Biosynthesis of nucleotides
De-Novo pathway & Salvage pathway
Nucleosides and Nucleotides
Components of nucleic acids
Types of nucleic acid
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Functions of Nucleic acids
Catabolism of Nucleic acid
Nucleic acids are polymers of nucleotides, joined together by
They contain information for inheritance and act as genetic
material of all living things as well as of viruses.
Elemental composition- carbon, hydrogen, oxygen, nitrogen
The instructions that govern all cellular activities and enzyme
synthesis are coded within the structure of nucleic acids.
DNA & RNA together known as nucleic acids. Most of the DNA
is in nucleus, some in the chloroplast and mitochondria, while
most of RNA is scattered in the entire cell.
The production of complex molecules from the smaller
monomers, the process known as Biosynthesis.
There are two pathways lead to nucleotides:
1) De-Novo pathways
2) Salvage pathways
In de-novo pathways, the nucleotide bases are assembled from simple
De-novo synthesis of Purine:
Synthesis is IMP, inosine monophosphate ( precursor of Adenine
Synthesis of Adenine and Guanine from IMP.
De-novo synthesis of Pyrimidine:
Synthesis of Uracil
Synthesis of Cytosine
Synthesis of Thymine
Synthesis of deoxy ribonucleic acid
1) De-novo pathways:
This route of nucleotide synthesis has a high requirement for energy
as compared that of salvage pathway.
The first reaction is formation of 5-phosphoribosyl-pyrophosphate
PRPP is required for the synthesis of Purine and Pyrimidine
By series of reactions, PRPP is converted into inosine
Salvage pathway also known as Recycle pathway, used to recover
bases and nucleosides formed during the degradation of RNA and
Salvage pathway of Purine synthesis:
Pre-existing Purine bases and nucleosides may be salvaged to form
Conversion of purine bases into nucleotides is catalyzed by two
(a) Adenine phosphoribosyl transferase ( APRT)
(b) Hypoxynthine-guanine phosphoribosyl transferase (HGPRT)
2) Salvage pathways:
Salvage pathway of Pyrimidine synthesis:
Cytosine base converts into Cytidine by adding ribose and
Uracil base converts into Uridine by adding ribose and
Thymine base converts into Thymidine by adding deoxy ribose
and phosphate group.
Cytosine Cytidine cMP
Uracil Uridine uMP
Thymine Thymidine tMP
The combination of pentose sugar with a base forms a
compound known as nucleosides.
The combination of a nucleoside with a phosphate group
results in a nucleotide.
Thus, a nucleotide consists of a phosphate group, a five carbon
sugar and a nitrogenous base.
Acidic property of nucleic acid is due to having phosphate
group in it.
Nucleotides are building blocks of Nucleic acids (DNA & RNA
Some nucleotides works as Co enzymes (NAD, FAD NADP,FMN)
Nucleosides and Nucleotides
Nucleotides have three characteristic
Components of Nucleotides
1) Nitrogenous base
Nitrogen containing chemical, further divided into two:
Large sized nitrogen containing biomolecules.
These are 9 membered double ring compounds.
Two types: Adenine and Guanine present in both DNA as
well as RNA.
Small sized nitrogen containing biomolecules.
These are 6 membered single ring compounds.
Three types: Cytosine, Thymine & Uracil, Cytosine present
in both DNA & RNA but Uracil only present in RNA. Thymine
only present in DNA.
Basic structure of Pyrimidine and Purine
C C N
N 3 4 5 CH N 1 6 5 C 7
HC 2 6 CH HC 2 4 C
1 3 9
N N N
2) Phosphate Group
Mono-, di- or triphosphate O
phosphate bind at C3 or C5 R O P O
atoms of sugar O
3) Pentose Sugar 5’
Pentose ( 5-carbon Sugar) O
Numbering of Sugar is primed 4’ 1’
There are two man types of Nucleic acid:
1) Deoxyribonucleic acid (DNA)
2) Ribonucleic acid (RNA)
Types of Nucleic acid
DNA is a long chain polymer.
It is a di-polynucleotide, double helix.
Both helices are antiparallel to each other, the antiparallality is
due to Phosphodiester bonds.
It means that one strand begins from 3’ end terminates at 5’ end;
and its complementary strand begins at 5’ end and terminated at
3’ end . Hence antiparallel to each other.
In double helix of DNA, Adenine always pair with the thymine.
Between Adenine and Thymine two hydrogen bond are present.
In double helix DNA, Guanine always pair with Cytosine.
Between Guanine and Cytosine Three hydrogen bond are present.
1) Deoxyribonucleic acid or DNA
Hydrolysis of nucleic acid DNA showed that it is
Phosphoric acid (H3PO3), a phosphate group
A 5-C sugar, later identified as Deoxyribose sugar
The Purine bases, Adenine (A) and Guanine (G).
The Pyrimidine bases, cytosine (C) and Thymine (T)
Both purine and pyrimidine bases contain significant amount of
nitrogen, therefore these are called nitrogenous bases.
Chemical nature & Structure of DNA
Watson and Crick in 1953 proposed the double helical model of DNA.
They used DNA x-ray diffraction patterns produced by Rosalind
Franklin and Murice Wilkins and data from Erwin Chargaff’s base
ratio. The x-ray data showed the DNA molecule to be long, thin and
helical (spiral-like) in shape.
They were awarded Noble prize in 1962
In 1868, Friedrich Miescher isolated DNA from the nucleus of
Pus cell and named it Nuclein.
Avery, Colin MacLeod and Maclyn McCarty experimented on
DNA and DNA was extracted from a virulent (disease causing)
strain of the bacterium Streptococcus pneumoniae and
injected into a nonvirulent of the same bacterium transformed
the nonvirulent strain into virulent strain.
They concluded that the DNA from the virulent strain carried
the genetic information for virulence. So, they found DNA as
DNA as Genetic Material
The form of DNA described in Watson-crick model, is called
It is right handed helix and narrower, more elongated helix
Its one complete helix is 34A.
Its helical diameter is 20A and has ten base pairs in one helix
It is right handed double helix, with shorter and more
compact helical structure (wider and less flexible), than B-
If the water content increase to about 75%, the A form DNA
In this helix there are more base pairs per turn as compared
It is a left-handed helix.
Sugar phosphate backbone forms a zigzag structure., so it is called
It has twelve base pairs in one helix.
It is obtained artificially.
RNA is the only macromolecule known to have a role both in the
storage and transmission of information.
It is polymer of nucleotides, backbone is sugar and phosphate
It is single stranded and it can be found inside or outside of the
RNA is the first discovered nucleic acid!
2) Ribonucleic Acid (RNA)
The building blocks of RNA are also nucleotides linked into
However , it differs from DNA in the following respects:
a) Ribose differs from the Deoxyribose sugar in that the Deoxyribose
has one less oxygen at C-2 position that does the ribose.
b) The pyrimidine base Uracil replaces thymine.
c) The polynucleotides that polymerize to make RNA are:
Chemical nature & structure of RNA
In every cell, RNA molecule is of three
types. These three types are responsible
for expression of genetic information.
Types of RNA
Term mRNA given by Jacob and Monad.
It is produced in nucleus.
Messenger RNA encode the amino acid sequence of one or more
polypeptide specified by a gene or set of gene.
It carries chemical information from DNA of the gene to ribosome
for protein synthesis.
The mRNA molecules are formed with the help of DNA template
during the process of transcription.
Molecules of mRNA are 75-3000 nucleotides long and are not folded
in any special way.
2). Messenger RNA or mRNA
Transfer RNAs read the information encoded in the mRNA and
transfer the appropriate amino acid to a growing polypeptide chain
during protein synthesis.
tRNA carries amino acids to ribosomes during protein synthesis,
It is also called adapter molecule.
All cells have at least 20 different kinds of tRNA molecules, and all
are very similar in shape.
The structure of tRNA molecule is nearly similar in bacteria and
It is smallest class of RNA molecule and consists of 75-90
3) Transfer RNA or tRNA
It is produced inside the nucleus within the nucleolus.
It is major component of Ribosomes and takes part in formation of
mRNA and tRNA.
Eukaryotic ribosomes contain four different rRNA molecules: 18S,
5.8S, 28S and 5S rRNA.
Three of the rRNA are synthesized in the nucleolus, and one is
In the cytoplasm, ribosomal RNA and protein combine to form a
nucleoprotein called ribosome.
The ribosomes binds mRNA and carries out protein synthesis.
Most of the RNA found in Eukaryotic cell is the rRNA, almost
comprises of 80% of RNA in cell.
1). Ribosomal RNA or rRNA
Nucleic acids (DNA & RNA) play an important role in all
DNA molecule is the carrier of genetic information. It contains
the information that organisms inherit from their parents in
the form of genes.
RNA molecules are involved in the synthesis of protein based
on the genetic information provided by DNA.
Some RNA molecules function as enzyme-like catalysts
Functions of Nucleic acids
RNA contain the bases are
Adenine, Uracil, Cytosine and
It is always single stranded.
RNA does not obey Chargaff's
Short lived than DNA.
It is involved in synthesis of
DNA contain the bases are
Adenine, Thymine Cytosine and
It is always double stranded
DNA Obeys Chargaff’s rule.
Life time of DNA comparatively
It is genetic and hereditary
material of cells.
Difference between RNA & DNA
A process of break up polymers into monomers, known as catabolism.
The polymer of nucleotides broken down into uric acid and this is
the second major organic waste product that we excrete in our
urine. (First is Urea).
Nucleic acids Nucleotides Uric acid
Nucleic acids polymer broken down into nucleotides
Nucleotides broken down into phosphoric acid (H3PO4) and
Nucleosides break down into pentose sugar and nitrogen base.
The hydrogen bond between nitrogen base breaks down purine and
Catabolism of Nucleic acids
in stomach Gastric acid and pepsin
Nucleic acid Protein
In small intestine Endonucleases: RNAse & DNase
Degradation of Nucleic acid
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