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1. M.Prasad Naidu
MSc Medical Biochemistry, Ph.D,.

2. Structure of DNA
 Watson & Crick model
 Double stranded helix
 Both strands are polar [5’ and 3’end]
 Both strands are Anti-parallel
 Both strands are complimentary .
 Strands are composed of A, T, C, G
 Strands are held by phosphodi-ester
and Hydrogen bonds
 Obeys base pair rule
 Obeys Chargaff’s rule

3. STRUCTURE OF DNA
The structure of DNA was proposed by Watson and Crick.
Salient features
1. DNA is a double stranded helix.
DNA has two chains , twisted around each other on a
common axis. It is commonly a right handed helix .
2. The 2 strands are polar with 5’ and 3’ ends.
3. The two strands are antiparallel, one strand
runs in 5‘ to 3' and another in 3' to 5' direction.
4. The two chains are not identical but complementary to
each other due to base pairing.

4. 5. Each turn of helix is 3.4 nm , with 10 pairs of
nucleotides , each pair placed at a distance of 0.34nm.
6. Each strand has hydrophilic deoxyribose phosphate
back bone on periphery and hydrophobic bases
stacked inside.
7. The two strands of helix are held by both hydrogen
bonds between bases and by vanderwaals and
hydrophobic interactions between adjacent base
pairs

5. 8. Obeys base pair rule:
Adenine always pairs with
Thymine and Guanine with Cytosine
Two purines , they would not fit into allowable space
Two pyrimidines – too far to form H- bonds.
So, purine always pairs with pyrimidine
.

6. A T Weak
Two Hydrogen bonds
G C Strong
Three Hydrogen bonds
Base Pairing

7. 9.CHARGOFF’s RULE:
Concentration of adenine = thymine , and
concentration of guanine = cytosine.
10. Genetic information is present on coding strand.
the other strand is called as non coding strand.

8.  Coding strand : the DNA strand that
gives the genetic information. Also called as non-
template
 Template strand: the DNA strand the
forms the mRNA.

9. 5`end coding strand[DNA] 3`end
AGTC------------------------------------
TCAG------------------------------------
template strand[DNA]
AGUC---------------------------------------
RNA
DNA

10. Structure of DNA : Watson & Crick model
1. Double stranded helix
2. Both strands are polar [5’ and 3’end]
3. Both strands are Anti-parallel
4. Both strands are complimentary .
5. Strands are composed of A, T, C, G
6. Strands are held by phosphodi-ester/ H bonds
7. Deoxy ribophosphates outside and bases inside
8. Obeys base pair rule
9. Obeys chargaff’s rule
10.Stores genetic message on coding strand.

11. TYPES OF DNA
A DNA B DNA Z DNA
• Helix handedness right right left
• Base pairs per Turn 11 10 12
• Pitch ( turn ) 2.46 nm 3.4 nm 4.56 nm
•Major groove present present convex shape
•Minor groove present present deep cleft

12. DNA structure

13.  .
Each strand has hydrophilic deoxyribose phosphate
back bone on periphery and hydrophobic bases
stacked inside

15. Denaturation of DNA
• The double stranded structure of DNA can be separated
into two component strands (melted) in solution by
increasing temperature or decreasing salt concentration.
• This phenomenon of loss of helical structure is called
denaturation.
• Disruption of hydrogen bonds occur.
•
•PDE bonds not broken.

17.  Melting temperature (Tm) :
 It is defined as temperature at which half of helical
structure is lost.
 Tm is influenced by base composition and by salt
concentration of solution. DNA rich in G-C pairs melts
at higher temperature than A-T pairs.

18. RENATURATION OF DNA
• Separated strands of DNA reassociate when
appropriate physiologic temperature and salt
conditions are achieved , a process called
as Renaturation (Reannealing)

19. RIBONUCLEIC ACID (RNA)
RNA is a polymer of ribonucleotides in a single chain
linked by phosphodiester bonds.
3 TYPES: mRNA, tRNA, rRNA
.

20. Messenger RNA
•m RNA is present in nucleus
•m RNA receives genetic information from DNA
• m RNA is the most heterogenous in size and stability.
• hnRNA on processing liberates functional mRNA which enter
cytoplasm and take part in protein synthesis.
.

21.  The mRNA formed from DNA is called the
primary transcript or hnRNA.
 It undergoes extensive modifications to
become active and mature mRNA.
These modifications are called as post
transcriptional modifications.

22. Post transcriptional modifications51 Capping and 31 Poly-A tailing
primary transcript [m RNA]
51 ---------------------- 31 [hn RNA]
7-methyl guanosine51 --------------------------------- 31 AAAAA
[ capping ] mature RNA [ tailing ]

23. Transfer RNA (soluble RNA):
• At least 20 species of tRNA in every cell ,
corresponding to each of 20 aminoacids required for
protein snythesis.
• Apart from regular nucleotides it has pseudouridine
and thymine.
• The primary structure of t-RNA allows extensive
folding to generate secondary structure ---
CLOVER LEAF MODEL.

24. Structure of t RNA : Clover leaf model with 5 arms

26. tRNA
1.Acceptor arm at 3 'end: Carries the aminoacid.
2. Anticodon arm : recognises codon present in mRNA
3. Pseudouridine arm (TψC): binds t RNA to ribosomes.
4. D arm : recognition site for amino acid
5. Variable arm : Most variable arm

27. RIBOSOMAL RNA (rRNA)
Ribosomes provide necessary infrastructure
for mRNA , tRNA, and aminoacids to interact
with each other for translation process.

28. Eukaryotic 80s ribosome
60s
40s

29. Differences between DNA and RNA
DNA RNA
• SITE NUCLEUS CYTOPLASM
• STRAND DOUBLE Single strand
• SUGAR DEOXYRIBOSE RIBOSE
• BASES THYMINE URACIL

30.  Chargaffs rule obeys do not obey
 Alkalihydrolysis stable susceptible
 Function genetic information protein synthesis
protein synthesis
 Types A,B, and Z mRNA, tRNA, rRNA
DNA RNA