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Nucleic Acid
Chemistry
BIOMEDICAL IMPORTANCE
 They form a part of many coenzymes.
 Serve as donors of phosphoryl groups (eg,
ATP or GTP)
 Regulatory nucleotides include the second
messengers cAMP and cGMP
 Synthetic purine and pyrimidine analogs
that contain halogens, thiols, or additional
nitrogen are employed for chemotherapy
of cancer and AIDS
BIOMEDICAL IMPORTANCE
 Suppressors of the immune response
during organ transplantation.
 Genetic diseases of purine metabolism
include gout
 There are few clinically significant
disorders of pyrimidine catabolism
Nucleic acid
DNA
or
RNA
Adenine
A
DNA or RNA
Guanine
G
DNA or RNA
Uracil
U
RNA
only
Cytosine
C
DNA or RNA
Thymine
T
DNA
Nucleic acid
↓ Nuclease
Nucleotides
↓ Nucleotidase
Nucleoside + Phosphoric acid (H3PO4)
↓ Nucleosidase
Pentose
+
Nitrogenous base
↓
↓
Ribose or Deoxyribose
RNA
DNA
Purines Pyrimidines
A, G
U, C, T
Nitrogenous bases
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 Nucleosides
 SUGAR (Pentose) + Nitrogenous Base
 Nucleotides
 SUGAR (Pentose) + Nitrogenous Base
 + Phosphate

They are formed by esterification of phosphoric acid to the OH
of C5` of Pentose sugar of the nucleoside.
Nucleic acid structure
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 The Polynucleotide structure of nucleic acids
is obtained by esterification of phosphate of
each nucleotide with the OH group of the
C3` of pentose of the adjacent nucleotide.
 The nucleic acid has 2 ends:
 The 5 ` end: has a `5 (P) group.
 The `3 end: has a `3 free (OH)
group.
Human DNA
 Human DNA is composed of TWO
POLYNUCLEOTIDE STRANDS.
 DNA PRIMARY STRUCTURE
 PENTOSE SUGAR:
 2-deoxyribose
 N BASES:
 A, G, C and T
 NUCLEOTIDES:
 dAMP, dGMP, dCMP, dTMP
DNA PRIMARY STRUCTURE
 Sugar and Phosphate form backbone
structure (OUTSIDE)
 Nitorgenous bases (INSIDE)
 3` --free OH group of pentose sugar.
 5` --free phosphate group
 Sequence is written from 5 ` → 3 `
direction.
DNA secondary structure
2 helices, opposite
Backbone (P & S)
Outside
N Bases (inside)
3.4 Å
10 base (34 Å)
20 Å diameter
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DNA secondary structure
Watson-Crick model of DNA
1. Two helical polynucleotide chains are coiled
around a common axis. (DOUBLE HELIX)
2. The chains run in opposite directions.
(ANTIPARALLEL)
3. The sugar-phosphate backbones are on the
OUTSIDE.
4. The purine and pyrimidine bases lie on the
INSIDE of the helix.
Complementary Bases A = T
-
G=C
Watson-Crick model of DNA
5. The bases are nearly perpendicular to the
helix axis
6. Adjacent bases are separated by 3.4 Å.
7. The helical structure repeats every 34 Å, so
there are 10 bases (34 Å per repeat/3.4 Å per
base) per turn of helix.
8. The diameter of the helix is 20 Å.
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DNA
 Site:
 Nucleus
 Mitochondria
 Functions:
 Carry genetic informations
 RNA synthesis
Nucleotides
 Are polyfunctional Acids
 Free purine or pyrimidine bases are uncharged
at physiologic pH
 Phosphoryl groups of nucleotides ensure that
they bear a negative charge at physiologic pH
 nucleotides absorb light at a wavelength close to
260 nm
 The concentration of nucleotides and nucleic
acids thus often is expressed in terms of
“ABSORBANCE AT 260 nm.”
Human RNA
 Site:
 Mainly in the cytosol, less common in
the nucleus.
 Sugar:
 Ribose
 Bases:
 A, G, C and U.
 Nucleotides:
 AMP, GMP, CMP and UMP.
RNA (Ribonucleic Acid)
 Ribosomal RNA (rRNA)

constituents of ribosomes, the intricate cellular machines
that synthesize proteins.
 Messenger RNA (mRNA)

encode the amino acid sequence of one or more specified
by a gene.
 Transfere RNA (tRNA)

read the information encoded in the mRNA and transfer
the appropriate amino acid to a growing polypeptide
chain during protein synthesis.
Ribosomal RNA (rRNA)
 Its about 80% of total RNA.
 Function, Synthesis of 2 ribosomal
subunits.
 RIBOSOSMES:
 Formed of ribonucleoproteins (rRNA + protein)
 2 subunits Small (40S) and Large (60S)
 40S + 60S form 80S ribosome,

responsible of Protein synthesis.
Messenger RNA (mRNA)
 Its about 5% of the total RNA.
 Carry the message, genetic information from
DNA to protein..
 Each 3 bases in the mRNA code for one

amino acid (CODON)
 Genetic code.
 AUG codes for methionine
 UCA codes for serine
Transfere RNA (tRNA)
 Its about 15% of the total RNA.
 Carry and transport amino acids to
ribosomes. (PROTEIN SYNTHESIS).
 Each tRNA is compsed of 70 - 90 nts.
 Clover leaf shape (3 lops and small lump).
 More than 20 different types of tRNA.
 ONE ON ONE. (aa : tRNA)
 Amino acid + tRAN = amino acyle tRNA.
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Image from: http://www.biology.iupui.edu/biocourses/N100/images/13translation.gif
DNA
Site
RNA
Cytosol
Shape
Nucleus &
mitochondria
Double helix
Strand
Double stranded
Single stramded
Purines
A, G
A, G
Pyrimidines
C, T
C, U
Types
One
Three
Functions
Carry genetic
information, RNA
synthesis
Protein synthesis
Variable