$4.(3$ points$)$ Researchers working on an artificial prokaryotic cell decide it should have a unique genetic code as a safety precaution in case of an accidental release, and also to make it immune from natural viruses.

To change the genetic code, what type of proteins need to be mutated and what about them should be altered? Be as specific and clear as possible.

The modified genetic code the researchers decide to adopt has exactly $3$ codons for each of the $20$ different amino acids. To create the genome for the new organism, researchers will need to assemble an entirely new DNA molecule whose sequences use the new code to express the protein sequences the organism needs. Given that, how many different DNA sequences are possible for one specific protein sequence that is $100$ amino acids long?