In this article we will discuss about the chemical nature of the genetic material of organisms.
The knowledge about the chemical nature of the genetic material was gathered from a number of experimental observations made with bacteria and viruses. These experimental observations led to the conclusion that all cellular organisms have DNA (deoxyribonucleic acid) as the genetic material. Only some viruses (the RNA viruses) have RNA (ribonucleic acid) instead of DNA as the genetic substance.
One of the most important experimental evidences is the transformation of pneumococci (Streptococcus pneumoniae). Two types of pneumococci were used in the original experiment, — one type which was pathogenic possessed a thick polysaccharide capsule around the cells and produced smooth coloni on agar medium (S-type), while the other variant which was non-pathogenic possessed non-capsulated cells and produced rough colonies (R-type).
The ability to form capsule is a stable genetic character, or, in the language of genetics, it is a true-breeding character. Similarly, lack of a capsule is also a true-breeding character. The R-type bacteria presumably originate from the S-type through a genetic change (mutation) at a very low frequency, roughly at a rate of 1 in 105 to 1 in 106 cells.
In the initial experiments, it was observed that on injecting laboratory mice with a mixture containing heat-killed S-type pneumococci and living R-type bacteria, the animals died of pneumonia. When the dead mice were autopsied, living S-type pneumococci could be recovered on culturing. The observations indicated that living R-type pneumococci were transformed into S-type bacteria.
The number of living S-type bacteria recovered in the total population (consisting of both R-type and S-type cells) was much more than what could be expected if R-type cells arose by spontaneous back-mutation (R—>S). This phenomenon of origin of S-type from R-type bacteria was designated as transformation. Later experiments made on a much simpler system without involving laboratory mice led to the same results.
In such experiments, a heat-killed suspension of capsulated pneumococci (S-type) was mixed with a suspension of living non-capsulated pneumococci (R-type) and the mixture was added to a growth medium.
On incubation, the living bacteria multiplied and on plating a sample produced both R-type and S-type colonies:
These observations strongly indicated that some substance coming out from the dead capsulated bacteria was taken up by the living non-capsulated bacteria and caused their transformation into capsulated ones. This substance was named as the transforming principle. Through later research the transforming principle was rigorously purified and it was identified as DNA.
The conclusion from the transformation experiments is that DNA of the killed cells could cause a change in a genetic character. This provided a strong evidence that the genetic material is DNA. By a later experiment, it was also possible to transform penicillin-sensitive pneumococci into penicillin-resistant ones.
Another experimental evidence to prove that DNA is the genetic material came from the study of infection of E. coli by the DNA-bacteriophage T2. Two types of radioactive T2-phages were produced experimentally by growing a susceptible strain of E. coli in a medium containing a radioactive salt. The bacteria were then infected by T2-phage.
From the lysate, the phage particles were isolated by centrifugation. In one set, the culture medium for growing E. coli contained 32P-labelled phosphate and, in a second set, 35S-labelled sulfate. The bacteria incorporated these salts and became labelled. The bacteriophages which infected these bacteria were likewise labelled. The bacteriophage T2 contains a protein coat consisting of a head, a contractile tail and other tail parts, and a genome (or chromosome).
As protein contains sulfur, but no phosphorus, and the genomic material contains phosphorus, but no sulfur, two types of phages were obtained. In one type the protein coat was labelled with radioactive sulfur (35S) and in the other the genomic substance was labelled with radioactive phosphorus (32P).
These two types of phages, viz. 35S-labelled ones, and 32P-labelled ones were allowed to infect nonradioactive E. coli cultures separately. After allowing some time for infection to occur, the culture was vigorously shaken in a blender which caused separation of the attached phage-particles from the bacterial cells.
The bacteria were next collected by centrifugation and radioactivity of the cells was measured. It was observed that radioactivity was present only in the bacteria which were infected by 32P-labelled phages, but not in those infected by 35S-labelled phage.
These results showed that in phage infection, the 35S-labelled protein coat does not penetrate into the cell and only the genomic material containing 32P-labelled substance enters into the cell making the bacteria radioactive. Since, in bacteriophage T2, DNA is present in its chromosome, the genetic material which controls infection and multiplication must be DNA.
The experiment is schematically presented in Fig. 9.1.
Certain viruses do not have DNA. Instead, they have RNA. For example, the tobacco mosaic virus (TMV) has a protein coat and an RNA core. It was found that the RNA isolated from TMV could produce the disease symptom in tobacco plants when it was injected into leaves, although the infectivity was less than intact virus. The naked RNA was designated as “infective RNA”. The ability of naked viral RNA to produce disease and progeny virus in the infected plant proved that it acts as the genetic material of TMV.
In all eukaryotic cells, major portion of the cellular DNA is present in the chromosomes. But DNA is also present in the cell organelles, like mitochondria and chloroplasts. Prokaryotic cells do not have these organelles, but they also have extra nuclear DNA in the form of plasmids.
Thus, except the RNA viruses, all cellular organisms and other viruses possess DNA as the genetic material. It has been suggested by some scientists that, about 4 billion years ago; the earliest living forms probably used RNA as the genetic material and DNA appeared later in the evolutionary process. It replaced RNA as the genetic material, because it serves more efficiently as the store-house of information than RNA.