I honestly think it Lymph,
Answer: When DNA is being copied for the Daughter cells
Explanation: There is a mistake when the DNA is being copied for the new daughter cells. When a cell divides, it must divide its DNA into 2 exact copies of the orginal DNA for the 2 new daughter cells. Sometimes the copy isn't correct and the cells mutates.
Limiting factors
for example: amount of food, water, shelter, predators, etc