DNA as the Primary Genetic Material

Section 1: DNA as the Primary Genetic Material

Early Speculations on Genetic Material

In the 1920s, it was already understood that proteins are biological macromolecules composed of various amino acids linked together. The different sequences of amino acids could form different proteins, leading people to naturally speculate that the diverse arrangements of amino acids might contain genetic information. At the time, studies on other biological macromolecules had not yet revealed similar structural characteristics. Consequently, most scientists believed that proteins were the genetic material of living organisms.

It wasn't until the 1930s that it became known that DNA is a biological macromolecule composed of many deoxyribonucleotides, with the chemical composition of these nucleotides including phosphate, bases, and deoxyribose (Figure 3-1). There are four types of deoxyribonucleotides that make up DNA, each with a specific base. This understanding could have highlighted the importance of DNA, but due to the lack of clear knowledge about its structure, the view that proteins were the genetic material still predominated.

Griffith's Transformation Experiment

The first challenge to the idea that proteins were the genetic material, supported by solid experimental evidence, came from American microbiologist Oswald Avery (O. Avery, 1877–1955), building on the earlier work of British microbiologist Frederick Griffith (F. Griffith, 1877–1941).

In 1928, Griffith used mice as experimental subjects to study the pathogenicity of Streptococcus pneumoniae (pneumococcus). He infected mice with two different types of Streptococcus pneumoniae. One type had a polysaccharide capsule and formed smooth colonies on culture media, known as S-type bacteria. S-type bacteria are pathogenic and can cause pneumonia in humans and mice, leading to septicemia and death in mice. The other type lacked the polysaccharide capsule and formed rough colonies on culture media, known as R-type bacteria. R-type bacteria do not cause disease in humans or mice, and thus are non-pathogenic. Griffith's experimental process is shown in Figure 3-2.

From the bodies of mice in the fourth group of experiments, live S-type bacteria were isolated, and their offspring were also pathogenic S-type bacteria. From this, it could be inferred that the heat-killed S-type bacteria contained some active substance—referred to as the "transforming factor"—that caused the live R-type bacteria to transform into live S-type bacteria.

But what exactly was this transforming factor?

In the 1940s, Avery and his colleagues crushed heat-killed S-type bacteria and attempted to remove most of the sugars, proteins, and lipids, creating a cell extract. When this extract was added to a culture medium containing live R-type bacteria, live S-type bacteria appeared (Figure 3-3, Group 1). Then, they treated the cell extract with different enzymes before conducting the transformation experiment again. The results showed that the cell extract retained its transforming activity after treatment with protease, RNase, or esterase (Figure 3-3, Groups 2 to 4), but lost its transforming activity after treatment with DNase (Figure 3-3, Group 5).

These experiments demonstrated that the cell extract contained the transforming factor described earlier, and this transforming factor was likely DNA. Avery and his team further analyzed the physicochemical properties of the cell extract and found that they were very similar to those of DNA. As a result, Avery proposed a conclusion that differed from the prevailing view of most scientists at the time: DNA is the substance that causes stable genetic changes in R-type bacteria.

The Hershey-Chase Experiment

In 1952, American geneticist Alfred Hershey (A.D. Hershey, 1908–1997) and his assistant Martha Chase (M.C. Chase, 1927–2003) conducted another compelling experiment using T2 bacteriophage (Figure 3-4) as their experimental material, utilizing radioactive isotope labeling technology.

The T2 bacteriophage is a virus that specifically parasitizes within Escherichia coli (E. coli). Its head and tail are both composed of protein, with DNA inside the head. When the T2 bacteriophage infects E. coli (Figure 3-5), it uses the materials within the E. coli cell to synthesize its own components and undergoes extensive replication under the influence of its own genetic material. Once the bacteriophages have proliferated to a certain number, the E. coli cell lyses, releasing a large number of bacteriophages.

Hershey and Chase first cultured E. coli in media containing radioactive isotopes S35 and P32, respectively, and then used these E. coli to culture T2 bacteriophages. This resulted in bacteriophages with protein labeled with S35 or DNA labeled with P32. They then used the S35 or P32 labeled T2 bacteriophages to infect unlabeled E. coli. After a short period of incubation, they used a blender to agitate and then centrifuge the mixture (Figure 3-6). The purpose of agitation was to separate the bacteriophages from the bacteria, and centrifugation was to precipitate the heavier E. coli cells, leaving the lighter T2 bacteriophage particles in the supernatant. After centrifugation, they checked for the presence of radioactive material in the supernatant and the precipitate. They found that in the infection experiment with S35-labeled bacteriophages, the radioactive isotopes were primarily found in the supernatant. In the experiment with P32-labeled bacteriophages, the radioactive isotopes were primarily found in the precipitate. What does this result indicate?

Further observation revealed that the bacteriophages released by the lysed bacteria contained P32-labeled DNA but no S35-labeled protein. What does this result suggest?

Hershey and Chase's experiment demonstrated that when bacteriophages infect bacteria, DNA enters the bacterial cell while the protein coat remains outside the cell. Therefore, the characteristics of progeny bacteriophages are inherited through the parental DNA. DNA is the genetic material of the bacteriophage.

Is DNA the Only Genetic Material?

Subsequent research has shown that genetic material is not limited to DNA; RNA can also serve as genetic material. Some viruses contain only protein and RNA, such as the tobacco mosaic virus (Figure 3-7). Protein extracted from the tobacco mosaic virus cannot cause tobacco plants to become infected, but RNA extracted from these viruses can. Therefore, in these viruses, RNA is the genetic material. However, since the vast majority of organisms use DNA as their genetic material, DNA is considered the primary genetic material.