DNA & RNA the basic bio molecules of heredity are very complex molecules with definite structure and functions. Few of the basic structures and functions are described in the presentation.

1. DNA & RNA STRUCTURE AND
FUNCTION

2. NUCLEIC ACID (DNA & RNA)
 Discoverer: Fredrich Mischer in 1870 from pus cells,
he named it nuclein.
 Renamed by Altman in 1889 as nucleic acid.
 Altman also discovered two types of nucleic acid- DNA
& RNA.
 In 1944, Avery, Macleod and McCarty confirmed that
DNA is a genetic material.
 Nucleic acids are polynucleotides or nucleotide
polymers.
 A nucleotide consists of
 Pentose sugar
 Nitrogenous base
 Phosphate group

3. DEOXYRIBONUCLEIC ACID (DNA)
 DNA is the genetic material in all organisms except certain
viruses.
 In prokaryotes DNA is present in cytoplasm while in
eukaryotes it is present in well defined nucleus.
 In prokaryotes DNA is circular while it is linear in
eukaryotes.
 The DNA content of a particular species is fairly constant.
 The total haploid genome content of an organism is called
as C value and it is specific for a particular species.
 It is a polymer of deoxyribonucleotides.
 Each nucleotide consist of- nitrogenous base, phosphate
group and deoxyribose sugar.
 Two nucleotides are joined together with help of
phosphodiester bond.

4. EUKARYOTIC DNA
 Watson and crick gave the double helical structure
of DNA in 1853.
 This model describes the features of B-DNA
 There are many other forms of DNA as well other
than B-DNA and are biologically important too like
A-DNA, Z-DNA.
 Z-DNA is smallest and left handed helix.

5. B-DNA: THE MOST COMMON FORM
 Major features of B-DNA are:
 Two long polynucleotide strands are coiled around a central
axis.
 Two strands form a right handed double helix
 The strands are antiparallel i.e. one strand is oriented in 5’-3’
direction while other in 3’-5’ direction.
 Two strands are joined together through complementary
base pairing-
A=T
G≡C
 Structural features of B-DNA have been explained in the
figure ahead

6. STRUCTURE OF DNA DOUBLE HELIX
Courtesy: genetics analysis and principles; Robert Brooker

7. COMPARISON OF A, B & Z DNA

8. CHARGAFF’S RULE
Erwin Chargaff proposed two rules which are called
Chargaff’s rule-
 First: in any double stranded DNA the number of G
residues are equal to C residues and A are equal to
G residues
 Second the composition of DNA varies from one
species to another.
Thus-
 A+G = C+T
 A-T base pairs = C-G base pairs.
 A=T & C=G
 A+T/C+G= constant for a species, varies from species
to species.

9. HERSHEY & CHASE
EXPERIMENT
 In 1952 Alfred Hershey and
Martha Chase conducted an exp.
to prove that DNA is the genetic
material.
 They used radioactive labeled
isotopes of sulfur and
phosphorous (S35 & P32).
 They used E. Coli and T2
Bacteriophage.
 They labeled the phages and
infected the E. Coli with labeled
viruses.
 When P32 labeled phage entered
the bacterial cell, then the next
generation of phages from
infected cells showed a significant
amount of P32 label.
 When S35 labeled phage entered
the bacterial cell, in the progeny
they found that most of labeled
S35 remained outside the
bacterial cell. Image Courtesy: principles of biochemistry;
Lehninger 5th ed.

10. CONCLUSION OF HERSHEY & CHASE
EXPERIMENT
 They showed that outer protein coat of phase
doesn't enter the bacterium.
 The inner phage material i.e. DNA enter the
bacterial cell.
 Since the DNA is responsible for production of new
phages during infection process, the DNA not
protein is genetic material.
 They shared Noble prize in medicine in 1969 for
their discoveries.

11. STRUCTURE OF RNA
 RNA is a polymer of ribonucleotide monophosphates
 1. purine bases are adenine and guanine; pyrimidine
bases are cytosine and uracil.
 2. RNA molecules can have extensive secondary
structure
 (a) intramolecular base pairing.
 (b) regions of base pairing in RNA form an A-type
double helix.
 (c) many secondary structures of RNA have defined
functional roles.

12. CLASSES OF RNA MOLECULES
 A. Messenger RNA (mRNA):
 transcribed by RNA polymerase II in eukaryotes; encode proteins
 B. Ribosomal RNA (rRNA)
 1. 18 S, 28 S, and 5.8 S rRNAs are transcribed by RNA
polymerase I in eukaryotes.
 2. 5 S RNA is another type of RNA associated with ribosomes
but is transcribed by RNA polymerase III in eukaryotes.
 3. rRNA serve structural and catalytic roles in ribosomes.
 C. Transfer RNA (tRNA): transcribed by RNA polymerase III in
eukaryotes.
 Note: all three types of RNA’s above are transcribed by the
same RNA polymerase in prokaryotes.
 D. Numerous other small RNA’s are also found in cells—in
eukaryotic cells these can be put into two general classes:
snRNA = small nuclear RNA & scRNA = small cytoplasmic RNA.
 snRNA’s and scRNA’s are found complexed with proteins and
carried a variety of cellular functions (snRNP and scRNP).

13.  It consists of about 5% of total RNA
 It is the longest of all three types of RNA.
 Synthesis of messenger RNA occurs in nucleus, with help of
enzyme RNA pol II.
 The mRNA formed after transcription called as pre mRNA
and it consists of exons and introns.
 It undergoes post transcriptional modifications to function as
coding mRNA, these modifications also occur in nucleus.
 The modification involve capping, poly A tail addition and
splicing.
 After modifications the mRNA is transported to cytoplasm.
 Eukaryotic mRNA is monocistronic while that of prokaryotes
is polycistronic.
 half life of mRNA is very short.
m RNA: messenger RNA

15. RIBOSOMAL RNA
 It comprises of about 80% of total RNA.
 rRNA consists of 2 subunits; (50s and 30 s in
prokaryotes and 60s & 40s in eukaryotes).
 There are 3 kinds of r RNA in prokaryotes- 23s, 16s
& 5s; while 4 kinds in eukaryotes- 28s, 18s, 5.8s &
5s.
 The synthesis of ribosomal RNA occurs in
nucleolus and enzyme involved is RNA pol I (28s,
18s, 5.8s) and RNA pol III for 5s rRNA.
 The rRNAs are combined with proteins in nucleolus
to form ribonucleoprotein (RNPs)
 These subunits unite to form ribosomes at time of
protein synthesis.
(rRNA)

16.  It forms about 15% of total
RNA.
 It is the smallest of all 3
RNAs.
 Its secondary structure in
clover leaf shaped.
 It has four arms:
 Amino acid binding site
 D arm (activating enzyme
site)
 Anticodon loop
 Tψc arm (ribosome recog.
site)
 The tRNA are the readers
of mesege written in
mRNA.
tRNA

17. SNRNA
 Small nuclear RNA are group of molecules confined
to nucleus.
 They range in 90-220 nucleotides.
 These are of 6 types: U1, U2, U3, U4, U5 & U6.
 These are not present as free nucleic acid, but are
complexed with proteins to form small nuclear
ribonucleoprotein particles (snRNPs) and form
spliceosome.
 They play a role in post transcriptional RNA
processing.

18. REFERENCES
 Jocelyn E. Kreb, Elliot S. Goldstein, Stephan T.
Kilpatrick; LEWIN’S GENES; 10th edition.
 David L. Nelson, Micheal M. Cox; LEHNINGER
PRINCIPLES OF BIOCHEMISTRY; 5th edition.
 Robert J. Brooker; GENETICS ANALYSIS AND
PRINCIPLES; 4th edition.
 Donald Voet, Judith G. Voet; BIOCHEMISTRY; 4th
edition.
 P. S. Dhami, J. K. Dhami; ZOOLOGY VOL II.