Transcript Nucleotides and Nucleic acids

Welcome to class of
Nucleotides and Nucleic acids
Dr. Meera kaur
Learning objectives
• To understand
– the names and one-letter symbols of the five major nitrogen
bases found in nucleic acids
– two differences between the molecular composition of DNA
and RNA
– the structure of DNA
– the process of replication, transcription and translation
– Concept of gene mutation and molecular disease
– Concept of recombinant DNA technology
Introduction to nucleic acids…
• In 1869, Johannes Miescher of the University of Germany
carried out the research on elemental analysis of the nucleus.
He studied cell containing large nuclei and little cytoplasm
(white blood cells). He collected white cells from pus, washed
from bandages of infected surgical patients. When he extracted
the nuclei with basic solution and acidified the alkaline extract,
he got a precipitate of stringy material that he analyzed further.
• His careful elemental analysis gave carbon, nitrogen and
phosphorus in proportions that were unlike of those of
carbohydrates, lipids or protein. Miescher named his new
materials nuclein to reflect its nuclear origin.
• Today, we called these compound the nucleic acids.
Introduction to nucleic acids
• Miescher made little progress in learning about nucleic acids, either
their functions or structure. But other scientists eventually learned that
there are two types of nucleic acids: deoxyribonucleic acid (DNA) and
ribonucleic acids (RNA).
Structure of Nucleic acids:
- Nucleotides are the fundamental building blocks of nucleic acids.
- DNA and RNA molecules are polynucleotides; i.e., polymers composed
of many repeating units of nucleotides.
- Each nucleotide consists a sugar unit, a nitrogen base and a
phosphate group attached to the sugar unit.
The combination of a base and sugar units makes a nucleosides. Adding
a phosphate group to the sugar unit of a nucleoside makes nucleotide.
Structure of nucleic acids
BASE + SUGAR UNIT
BASE------SUGAR + PHOSPHATE
A nucleoside
BASE---SUGAR
A nucleoside
BASE— SUGAR---PHOAPHATE
A nucleotides
Structure of the sugar units
• The sugar unit of the nucleotides strung together to
make RNA is  -D- ribose -- hence the name
ribonucleic acid
• The sugar unit of DNA is  -D-2-deoxyribose—
hence the name deoxyribonucleic acid
Structure of the base units
• Four different nitrogen bases (heterocyclic amines) are found in
DNA.
• Two of these bases - adenine (A) and guanine (G) - are
derivatives of purine. They are called purine bases.
• The other two cytosine (C) and thymine (T) are derivatives of
pyrimidine. They are called pyrimidine bases.
• Except for thymine, the same bases are found in RNA. Instead
of thymine, uracil — a pyrimidine base is found in RNA.
Basic structure of nucleotide
(nitrogenous base, pentose, and phosphate)
Carbons of
the sugar ring
are numbered
1, 2, 3, 4 
and 5(read
as “one
prime”, “Two
prime” and so
forth)
Nitrogenous bases of nucleotides
The ring atoms of the nitrogen bases are
numbered as 1, 2, 3, 4, 5 and so forth
Major purine and pyrimidine bases of nucleic acids
Structure of nucleosides…
• The base and sugar units of nucleosides are held together by a
covalent bond between the nitrogen of a purine or pyrimidine
bases and the ring carbon of the sugar unit.
• Ribonucleosides (nucleosides found in RNA) are similar in
structure to the deoxyribonucleosides, except that ‘ribose’
rather than ‘deoxyribose’ is the sugar and uracil replaces
thymine.
• Purine nucleosides are formed by a covalent bond between
nitrogen 9 of the base and 1 of the sugar unit.
• The base and sugar of the pyrimidine nucleosides are joined by
a covalent bond between nitrogen 1 of the base and carbon 1
of the sugar.
Structure of nucleotides
• Addition of a phosphate group to a sugar hydroxyl group
forms a nucleotide.
Some adenosine monophosphates
Examples of nucleotides
The covalent backbone structure of DNA and RNA
In DNA and RNA molecules, nucleotides
are linked together through phosphate
ester bridge between the 3hydroxyl
group of one nucleotide and the
5hydroxyl group of the next nucleotide
in the chain. These bridges are called
phosphodiesters. Even though two of the
four oxygen attached to the phosphorus
of each bridge are tied up as phosphate
esters, and one is present in
phosphorus—oxygen double bond, one
oxygen is free to lose a proton. It is the
presence of many such dissociating
groups that gives DNA and RNA their
highly acidic character.
Shorthand structures for nucleotide sequence
•
DNA and RNA molecules are too complex to write in full. So biochemists
often use a shorthand form.
•
The basis for the shorthand is the differing order of their bases. The sugar
units and the phosphate groups are identical in all the nucleotides.
•
We can write the structure of an RNA by ignoring the sugars and the
phosphodiester bridges, and by writing only the sequence of nitrogen
bases. Start at the left with the end of the molecule that has a free 5end
and work toward the end that has a free 3hydroxyl group
U—A—G—C—U—G—C—C
5 ________________ 3
RNA
dA—A—T—G—T—C—A—C
5 __________________ 3
DNA
Avery-MacLeod-McCarty experiment…
• 1944
• First direct evidence that DNA is the bearer of
genetic information
• PROBLEM: not universally accepted, since
protein impurities in the DNA could have
carried the message
The Avery-MacLeod-McCarty Experiment…
Adding heat killed
virulent bacteria to a
live nonvirulent
strain permanently
transformed the
latter into lethal,
virulent,
encapsulated
bacteria. He
concluded that a
transforming factor
in the heat killed
virulent bacteria had
gained entrance into
the live nonvirulent
bacteria and
rendered them
virulent and
encapsulated.
Avery and
his
colleagues
identified this
transforming
factor as
DNA.