Suppose that you could designan RNA polymerase that would transcibe

both strands of a DNA doublehelix at the same time. Do you think

this would be useful for acell?

No it would not be useful for cell.

Explanation:

RNA polymerase uses only one strand of DNA which is called the template strand for the synthesis of mRNA. The other strand which is not used to make mRNA is called the coding strand.

If an RNA polymerase is designed which transcribe both the strand than two different strands of mRNA will be generated that will have different nucleotide sequences and will code for different proteins. So one strands will generate a protein which have different amino acids sequence that will complicate the overall process for the cell.

Another problem is if two RNA strands are formed then they can form a double helix and the ribosome might not be able to bind on them and translation might stop. Therefore it would not be useful for cell.