GENETIC MATERIAL refers to the material of which genes are made up of. It includes both DNA and RNA. Though in most of the organism DNA is playing this role, but in certain viruses RNA is storing all the genetic information of the individual. Here we are discussing about the discovery and property of these genetic material.

1. THE GENETIC MATERIAL
Prepared By:
Dr. Asit Prasad Dash
Assistant Professor
DEPARTMENT OF PLANT BREEDING AND GENETICS
INSTITUTE OF AGRICULTURAL SCIENCES
SIKSHA ‘O’ ANUSANDHAN (DEEMED TO BE UNIVERSITY), BHUBANESWAR,
751029

2. THE GENETIC MATERIAL:
It refers to the material of which genes are made.
Properties:
• High fidelity replication.
• Ability to express itself.
• Ability of store information.
• Must provide some error for origin of genetic variation.
IDENTIFICATION OF THE GENETIC MATERIAL
v The process of identification of genetic material began in 1928
with the experiments of Griffith and concluded in 1952 with the
studies of Hershey and Chase.
v Another ingenious experiment by Frankel-Conrat and Singer in
1957 established that in some viruses RNA functions as the
genetic material.

3. v But nuclic acids were discovered much earlier in 1871 by
Meischer who called them nuclein.
v There are two types of nucleic acids, viz., deoxyribose nucleic
acid (DNA) and ribose nucleic acid (RNA).
v In eukaryotes, chromosomes contain genes, and they are made
up of chromatin, i.e., DNA + proteins.
v Obviously, either DNA or protein would be the genetic material.
There was a prolonged controversy before DNA was
unequivocally accepted as the genetic material.

4. Experiment of Griffith
v Griffith discovered the phenomenon of transformation through
his studies on Diplococcus pneumoniae, which causes pneumonia
in most of the mammals.
v Different strains of Diplococcus form one of the following two
types of colonies: (1) smooth and (2) rough.
v The cells of strains forming smooth colonies are able to produce
pneumonia and are called virulent.
v But strains producing rough colonies are avirulent since they
cannot produce pneumonia.
v When live cells of the avirulent strain IIR (R is for rough
colonies) were injected into mice, all the mice survived as they did
not suffer from pneumonia.
v On the other hand, when mice were injected with live IIIS (S is for
smooth colonies; virulent) cells, all the mice died due to
pneumonia.

6. v Further, mice injected with heat-killed cells of the virulent strain
IIIS did not develop pneumonia.
v However, when mice were injected with a mixture of heat-killed
IIIS cells and live IIR cells, some of them died due to pneumonia.
v Diplococcus cells isolated from the dead mice were of the type IIIS.
v Since all the cells of the heat-killed IIIS culture were dead, it was
postulated that some of the cells of IIR changed into the IIIS type
due to the influence of dead IIIS cells present in the mixture.
v This phenomenon was called transformation. The component of
IIIS cells, which induced the conversion of IIR cells into IIIS cells
was named as the transforming principle.
v The experiments of Griffith demonstrated transformation, but they
did not hint at the identity of the transforming principle. It was
later shown by Avery and co-workers that DNA is the
transforming principle.

7. Experiments of Avery,
MacLeod and McCarty
Avery and associates
carried out the
experiments of Griffith in
vitro on a glass vessel in
the place of mice (in
vivo).

8. Experiments of Hershey and Chase
v The results of these experiments, led to the universal acceptance
of DNA as the genetic material.
v Hershey and Chase studied the life cycle of T2 phage of E. coli;
they clearly showed that only the DNA component of T2 particles
is transmitted to the progeny phage particles.
v T2 and other bacteriophages are composed of protein and DNA.
v Head coat and tail are made up of protein, while the DNA is
packed inside the head coat.
v DNA contains phosphorus (P) but no sulphur (S), while
proteins contain S but no P.
v Therefore, they labelled T2 DNA with 32P, while proteins were
labelled with 35S.

11. RNA as Genetic Material
v In several viruses, e.g., TMV (tobacco mosaic virus), DNA is absent.
These viruses are composed of RNA and protein.
v TMV particles are like hollow cylinders. Their RNA is coiled like a
spring, while the protein molecules are arranged on the outside of the
coil.
v Frankel-Conrat and Singer demonstrated that RNA functions as
the genetic material in TMV.
v Proteins and RNA of TMV can be separated chemically; when
they are remixed under appropriate conditions, they reassociate to
produce active TMV particles.
v In one experiment, Frankel-Conrat and Singer used either RNA or
the proteins isolated from TMV for infection of tobacco leaves.
v Mosaic symptoms developed only when RNA was used for
infection (and not when the proteins were used).
v Clearly, only RNA fraction of TMV is capable of producing the
disease, and hence appears to be the genetic material.

13. Components of DNA
Chemical analyses have shown that nucleic acids (DNA and RNA) are composed of
the following three types of molecules
1. Phosphoric Acid
Phosphoric acid (H3PO4) is involved in forming the sugar-phosphate backbone of
DNA, which is linked to the 5'C of one and the 3’C of the other neighbouring
pentose sugar molecule of DNA to produce the phosphodiester (5'C-0-P-0-C3')
linkage.
2. Pentose Sugar
The pentose present in RNA is called ribose from which this nucleic acid gets its
name. Similarly, DNA contains deoxyribose, which is the reason for the name
deoxyribose nucleic acid.
3. Organic Bases/ Nitrogenous bases
DNA ordinarily contains four different bases called, adenine (A), guanine (G),
cytosine (C) and thymine (T). RNA contains the same bases except for thymine
in it has uracil (U). These five bases (A, G, C, T, U) are grouped into two classes:
(I) pyrimidine (C, T, U) and (2) purine (A, G).

16. Nucleosides
Nucleosides are formed by the linkage of an organic base to the
pentose sugar with the help of a covalent bond.
Organic base + Ribose Riboside + 1H20
Organic base + Deoxyribose Deoxyriboside + 1H20
Ribosides and deoxyribosides = Nucleosides
Nucleotides
A Nucleotide is formed when a phosphate group is attached to the
pentose molecule of a nucleoside.
Organic base + Ribose + phosphate Ribotide + 2H20
Organic base + Deoxyribose + phosphate Deoxyribotide+ 2H20
Ribotides and deoxyribotides = Nucleotides

18. PRIMARY STRUCTURE OF DNA
• A native DNA molecule is double-stranded.
• Each of the two strands of a DNA molecule has many deoxyribonucleotides, and
is known as a polynucleotide.
• These nucleotides are joined with each other by phosphodiester linkages.
• In a phosphodiester linkage, the 5'C of the pentose of one nucleotide is linked
with one -O- of the phosphate, while the 3'C of pentose of the other nucleotide
is linked with another -O- of the same phosphate residue. This produces a 5'C-
0-P-0-C3' linkage.
• Thus a polynucleotide chain consists of a backbone made-up of several alternating
pentose and phosphate molecules linked with each other; this is called sugar-
phosphate backbone.
• Each pentose has any one of the four organic bases.
• At one end of the polynucleotide chain, the 5'C of pentose has a free -OH (-OH of
the phosphate group attached to the 5'C), while the 3'C of the pentose at the
opposite end has a free -OH; these ends are known as 5'- and 3'-ends,
respectively.

20. THE DNA DOUBLE HELIX
Chemical analyses of DNA by Chargaff to and others during 1940s
clearly demonstrated the following features. (Chargaff’s principle)
v The quantity of A is always equal to that of T, while the
quantity of G is equal to that of C.
v As a rule, the number of pyrimidine bases, i.e., C+T, is equal to
that of purine bases, viz., A + G.
v Similarly, A + C content of DNA is equivalent to that of G + T.
These findings were available by early 1950s, and were used by
Watson and Crick to develop the double helix model of DNA
formally proposed in 1953. This model was soon universally
accepted.

21. Main features of double helix model of DNA (Watson and Crick)
v A DNA molecule is made up of two polydeoxyribonucleotide (or simply
polynueleiotide) strands or chains.
v Each polynucleotide strand is composed of many deoxyribonucleotiries.
v The two strands of a DNA molecule are oriented antiparallel to each
other. This antiparallel orientation of the two strands is essential for the
formation of hydrogen bonds between the pairs of DNA bases
v The base sequences of the two strands of a DNA molecule show the
following universal relationship.
(a) Wherever adenine occurs in one strand, thymine is present in the
corresponding position of the other strand and vice-versa.
(b) the sites at which guanine is present in one strand are occupied
by cytosine in the second strand and vice-versa.
v Therefore, the two strands of a DNA molecule are called complementary
strands. Thus, if the base sequence of one strand of DNA is known, the
base sequence of its complementary stand can be easily deduced.

22. v The adenine present in one strand of a DNA molecule is linked by two hydrogen
bonds with the thymine located opposite to it in the second strand, and vice-versa.
v Similarly, G located in one strand forms three hydrogen bonds with the C present
opposite to it in the second strand and vice-versa.
v The two strands of a DNA molecule are coiled together in a right-handed helix
forming the DNA double helix.
v The diameter of this helix is 20 Å, while its pitch (length of helix required to
complete one turn) is 34 Å.
v In each DNA strand, the bases occur at a regular interval of 3.4 Å so that about 10
base pairs are present in one pitch of a DNA double helix. The base pairs in a
DNA molecule are stacked between sugar phosphate backbones of the two strands.
v During replication, the two strands of a DNA molecule uncoil. The unpaired bases
in the single-stranded regions of the two strands pair with their complementary
bases. These nucleotides become joined with each other and yield the
complementary strands of the old ones. This provides for an almost error-free
replication of the genetic material.
v Sometimes, errors in base-pairing may occur during replication. This would
account for the occurrence of mutations.

24. THE A, B, C AND Z FORMS OF DNA
The above description is of the B-from of DNA. DNA is also known
to occur in three other forms (Table 1).
TABLE 1: Main differences between different forms of DNAs
Characteristic A-DNA B-DNA C-DNA Z-DNA
Coiling Right-
handed
Right-
handed
Right-
handed
Left-handed
Pitch 28 Å 34 Å 31 Å 69 Å
Base pairs per turn 11 10 9.33 12
Diameter 23 Å 19 Å 19 Å 18 Å
Vertical rise per
base pair
2.56 Å 3.38 Å 3.32 Å 5.71 Å
Sugar-phosphate
backbone
Regular Regular Regular Zig-zag

25. STRUCTURE OF RNA MOLECULES
v RNA, like DNA, is a polynucleotide.
v It is produced by phosphodiester linkages between
ribonucleotides in the same manner as in the case of DNA.
v RNA nucleotides have ribose sugar (in place of deoxyribose in
DNA), which participates in the formation of sugar-phosphate
backbone of RNA.
v Thymine is usually absent in RNA, and uracil is found in its place.

26. COMPARISON BETWEEN DNAAND RNA
Characteristics DNA RNA
Pentose sugar Deoxyribose Ribose
Organic base Ordinarily, thymine present
and uracil absent
Ordinarily, uracil present and
thymine absent.
Number of
strands
Generally, double-stranded Generally, single-stranded
Function Genetic material only (i) Generally, nongenetic functions,
e.g., mRNA rRNA, tRNA, etc.
(ii) In some viruses, genetic
material
Origin (i) Replication of pre-existing
DNA (ii) In case of infection
by RNA viruses, reverse
transcription of genetic RNA
(i) Transcription of DNA, or
(ii) Through replication of RNA by
RNA-dependent RNA polymerase
Native form Double-stranded DNA usually
in B-form
Double-stranded RNA usually in
A-form

27. Thank You