But mice injected with heat killed cells of virulent strain IIIS did not develop pneumonia indicating that cells were killed by the heat treatment.
However, when the mice were injected with a mixture of heat killed IIIS cells and live IIR cells, some of them died due to pneumonia; Diplococcus cells isolated from the dead mice were of the type of IIIS.
Since all the cells of the heat killed IIIS culture were dead it was postulated that some of the cells of IIR had changed into the IIIS type due to the influence of dead IIIS present in the mixture.
This phenomenon called as transformation and components of IIIS cells, which induced the conversion of IIR cells into IIIS was named the transforming principle.
Griffith demonstrated the transformation but they did not hint as the identity of transforming principle.
Confirmation by Hershey and Chase (1952):
DNA is the genetic material could also be demonstrated with the help of infecting bacteria with T2 bacteriophage.
In 1952, they reported that DNA of the phage enters the host cells and this DNA carried all the genetic information necessary for assembly of new phage progeny. They labeled T2 DNA and protein with 32P and 35S respectively.
DNA contains phosphorus (P) but not sulphur (S) and protein contain sulphur only.
Then allowed both kinds of labelled phage particles to infect bacteria. After shaking, only radioactive 32p was found associated with bacterial cells and 35s found only in surrounding medium and not in bacterial cell.
When phage progeny was studied for radioactivity, it was found that the phage progeny carried label with 32p but not label with 35s.
This indicates that only DNA enters into bacterial cells and not in protein coat and cause the synthesis of new phage particles. So it proved that DNA is the genetic material.
Components and DNA double helix structure:
Deoxyribonucleic acid is an extremely long polymer made from units called deoxyribonucleotides, which are often simply called nucleotides.
These nucleotides differ from those in one respect: the sugar is deoxyribose, not ribose. Note that deoxyribose, unlike ribose, has no OH group on its 2_ carbon.
Four bases are found in DNA; they are the two purines, adenine (A) and guanine (G) and the two pyrimidines, cytosine (C) and thymine (T).
The combined base and sugar is known as a nucleoside to distinguish it from the phosphorylated form, which is called a nucleotide. Four different nucleotides join to make DNA.
They are 2_-deoxy-ad- enosine-5_-triphosphate (dATP), 2_-deoxyguanosine-5_-triphos- phate (dGTP), 2_-deoxycytidine-5_-triphosphate (dCTP), and 2_- deoxythymidine-5_-triphosphate (dTTP).
The DNA molecule is a double helix:
In 1953, Rosalind Franklin used X-ray diffraction to show that DNA was a helical (i.e. twisted) polymer.
James Watson and Francis Crick demonstrated by building three dimensional models that the molecule is a double helix.
Two hydrophilic sugar-phosphate backbones lie on the outside of the molecule and the purines and pyrimidines lie on the inside of the molecule.
There is just enough space for one purine and one pyrimidine in the center of the double helix. The Watson and Crick model showed that the purine guanine (G) would fit nicely with the pyrimidine cytosine (C) forming three hydrogen bonds.
The purine adenine (A) would fit nicely with the pyrimidine thymine (T) forming two hydrogen bonds. Thus A always pairs with T, and G always pairs with C.
The three hydrogen bonds formed between G and C produce a relatively strong base pair. Because only two hydrogen bonds are formed between A and T this weaker base pair is more easily broken.
The difference in strengths between a GC and an AT base pair is important in the initiation and termination of RNA synthesis.
The two chains of DNA are said to be anti-parallel because they lie in the opposite orientation with respect to one another with the 3’_-hydroxyl terminus of one strand opposite the 5?-phosphate terminus of the second strand.
The sugar phosphate backbones do not completely conceal the bases inside. There are two grooves along the surface of the DNA molecule.
One is wide and deep the major groove and the other is narrow and shallow the minor groove. Proteins can use the grooves to gain access to the bases.
The coiling of helix is right handed and full length of one spiral turn is 34° A with 10 base pairs.
Modes of replication:
During replication the two strands of the double helix unwind. Each then acts as a template for the synthesis of a new strand.
This process generates two double-stranded daughter DNA molecules, each of which is identical to the parent molecule.
The base sequences of the new strands are complementary in sequence to the template strands upon which they were built.
This means that G A, C, and T in the old strand cause C, T, G and A, respectively, to be placed in the new strand.
The replication of double stranded DNA could be:
The double stranded DNA molecule is conserved as such and a new copy is synthesized from old molecule.
2) Dispersive replication:
The old molecule should disintegrate and two new molecules would be synthesized.
3) Semi conservative replication:
The two strands could separate from one another to maintain their integrity and each will synthesize (from the pool of nucleotides) its complementary strand.
The newly synthesized molecule will conserve one of the two stands from parent molecule and the other strand would be newly assembled.