Why the two form of Dna occur in a cell?

I believe what you wished to ask is why two forms of nucleic acids occur in the cell.

In the cell, there are both DNA and RNA. These are two types of nucleic acids and are very important if the cell is going to grow, function normally and reproduce to make more cells of its kind.

DNA is responsible for storing of genetic information for production of new cells and for making of proteins that the cell needs to function.

RNA is responsible for taking this information from the DNA in the nucleus and into cell organelles in the cytoplasm responsible for making proteins.

These two types of nucleic acids work together to ensure the cell has the necessary proteins to enable it to grow, to function normally and to reproduce.