M.Prasad NaiduMSc Medical Biochemistry, Ph.D,.

Structure of DNA

Watson & Crick model Double stranded helixBoth strands are polar [5’ and 3’end]Both strands are Anti-parallelBoth strands are complimentary .Strands are composed of A, T, C, G Strands are held by phosphodi-ester

and Hydrogen bonds Obeys base pair ruleObeys Chargaff’s rule

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 one strand

runs in 5runs in 5‘ ‘ to 3to 3'' and another in 3 and another in 3'' to 5 to 5'' direction. direction.

4. The two chains are not identical but 4. The two chains are not identical but complementarycomplementary to to

each other due to base pairing.each other due to base pairing.

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 phosphateback bone on periphery and hydrophobic bases stacked inside.

7. The two strands of helix are held by both hydrogen The two strands of helix are held by both hydrogen bonds between bases and by vanderwaals and bonds between bases and by vanderwaals and hydrophobic interactions between adjacent base hydrophobic interactions between adjacent base pairspairs

8. Obeys base pair rule8. Obeys base pair rule: :

Adenine always pairs with

Thymine and Guanine with Cytosine Two purines , they would not fit into allowable spaceTwo pyrimidines – too far to form H- bonds.So, purine always pairs with pyrimidine

..

A T Weak

Two Hydrogen bonds

G C Strong

Three Hydrogen bonds

Base Pairing

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.

Coding strand : the DNA strand that gives the genetic information. Also called as

non-template Template strand: the DNA strand the forms the mRNA.

5`end coding strand[DNA] 3`end

AGTC------------------------------------ TCAG------------------------------------

template strand[DNA]

AGUC--------------------------------------- RNA

DNA

Structure of DNA : Watson & Crick model

1.Double stranded helix2.Both strands are polar [5’ and 3’end]3.Both strands are Anti-parallel4.Both strands are complimentary .5.Strands are composed of A, T, C, G 6.Strands are held by phosphodi-ester/ H

bonds7.Deoxy ribophosphates outside and bases

inside 8.Obeys base pair rule9.Obeys chargaff’s rule 10.Stores genetic message on coding strand.

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

DNA structure

.

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

Denaturation of DNADenaturation of DNA

• The double stranded structure of DNA can be separated The double stranded structure of DNA can be separated

into two component strands (melted) in solution by into two component strands (melted) in solution by

increasing temperature or decreasing salt concentration.increasing temperature or decreasing salt concentration.

• This phenomenon of loss of helical structure is called This phenomenon of loss of helical structure is called denaturation.denaturation.

• Disruption of hydrogen bonds occur.Disruption of hydrogen bonds occur.

•

•PDE bonds not broken.PDE bonds not broken.

Melting temperature (Melting temperature (Tm)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.

RENATURATION OF DNARENATURATION OF DNA

• Separated strands of DNA reassociate when

appropriate physiologic temperature and salt

conditions are achieved , a process called

as Renaturation (Reannealing)

RIBONUCLEIC ACID (RNA)

RNA is a polymer of ribonucleotides in a single chain linked by phosphodiester bonds.

3 TYPES: mRNA, tRNA, rRNA

.

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.

.

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.

Post transcriptional modifications 51 Capping and 31 Poly-A tailing

primary transcript [m RNA]

51 ---------------------- 31 [hn RNA]

7-methyl guanosine51 --------------------------------- 31 AAAAA [ capping ] mature RNA

[ tailing ]

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 pseudouridine and thymineand thymine.

• The primary structure of t-RNA allows extensive folding to generate secondary structure --- CLOVER LEAF MODEL.

Structure of t RNA : Clover leaf model with 5 arms

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

.

RIBOSOMAL RNA (rRNA)

Ribosomes provide necessary infrastructure

for mRNA , tRNA, and aminoacids to interact

with each other for translation process.

Eukaryotic 80s ribosome

60s

40s

Differences between DNA and RNA

DNA RNA

• SITE NUCLEUS CYTOPLASM

• STRAND DOUBLE Single strand

• SUGAR DEOXYRIBOSE RIBOSE

•BASES THYMINE URACIL

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