What happens to the amount of dna in the nucleus just before the beginning of mitosis?

The amount of DNA doubles.

Explanation:

Interphase is the stage in which cells spend most of its time. Interphase has three phases G1, S, and G2 phase. During G1 phase cells prepare those enzyme that is required for the replication of DNA.

In the synthesis phase, the DNA present in the cell nucleus gets doubled by the process of replication so that after mitosis each cell gets an equal amount of DNA. In G2 phase cells continues to grow and produce proteins and prepare the itself to move in the mitotic phase.

Therefore before the beginning of mitosis, the amount of DNA in the nucleus gets doubled.