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DNA (Deoxyribonucleic acid)
Structure and Functions
Namrata Chhabra
M.D.Biochemistry 1

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An international team of researchers from UK, Germany, and China discovered the oldest ancestor of man
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Researchers have deciphered the information of
ancient DNA extracted from the thigh bone of an early
human who died 400,000 years ago in Spain.
Researchers say DNA strands of
400,000-year-old early human can help build
clearer picture of human family tree.
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Oldest
Human DNA
Reveals
Mysterious
Branch of
Humanity
An artist's impression of the early humans that are estimated to have
lived 400,000 years ago.
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DNA passes on genetic information through its chemical structure and molecular behavior.
A simple fertilized egg becomes a complex
human being of forty trillion cells.
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DNA- the molecular basis of inheritance
● DNA is a molecule that contains the
information an organism requires to
develop, live and reproduce.
● These ‘instructions’ are found inside
every cell and are transferred from
parents to their children.
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Learning objectives
● To conceptualize the
key structural features
of DNA, and
● apply the learned
concepts in illustrating
the functions of DNA
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DNA
● A Storehouse of genetic
information.
● found in chromosomes,
mitochondria and
chloroplasts
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The DNA structure is probably the most
iconic of all biomolecules.
It has inspired the architects to design the
staircases, decorations, pedestrian bridges
(as in Singapore, and more.
The Helix Bridge, Singapore, at night.
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History of DNA Structure discovery
● Until the 1950s, the
structure of DNA
remained a mystery.
● The double-helical
structure of DNA was
discovered through the
work of James Watson,
Francis Crick, Rosalind
Franklin, and other
researchers.
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Double helical
structure of DNA
By Bdna.gif: Spiffistanderivative work: Jahobr (talk) - Bdna.gif, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=17436581
In the model, the orange and red
atoms mark the phosphates of the
sugar-phosphate backbones, while
the blue atoms on the interior of the
helix belong to the nitrogenous
bases.
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DNA structure
DNA is composed of subunits called
nucleotides
A nucleotide is made up of a sugar
(deoxyribose), a phosphate group, and one of
four nitrogenous bases: adenine (A), thymine
(T), guanine (G) or cytosine (C).
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Ribose and deoxyribose
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Bases present in DNA
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Pyrimidines
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Purines
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Nucleosides and Nucleotides
https://www.creative-proteomics.com/services/nucleoside-nucleotide-analysis-service.htm
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Vote
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Vote 23

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N-Glycosidic linkage
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Anti and syn configuration in glycosidic bonds
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Primary structure of DNA
● A linear sequence of
deoxyribonucleotides
linked together by 3'-5'
phosphodiester linkages
● The informational
content of DNA resides
in the sequence in
which
monomers—purine and
pyrimidine
deoxyribonucleotides—
are ordered
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Primary structure of DNA
● The polymer possesses
a polarity; one end has a
5'-hydroxyl or phosphate
terminal while the other
has a 3'-phosphate or
hydroxyl terminal.
● Traditionally, a DNA
sequence is drawn from
5’ to 3’ end.
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What makes DNA
an acid ?
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Watson and Crick's model of DNA
The structure of DNA, as represented in Watson and Crick's model, is a
double-stranded, antiparallel, right-handed helix.
The sugar-phosphate backbones of the DNA strands make up the outside of the
helix, while the nitrogenous bases are found on the inside and form
hydrogen-bonded pairs that hold the DNA strands together.
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Anti-parallel strands of DNA
● The two strands of the double-helical
molecule, each of which possesses a
polarity, are antiparallel; ie, one strand
runs in the 5' to 3' direction and the other
in the 3' to 5' direction.
● Sugar-phosphate chains wrap around
the periphery.
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Sugar-Phosphate backbone
DNA nucleotides are linked by
covalent bonds, formed
between the deoxyribose sugar
of one nucleotide and the
phosphate group of the
subsequent.
This arrangement makes an
alternating chain of deoxyribose
sugar and phosphate groups in
the DNA polymer, a structure
known as the
sugar-phosphate backbone
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Bases in DNA
● Bases (A,T, C and G) occupy
the core, forming
complementary A · T and G · C
Watson-Crick base pairs.
● The DNA double helix is held
together mainly by- Hydrogen
bonds.
● Two hydrogen bonds between
A:T pairs while three hydrogen
bonds between C: G pairs.
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Hydrogen bonds
Sharing of a hydrogen
atom covalently attached
to an electronegative
element (typically O-H and
N-H groups) between a
lone pair of electrons on
another electronegative
element.
● The electronegative atom with the lone pair
electrons is called the Hydrogen Bond Acceptor
● The electronegative atom bonded to the
hydrogen is called the Hydrogen Bond Donor
● The Hydrogen Bond Donor must be aligned 180
degrees to the Hydrogen Bond Acceptor
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Vote
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Each pair of bases lies flat, forming a "rung"
on the ladder of the DNA molecule.
The bases in DNA are planar and have a
tendency to "stack".
Major stacking forces: hydrophobic
interaction and Vander Waals forces.
The Bases in DNA
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Right handed helix
In Watson and Crick's model, the
two strands of DNA twist around
each other to form a
right-handed helix.
As one looks down the double
helix, the base residues form a
spiral in a clockwise direction.
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Chargaff’s rules
Erwin Chargaff was an Austro-Hungarian
Biochemist who was a professor of
Biochemistry at Columbia University
medical school.
Chargaff discovered two rules that helped
to the discovery of the double helix
structure of DNA.
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Chargaff’s rules
● The first rule was that in DNA the number of guanine units is equal to the
number of cytosine units, and the number of adenine units is equal to the
number of thymine units. This hinted at the base pair makeup of DNA.
● The second rule was that the relative amounts of guanine, cytosine, adenine
and thymine bases vary from one species to another. This hinted that DNA
rather than protein could be the genetic material.
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The impact of double stranded structure of DNA
Template strand - In the double-stranded DNA
molecules, the genetic information resides in the
sequence of nucleotides on one strand, the
template strand. This is the strand of DNA that is
copied during ribonucleic acid (RNA) synthesis. It is
sometimes referred to as the noncoding strand.
Coding strand- The opposite strand is considered
the coding strand because it matches the sequence
of the RNA transcript (but containing uracil in place
of thymine) that encodes the protein.
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Major and Minor grooves
● Major and minor grooves wind along
the DNA molecule parallel to the
phosphodiester backbones.
● In these grooves, proteins can interact
specifically with exposed atoms of the
nucleotides (via specific hydrophobic
and ionic interactions) without disrupting
the base pairing of the double-helical
DNA molecule.
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Forms of DNA
Property A-DNA B-DNA Z-DNA
Helix Handedness Right Right Left
Base Pairs per turn 11 10.4 12
Rise per base pair along
axis
0.23nm 0.34nm 0.38nm
Pitch 2.46nm 3.40nm 4.56nm
Diameter 2.55nm 2.37nm 1.84nm
Conformation of
Glycosidic bond
anti anti Alternating anti and syn
Major Groove Present Present Absent
Minor Groove Present Present Deep cleft 41

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Forms of DNA
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Physiological form of DNA- B DNA
● The horizontal arrow indicates the
width of the double helix (20 Å).
● The vertical arrow indicates the
distance spanned by one complete
turn of the double helix (34 Å).
● The major and minor grooves are
depicted.
● Hydrogen bonds between A/T and
G/C bases indicated by short
horizontal lines.
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How long is our DNA ?
● If you stretched the DNA in one cell all the way out, it would be about 2m long.
● All the DNA in all our cells put together would be about twice the diameter of
the Solar System.
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Amazing facts
● There are about 3 billion nucleotides in human DNA.
● The average length of a human nucleotide is 0.6 nanometers, or
0.0000000006 meters, so human DNA is about 1.8 meters (5 feet) long.
● The 5 feet of DNA is being packed into each cell in our body, and the average
diameter of the a nucleus in a human cell is only 10 microns, or 0.00001
meters. Clearly, DNA is packaged (twisted, wrapped and folded) so that it is
very compact.
● There are about 50 to 75 trillion cells in a human body, so if the DNA from
each cell were placed end to end, the chain would be from 90 to 135 trillion
meters long.
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Prokaryotic versus Eukaryotic DNA
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Prokaryotic versus Eukaryotic DNA
● EuKaryotic cells contain a very large quantity of DNA (human cells have at
least a thousand times more DNA than a typical bacteria cell).
● E.coli chromosomes have about 4.7 x 106 base pairs which results in a length
of 1.6 mm.
● An E.coli cell, however is only 0.002 mm long .
● That explains why the DNA must be folded in order to fit into the cell.
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Tertiary structure of DNA
In eukaryotic cells, DNA is folded into
chromatin.
Chromatin consists of very long
double-stranded DNA molecules and a nearly
equal mass of rather small basic proteins
termed histones as well as a smaller amount
of nonhistone proteins (most of which are
acidic and larger than histones) and a small
quantity of RNA.
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Levels of organization of DNA
● Nucleosomes are composed of DNA
wound around a collection of histone
molecules.
● The disk-like nucleosome structure
has a 10-nm diameter and a height
of 5 nm.
● The 10-nm fibril consists of
nucleosomes arranged with their
edges separated by a small distance
(30 bp of DNA) with their flat faces
parallel with the fibril axis.
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Levels of organization of DNA
● The 10-nm fibril is probably further
supercoiled with six or seven
nucleosomes per turn to form the
30-nm chromatin fiber.
● In interphase chromosomes,
chromatin fibers appear to be
organized into 30,000–100,000 bp
loops or domains anchored in a
scaffolding (or supporting matrix)
within the nucleus.
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Chromosomes
● At metaphase, mammalian
chromosomes possess a
twofold symmetry, with the
identical duplicated sister
chromatids connected at a
centromere, the relative position
of which is characteristic for a
given chromosome.
● The 3 x 109 base pairs of DNA
in humans are organized into
the haploid complement of 23
chromosomes.
●
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Functions of DNA
1. DNA Replication
It provides the information inherited
by daughter cells or offspring.
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Functions of DNA
2. Storehouse of genetic code
The DNA holds the information for all the
proteins to be created for the cell.
The bases are grouped in triplets called
codons that code for a specific amino acid.
The whole genetic code stores the
information for all cell types to reproduce.
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Functions of DNA
3. Mutation and Recombination
● During DNA replication, different DNA segments can be
spliced through gene linkage.
● The process can create new combinations of traits in
offspring.
● If the protein helps the species survive, it may evolve over
time.
● However, some mutations may be non-beneficial or lethal.
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Human evolution
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Functions of DNA
4.Gene expression
● Each cell contains a full set of
genes.
● Cells from different tissues and
organs appear and function
differently.
● The reason is that only some of the
DNA of each cell is used to make
proteins.
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Summary
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Summary
DNA consists of four bases—A, G, C, and T—that are held in linear array by
phosphodiester bonds through the 3' and 5' positions of adjacent deoxyribose
moieties.
DNA is organized into two strands by the pairing of bases A to T and G to C on
complementary strands.
These strands form a double helix around a central axis.
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Summary
The 3 x 109 base pairs of DNA in humans are organized into the haploid
complement of 23 chromosomes.
DNA provides a template for its own replication and thus maintenance of the
genotype and for the transcription of the roughly 30,000 human genes into a
variety of RNA molecules.
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Key references
1. Harper’s illustrated Biochemistry 31st edition.
2. Lehninger principles of Biochemistry 7the edition
3. Lippincott Illustrated Review Biochemistry, 5th edition.
4. http://scienceline.ucsb.edu/getkey.php?key=144
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LET US CHECK YOUR UNDERSTANDING
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Thank
you
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