DNA as a genetic material

It is evident that chromosomes are the carriers of genetic material. Chromosomes contain proteins, DNA and RNA. It is universally accepted that DNA is the genetic material in most of the organisms and higher organisms. In most of the plant viruses, RNA is the genetic material. There are many direct evidences for DNA being the genetic material. Here, we will discuss one of the evidences illustrated by Frederick Griffith.

Hereditary role of DNA – Bacterial transformation

In 1928, the bacteriologist Frederick Griffith conducted an experiment using Diplococcus pneumoniae. He studied two strains of virulent Diplococcus causing pneumonia. The virulent strain synthesized a smooth polysaccharide coat and produces smooth colonies. This strain was called strain-S. Another strain which lacked the proper polysaccharide coat is harmless and produces rough colonies. This strain was called strain-R.

When Griffith injected S-type of cells into the mouse, the mouse died. When R-type cells were injected into the mouse, the mice did not die. He injected heat killed S-type cells into the mouse. The mouse did not die. Griffith killed some smooth strain bacteria and mixed it with live rough strain bacteria. When the mixture of heat killed S-type cells and R-type

cells was injected into the mouse, the mouse was dead. The living rough strain of Diplococcus had been transformed into S-type cells. That is the hereditary material of heat killed S-type cells had transformed R-type cells into virulent smooth strains. Thus the phenomenon of changing the character of one strain by transferring the DNA of another strain into the former is called transformation.

Structure of DNA

DNA and RNA are identified in the nucleus. They are complex macro molecules and made up of millions of smaller units called nucleotides. Hence, DNA is a macromolecular substance with double stranded polynucleotide. Each nucleotide is made up of pentose sugar, a phosphate group and a nitrogenous base. Ribose is the constituent sugar in RNA and Deoxyribose in DNA. The nitrogenous bases are of two kinds – purines and pyrimidines. Adenine and guanine are the purines and thymine and cytosine are pyrimidines. The nitrogenous bases found in DNA are adenine, guanine, cytosine and thymine, whereas in RNA thymine is replaced by uracil. The sub-unit containing only sugar and nitrogenous base is known as nucleoside. A nucleoside combines with phosphate to form a nucleotide. Thus, four kinds of nucleotides are seen in DNA molecule. They are adenine nucleotide, guanine nucleotide, thymine nucleotide and cytosine nucleotide. Hence, nucleotides are building blocks of DNA.

In 1953, James Watson and Francis Crick proposed double helix DNA model on the basis of x-ray diffraction studies with photographs of DNA taken by Wilkins and Franklin. DNA is a double stranded structure in which the two strands are coiled around each other forming a double helix. The DNA duplex is “coil of life”. There are two grooves found in DNA molecule
namely major and minor grooves. The backbone of the helix is formed of sugar and phosphate molecule. The nitrogenous bases are attached to sugar molecules. The two nucleotide strands are held together by unstable hydrogen bonds. Erwin Chargaff in 1949 showed that

(i) The bases pair in specific manner. Adenine always pairs with thymine and guanine pairs with cytosine.
(ii) Total amount of purine nucleotides is always equal to the total amount of pyrimidine nucleotides i.e.[A] + [G] = [T] + [C].
(iii) The proportion of adenine is equal to thymine and so also of guanine is equal to cytosine. But the [A] + [T] need not necessarily be equal to [G] + [C].

These empirical rules regarding the composition of bases in DNA is collectively known as Chargaff’s law or Base pair rules. There are two hydrogen bonds between adenine and thymine (A= T) and there are three hydrogen bonds between guanine and cytosine (G≡C) pairing. The two strand run antiparallely in opposite directions ie. they run in opposite direction 5’ to 3’ end and 3’ to 5’ end. The two strands are interwined in clockwise direction. The width of DNA molecule is 20 A. The strand completes a turn every 34 A along its length. There are ten nucleotides per turn. The internucleotide distance is 3.4 A. Watson and Crick model of DNA is called B-form DNA. The chains in B-form DNA are in right handed orientation.

Functions of DNA

It controls all the biochemical activities of the cell. It carries genetic information from one generation to other. It controls protein synthesis and synthesize RNAs.