Answer:
DNA helicase separates the two strands of DNA at a replication fork.
Explanation:
To allow DNA replication, two DNA strands should be separated to serve as a template strand. DNA helicases are the enzymes that separate the two parental DNA strands in an energy-dependent way. DNA helicases move along the DNA and separate the two strands by breaking the hydrogen bonds between the complementary base pairs of two DNA strands. The energy released from ATP hydrolysis is used to break the hydrogen bonds.
The Griffith's experiment, the Avery-MacLeod-McCarty experiment, and the Hershey–Chase experiments were the set of experiments that established DNA as the key hereditary molecule. The Avery-MacLeod-McCarty experiment was an extension to the Griffith's experiment. The heat killed virulent S strain cells of the Griffith's experiment were lysed to form a supernatant containing a mix of RNA, DNA, proteins and lipids from the cell. The supernatent was equally divided into 3 parts after the removal of the lipids. The 3 parts were respectively treated with an RNAase to degrade the RNA, DNAase to degrade the DNA and proteinase to degrade the proteins. The treated supernatant was then added into the culture containing the non-virulent R cells. In case of the supernatant treated with the DNAse, no transformation of R cells into S cells occurred. The transformation of R cells to S cells occurred in the proteinase and the RNAse cases. This indicated that DNA was the hereditary molecule and not protein or RNA.

I'm pretty sure it's the third option.
Answer:
Streptococcus mutans.
Explanation:
Streptococcus mutans is the bacteria (it is still a microorganism) that predominatly causes tooth decay. It is present in all areas of the mouth.