Griffith’s experiments have shown that certain factors (which may be genes) can be transferred from heat-killed bacteria to harmless bacteria, which causes them to “transform” into a disease-causing version of the bacteria.

Avery’s experiments have shown that genes are composed of DNA and that DNA is the nucleic acid that stores genetic information. The information stored in DNA can also be transmitted from one generation of an organism to the next.

Lastly, Hershey and Chase’s experiments have shown that the genetic material that was contained in bacteriophages is DNA, which was observed to be injected into bacteria.

The Watson and Crick’s model of DNA is a double helix structure of the deoxyribonucleic acid (DNA). It consists of two sugar phosphate backbones that make up the sides and nitrogen bases, which are held by hydrogen bonds, that make up the steps.

The structure of DNA consists of nucleotides that contain any of the four different types of bases. These bases include adenine, thymine, guanine, and cytosine – known as A,T, G, and C.

Additionally, each base in the structure of DNA pairs specifically with another type of base. According to the DNA model, adenine pairs with thymine, while guanine pairs with cytosine.

Adenine, thymine, guanine, and cytosine

Watson and Crick discovered that hydrogen bonds can only form between certain base pairs to make it stable. Cytosine and guanine form three hydrogen bonds, whereas thymine and adenine form two hydrogen bonds. According to base pairing, there should be equal amounts of thymine and adenine in DNA. For every adenine molecule, there had to be one thymine molecule paired to it.

The radioactive substances serve as markers to label the DNA and proteins of the bacteriophage during the experiment. If sulfur appeared in the bacteria, that means the virus injected protein to the bacteria, since DNA does not contain sulfur. However, if phosphorus appeared, then it means the DNA was injected to the bacteria, since protein contains the smallest amount of phosphorus.

if both scientists only used one radioactive substance to mark the virus, they would not be able to detect the presence of either the protein or the DNA in the bacteria, and the results would not give them enough evidences to make a conclusion.