Nucleic Acids
Dr.B.RENGESH | M.Tech., Ph.D.
Associate Professor, Department of Pharmaceutical Technology,
Mahendra Engineering College (Autonomous),
Namakkal District, Tamil Nadu, India
Nucleic Acids
vThe transfer of genetic information to new cells is accomplished through the
use of biomolecules called nucleic acids:
– ribonucleic acid (RNA) — found mainly in the cytoplasm of living cells
– deoxyribonucleic acid (DNA) — found
mainly in the nucleus of living cells
v DNA and RNA are polymers consisting of
repeating subunits called nucleotides, which
are made of three components:
– a heterocyclic base
– a sugar
– phosphate
Components of Nucleic Acids - Heterocyclic Bases
• A ring that contains elements other than carbon is called a heterocyclic ring.
• The bases found in RNA and DNA contain two types of heterocyclic rings:
pyrimidine and purine.
– pyrimidine bases: uracil (U), thymine (T), cytosine (C)
– purine bases: adenine (A), guanine (G)
– DNA contains A, G, T, C; RNA replaces T with U
Components - Heterocyclic Bases
Components - Sugars, and the Phosphate Group
• In RNA, the sugar component is D-ribose, and in DNA the sugar is D-
deoxyribose. (Note that both sugars are in the β-anomeric form.)
• The phosphate group in nucleotides is derived from phosphoric acid, H3PO4,
and at physiological pH exists in the ionic form
Putting the Pieces Together - Nucleotides
• Nucleotides are formed from the combination of a sugar with a phosphate
group at the 5′ position and a heterocyclic base at the 1′ position.
Generalized form
The Structure of DNA - Primary Structure
• DNA is one of the largest molecules known, containing between 1 and 100
million nucleotide units.
• The nucleotides in DNA are linked by phosphate groups that connect the 5′
carbon of one nucleotide to the 3′ carbon of the next.
– Because these connections occur on two oxygen atoms of the phosphate
group, they are called phosphodiester bonds.
• The nucleic acid backbone then is a sequence of sugar-phosphate groups,
which differ only in the sequence of bases attached to the sugars along the
backbone (the primary structure of DNA):
The Structure
of DNA -
Primary
Structure
The Structure of DNA - Secondary Structure
• This allows two separate strands of sugar-phosphate backbones to run
alongside each other, held together by the hydrogen bonds between the
complementary base pairs (A-T) & (G-C):
DNA – The Discovery
• DNA was discovered in 1869 by the Swiss physician Friedrich Miescher in
the pus of discarded surgical bandages; he named it “nuclein” because it
was located in the nucleus of the cell.
• In 1878, Albrecht Kossel isolated the pure nucleic acid, and later isolated
the five nitrogenous bases.
• In 1943, Oswald Avery, Colin MacLeod, and Maclyn McCarty identified
DNA as the carrier of genetic information.
• In 1952, Rosalind Franklin obtained an X-ray crystal structure (“Photo 51”)
of a sample of DNA which contained structural features which lead James
D. Watson and Francis H. C. Crick to deduce the double helix structure of
DNA (Nobel Prize in Medicine, 1962).
DNA – The Double Helix structure
We wish to suggest a structure for the salt of
deoxyribose nucleic acid (DNA). This structure has
novel features which are of considerable biological
interest. ...
It has not escaped our notice that the specific
pairing we have postulated immediately suggests a
possible copying mechanism for the genetic
material.
“Molecular Structure of Nucleic Acids: A Structure
for Deoxyribose Nucleic Acid,” Nature (April 25,
1953)
Molecular Configuration in
Sodium Thymonucleate
R. Franklin, and R. G. Gosling,
Nature 171, 740-741 (1953).
DNA – The Double Helix structure
• The “ladder-like”
structure folds in on itself
to form a double heli.
• The sugar-phosphate
backbone runs along the
outside of the helix, with
the bases pointing
inwards, where they form
hydrogen bonds to each
other.
DNA – The Double Helix structure (cont…)
• The two intertwined polynucleotide chains run in opposite
(antiparallel) directions, with the 5′ end of one chain on the
same side as the 3′ end of the other.
– The base sequence of a DNA strand is always written
from the 5′ end to the 3′ end.
• The bases are nearly perpendicular to the helix axis, and
adjacent bases are separated by 3.4 Å. 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. There is a rotation of
36 degrees per base (360 degrees per full turn/10 bases per
turn).
• The diameter of the helix is 20 Å.
Chromosomes
• A normal human cell contains 46
chromosomes, each of which
contains a molecule of DNA coiled
tightly around a group of small
basic proteins called histones.
Chromosomes
• Individual sections of DNA molecules
make up the genes, which are the
fundamental units of heredity that
direct the synthesis of proteins.
– Viruses contain a few to several
hundred genes.
– Escherichia coli (E. coli)
contains ~1000 genes.
– Humans cells contain ~25,000
genes.
RNA - Ribonucleic Acid
• RNA is a long unbranched polymer consisting of nucleotides joined by 3′ to
5′ phosphodiester bonds.
• RNA strands consist of from 73 to many thousands of nucleotides.
• Whereas DNA is only found in the nucleus, RNA is found throughout cells:
in the nucleus, in the cytoplasm, and in the mitochondria.
• Differences in RNA and DNA primary
structures:
– In RNA the sugar is ribose instead
of deoxyribose.
– In RNA, the base uracil (U) is used
instead of thymine (T).
RNA - Structure of RNA
• Most RNA molecules are single-stranded, although many contain regions of
double-helical structure where they form loops. (A::U, G:::C)
RNA - Types
• There are three kinds of RNA: messenger RNA (mRNA), ribosomal RNA
(rRNA), and transfer RNA (tRNA).
Types of RNA – messenger RNA (mRNA)
• functions as a carrier of genetic information from the DNA in the cell
nucleus to the site of protein synthesis in the cytoplasm.
– The bases of mRNA are in a complementary sequence to the base
sequence of one of the strands of nuclear DNA.
– mRNA has a short lifetime (usually less than one hour); it is
synthesized as it is needed, then rapidly degraded to the constituent
nucleotides.
Types of RNA – ribosomal RNA (rRNA)
• Ribosomal RNA (rRNA) — the main component of ribosomes that are the
site of protein synthesis.
– rRNA accounts for 80-85% of the total RNA of the cell.
– rRNA accounts for 65% of a ribosome’s structure (the remaining 35%
is protein).
Types of RNA – transfer RNA (tRNA)
• Transfer RNA (tRNA) — delivers individual amino acids to the site of
protein synthesis.
– tRNA is specific to one type of amino acid; cells contain at least one
specific type of tRNA for each of the 20 common amino acids.
– tRNA is the smallest of the nucleic acids, with 73-93 nucleotides per
chain.
– tRNA has regions of hydrogen bonding between complementary base
pairs
Types of RNA – transfer RNA (tRNA) (cont…)
• Two regions of tRNA have important functions:
ü the anticodon is a three-base sequence
which allows tRNA to bind to mRNA
during protein synthesis. (It is
complementary to one of the codons in
mRNA.)
ü the 3′ end of the molecule binds to an
amino acid with an ester bond and
transports it to the site of protein
synthesis. An enzyme matches the tRNA
molecule to the correct amino acid,
“activating” it for protein synthesis.
Types of RNA – transfer RNA (tRNA) (cont…)
The Flow of Genetic Information
The Central Dogma of Molecular Biology
• The central dogma of molecular biology states that genetic information
contained in the DNA is transferred to RNA molecules and then expressed
in the structure of synthesized proteins.
• Each protein in the body corresponds to a DNA gene.
DNA – Replication
• Replication is the process by which an exact copy of DNA is produced.
– Two strands of DNA separate, and each one serves as the template for
the construction of its own complement, generating new DNA strands
that are exact replicas of the original molecule.
– The two daughter DNA molecules have exactly the same base sequences
of the parent DNA.
– Each daughter contains one strand of the parent and one new strand that
is complementary to the parent strand. This type of replication is called
semiconservative replication.
DNA – Replication
Three Steps are involved:
1. Unwinding of the double helix
2. Synthesis of DNA segments
3. Closing the nicks
Transcription & Translation
Transcription
Ø in eukaryotes, the DNA containing the stored information is in the nucleus of
the cell, and protein synthesis occurs in the cytoplasm. The information stored
in the DNA must be carried out of the nucleus by mRNA.
Translation
Ø mRNA serves as a template on which amino acids are assembled in the
sequence necessary to produce the correct protein. The code carried by mRNA
is translated into an amino acid sequence by tRNA.
Transcription
&
Translation
Transcription: RNA Synthesis
• Under the influence of the enzyme RNA polymerase, the DNA double helix
unwinds at a point near the gene that is being transcribed (the initiation
sequence). Only one strand of the DNA is transcribed.
• Ribonucleotides are linked along the DNA strand in a sequence determined by
the base pairing of the DNA and ribonucleotide bases (A::U, G:::C).
• mRNA synthesis occurs in the 3′ to 5′ direction along the DNA strand (in the
5′ to 3′ direction along the RNA strand) until the termination sequence is
reached.
• The newly-synthesized mRNA strand moves away from the DNA, which
rewinds into the double helix.
• Synthesis of tRNA and rRNA is similar to this.
Transcription:
RNA Synthesis
