Answer:
The amount of DNA doubles in the nucleus just before mitosis. The DNA makes a copy of itself. Due to this, the chromosomes are sometimes called 'double chromosomes' just before mitosis.
The DNA material doubles before mitosis so that each daughter cells receives the same amount of DNA. This process ensures that each of the daughter cells formed has the same type and amount of genome as the parent cell. So, it is very important for the DNA to undergo replication before the process of mitosis begins.