DNA is made up of nucleotides, each containing a nitrogenous base, a sugar (deoxyribose), and a phosphate group. The bases are either purines (adenine and guanine) or pyrimidines (cytosine, thymine, and in RNA, uracil). Watson and Crick discovered that DNA exists as a double helix with the bases pairing together (A with T and C with G) between antiparallel strands. The structure allows DNA to store and replicate genetic information accurately and transmit it to daughter cells.

1. B.PHARMA 1ST YEAR 2ND SEM
SUBJECT: HUMAN ANATOMY &
PHYSIOLOGY
FACULTY – DEEPA LASHKARI

2. Unit 5
DNA STRUCTURE
&
FUNCTION

3. DEOXY RIBOSE NUCLEIC ACID (DNA)
• In 1869, Friedrich Meischer was the first person who separated cell nuclei from
the cytoplasm and extracted an acidic material, nuclein, from the nuclei of pus
cells.
• He found that the acidic material contained unusually large amounts of
phosphorous and no sulphur.
• Later on in 1889, Richard Altmann used the term nucleic acid in place of
nuclein.
• Nucleic acids were found to be associated with various proteins called
nucleoproteins.
• There are two types of nucleic acids viz.,
 Deoxy ribose Nucleic acid (DNA) &
 Ribose Nucleic acid (RNA).
• DNA is the genetic material in most of the organisms.
• RNA acts as genetic material only in some viruses.
• DNA is mainly found in the chromosomes in the nucleus, while RNA is mostly
found in the ribosomes in the cytoplasm.

4. • Levene showed that nucleic acid can be broken into smaller
molecules called nucleotides.
• Each nucleotide consists of a sugar, phosphate group and a
nitrogenous
base.
• The combination of nitrogenous base and sugar without the
phosphate group is called nucleoside (riboside and
deoxyriboside)
• where as the combination of nitrogenous base, sugar and
the phosphate group is called nucleotide (ribotide and
deoxyribotide)
 nucleotide = nucleoside + phosphate

5. • The 5-carbon (pentose) sugar could be either ribose as
in case of RNA or deoxyribose in case of DNA.
• Associated with each sugar is a nitrogenous base
with one or two carbon–nitrogen rings.
• Bases containing one carbon–nitrogen ring are called
pyrimidines.
• The common pyrimidines present in DNA are
thymine(T) and cytosine (C), while in case of RNA
pyrimidine base thymine(T) is replaced by uracil(U).
• Bases containing two carbon-nitrogen rings are called
purines.
• The common purines present in nucleic acids are
adenine (A) & guanine(G).

6. Purine Bases
Adenine and guanine are purines. Purines are the larger of the
two types of bases found in DNA Structures are shown below:
Structure of A & G
The 9 atoms that make up the fused rings (5
carbon, 4 nitrogen) are numbered 1-9. All ring
atoms lie in the same plane.

7. Pyrimidine Bases
Cytosine and thymine are pyrimidines. The 6 stoms (4 carbon,
2 nitrogen) are numbered 1-6. Like purines, all pyrimidine ring
atoms lie in the same plane:-
Structure of C & T

8. Deoxyribose Sugar
• The deoxyribose sugar of the DNA backbone has 5 carbons & 3 oxygens.
• The carbon atoms are numbered 1', 2', 3', 4', and 5' to distinguish from the
numbering of the atoms of the purine and pyrmidine rings.
• The hydroxyl groups on the 5'- and 3'- carbons link to the phosphate
groups to form the DNA backbone.
• Deoxyribose lacks an hydroxyl group at the 2'-position when compared to
ribose, the sugar component of RNA.
• Structure of deoxyribose

9. Differences between pyrimidines & purines
Pyrimidines
• These are single ring (six
member)compounds.
• They are of three types, viz.,
cytosine, thymine and uracil.
• They occupy less space in
DNA structure.
• Deoxyribose is linked at
position 3 of pyrimidine.
Purines
• These are double ring (nine
member)compounds.
• They are of two types, viz.,
adenine and guanine.
• They occupy more space in
DNA structure.
• Deoxyribose is linked at
position 9 of purine.

10. The DNA Double Helix
• Taking into account the facts known at that time Watson and Crick in 1953 proposed
a “double helix” structure of DNA which quickly gained wide acceptance.
• The DNA molecule consists of two polynucleotide chains wound around each other
in a right-handed double helix.
• The two strands of a DNA molecule are oriented anti-parallel to each other and run
in opposite directions.
• The sugar-phosphate backbones of the two DNA strands wind around the helix axis
like the railing of a spiral staircase.
• The bases of the individual nucleotides are on the inside of the helix, stacked on top
of each other like the steps of a spiral staircase.
• Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite
strand, and G forms 3 hydrogen bonds with C on the opposite strand.

11. J. D. Watson & F. H. C. Crick proposed structure
of DNA “Double Helix” in 1953& for that they
were awarded by Nobel Prize 1962.

12. It contains two polynucleotide strands wound
around each other.
The backbone of each consists of alternating
deoxyribose and phosphate groups.
The phosphate group bonded to the 5' carbon atom of
one deoxyribose is covalently bonded to the 3' carbon of
the next.
The two strands are "antiparallel“ ; that is, one strand
runs 5′ to 3′ while the other runs 3′ to 5′.
The DNA strands are assembled in the 5′ to 3′ direction
[More] and, by convention, we "read" them the same way.
The purine or pyrimidine attached to each
deoxyribose projects in toward the axis of the helix.
Each base forms hydrogen bonds with the one directly
opposite it, forming base pairs (also called nucleotide
pairs).
3.4 Å separate the planes in which adjacent base
pairs are located.
The double helix makes a complete turn in just over 10
nucleotide pairs, so each turn takes a little more (35.7 Å
to be exact) than the 34 Å.

13. G forms 3 hyrdorgen bonds
with C on the opposite
strand.
A forms 2 hydrogen bonds
with T on the opposite
strand

14. A B & Z forms of DNA
• In a DNA molecule, the two strands are not parallel, but intertwined
with each other. Each strand looks like a helix. The two strands form a
"double helix" structure, which was first discovered by James D.
Watson and Francis Crick in 1953.
• In this structure, also known as the B form.
• In a solution with higher salt concentrations or with alcohol added, the
DNA structure may change to an A form.
• Which is still right-handed, but every 2.3 nm makes a turn and there
are 11 base pairs per turn.
• Another DNA structure is called the Z form. Because its bases seem
to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising
12 base pairs. The DNA molecule with alternating G-C sequences in
alcohol or high salt solution tends to have such structure.

15. Figure 3-B-3. The
normal right- handed
"double helix"
structure of DNA, also
known as the B form.
Figure 3-B-4.
Comparison between
B form and Z form.

16. Comparison of B-DNA & Z-DNA
Characteristic B-DNA Z-DNA
Coiling Right handed Left handed
Pitch 340 A 450 A
Base pairs / pitch 10.4 12.4
Diameter ~ 20 0 A ~ 180 A
Rise per base pair 3.40 A 3.70 A
Sugar – phosphate
backbone
Regular Zigzag
(Pitch – The length of the helix required to complete one turn)
Denaturation: The hydrogen bonds between the DNA strands break on
heating the DNA to high temperature (nearly 100oC). The process of
separation of DNA strands is known as denaturation.
Renaturation: Reunion of the separated or denatured DNA strands on
cooling is called renaturation or annealing. The optimum temperature for
renaturation is 20 – 25oC.

17. The DNA molecule satisfies the requirement of genetic material
in the following ways:-
1.It can replicate itself accurately during cell growth and
division.
2. Its structure is sufficiently stable so that heritable charges i.e.,
mutations can occur only very rarely.
3.It has a potential to carry all kinds of necessary
biological information.
4. It transmits all the biological information to the daughter cells.
Thus the essential functions of DNA are the storage and
transmission of genetic information and the expression of
this information in the form of synthesis of cellular
proteins.