Question

When does DNA copying occur?
Explain ...

Answer 1

During Mitosis, DNA is replicated during the S phase (Synthesis phase) of Interphase. Interphase is basically the daily life cycle of the cell. Cells spend most of their life in Interphase before Mitosis occurs (M phase).

Answer 2

Hey Friend,

DNA replication occurs in the S phase...

Explanation - 
The Interphase is the preparatory phase of the cell cycle. It has three substages - G1 phase, S phase, G3 phase.

S phase - It is the synthesis phase. During this phase two important things occur. 
(i) Synthesis of Histones
(ii) Replication of DNA

As a result, each chromosome has two chromatids (thread like structures joined by centromere) 

Note : DNA replication takes place is S phase in both Mitosis and Meiosis.

Hope it helps!