acidic and alkaline conditions may denature the enzyme, instead of making them inactive.

Other not-so-common factors may include the amount of substrate avaliable

Dna strands are split apart to allow an opening for replication. the opening has two replication forks so synthesis goes toward them in 5' to 3' direction.