THE NUCLEIC ACIDS

DR SAKINA ROOHI MBBS. M.D

OUT LINE

• INTRODUCTION

• NUCLEOSIDE

• NUCLEOTIDE

• DNA STRUCTURE

• RNA STRUCTURE

Nucleic Acid

Nucleotides are precursors of nucleic acid DNA & RNA

Nucleic acids were first isolated by Friedrich Miescher in 1869.

suga r base

sug a r base

phospha te

sug ar ba se

phospha te

su ga r base

phospha te

su ga r base

phospha te

Nucleoside

Nucleotides

Nucleic acids

1 23

2

1

PENTOSE

C1

C5

C4

C3 C2

O

Nitrogenous Bases

Purines (double-ring structures)

A and G

Pyrimidines (single-ring structures)

C, T, and U

hyperphysics.phy-astr.gsu.edu

PURINES AND PYRIMIDINES

Purines and pyrimidines are AROMATIC HETEROCYCLIC COMPOUNDS contain both carbon and other elements (hetero atoms).

Purine & Pyrimidine

BIOLOGICALLY IMPORTANT BASES

NUCLEOSIDE• Nucleosides are derivatives of purines and pyrimidines that

have a sugar linked to a ring nitrogen.

The sugar is linked to the heterocyclic base via a β-N-glycosidic bond, almost always to N-1 of a pyrimidine or to N-9 of a purine.

NUCLEOTIDE STRUCTUREPHOSPATE SUGAR

Ribose or Deoxyribose

NUCLEOTIDE

BASEPURINES PYRIMIDINES

Adenine (A)Guanine(G)

Cytocine (C)Thymine (T)Uracil (U)

NUCLEOTIDE

PHOSPHODIESTER BOND

Tautomerism of the Bases

Important Analogs of Purine Pyrimidine &Nucleotides

• 1.Allopurinol-used in treatment of gout

• 2.Azathioprine-suppress immunological rejection during transplantation

• 3.Arabinosyladenine-used in treatment of neurological diseases

• 4.Arabinosylcytosine-used in cancer therapy

• 5.Drugs used in AIDS are sugars modified synthetic nucleotide Analogs

DNA-The Double Helix (1953)

The DNA Double Helix

THE SUGAR-PHOSPHATE BACKBONE

• The nucleotides are all orientated in the same direction

• The phosphate group joins the 3rd Carbon of one sugar to the 5th Carbon of the next in line.

P

P

P

P

P

P

ADDING IN THE BASES

• The bases are attached to the 1st Carbon

• Their order is important It determines the genetic information of the molecule

P

P

P

P

P

P

G

C

C

A

T

T

DNA IS MADE OF TWO STRANDS OF POLYNUCLEOTIDE

P

P

P

P

P

P

C

G

G

T

A

A

P

P

P

P

P

P

G

C

C

A

T

T

Hydrogen bonds

Chargaff’s Rules:

A=T and G=Cpurines = pyrimidines

The DNA Double Helix

A, B and Z Forms of DNA

OTHER FORMS OF DNA STRUCTURE

BENT DNA

TRIPLE STRANDED DNA

FOUR STRANDED DNA

IT IS BELIEVED THAT SUCH STRUCTURES ARE IMPORTANT FOR MOLECULAR RECOGNITION OF DNA BY PROTEINS AND ENZYMES

OTHER FORMS OF DNA

Hoogsteen Hydrogen bonds

The Nucleosome -DNA (146 bp) wrapped around octamer of core histone proteins (+ linker DNA = ~200 bp)

ORGANISATION OF DNA

• Much more abundant than DNA

• Single stranded structure

• Contains ribose (DNA 2’-deoxy ribose)

• Contains uracil base• Does not obey Chargaff ’s rule

• Susceptible to alkaline hydrolysis

• Retains all information of DNA

• Specific RNA for specific functions

• Structure similar to A-form DNA

• 3 major types• mRNA : 5-10%

• tRNA: 10-20 %

• rRNA: 50-80 %

10/27/14Dr Aminu

33