2. DNA stands for deoxyribose nucleic acid
DNA is a polymer of deoxyribonucleotides
The monomeric deoxynucleotides in DNA are held
together by 3’,5’ -phosphodiester bridges
3.
4. This chemical substance is present in the nucleus of all
cells in all living organisms
DNA exist predominately in its double helix form
The double helix consists of two strands running in an
antiparallel fashion
These double strands are held together by variety of
interactions, including hydrogen bonding, London
dispersion forces and hydrophobic interactions
5. The inside of the helix consists of non-polar nitrogenous
bases
The exterior of the helix contains negatively charged
phosphate group and deoxyribose sugar
Each strands is complementary to the other one
6.
7. Composition of DNA
DNA consists of:
• Deoxy Ribose Sugar
• Nitrogenous bases
• A phosphate group
8. 5- carbon Sugar
Ribose is a Five carbon sugar
Deoxyribose is almost the same as ribose sugar but lacks
one oxygen atom at carbon number 2 of the five membered
ring
9. Phosphate Group:
Phosphate group is a component of a nucleotide is
derived from phosphoric acid (H3PO4)
Phosphate groups are essential for nucleotide
polymerization
10. Nitrogenous bases
The bases that occur in nucleic acids are aromatic
heterocyclic compounds derived from
Pyrimidine
Purine
Pyrimidine is a six-membered heterocyclic ring
Purine is a fused-ring compound containing a six-
membered ring (pyrimidine) connected to a five
membered ring (imidazole)
11.
12. The two purine derivatives found in nucleic acids
are adenine (A) and guanine (G)
The two pyrimidine bases are Cytosine(C) and
thymine(T)
13.
14. Structure of DNA
The structure of DNA is understood in terms of
three levels of structure
Primary
Secondary
Tertiary
15. Primary structure of DNA
Sequence of nucleotides in the DNA
Each phosphate is linked to 3’ and 5’
of two sugars
16.
17. Secondary Structure
Secondary structure is the set of interactions
between bases
DNA double helix, the two strands of DNA are
held together by hydrogen bonds
The nucleotides on one strand base pairs with the
nucleotide on the other strand
18. The Double Helix
Initial “like-with-like”, parallel helix:
Does not fit with Chargaff’s Rule: A = T, G = C
19. Chargraff’s Rule
Erwin Chargaff’s in 1940 observed the DNA from
different species
He observed that DNA have equal number of
adenine and thymine residues (A=T) and Equal
number of guanine and cytosine residues (G=C)
This is known as Chargaff’s rule of molar
equivalence between the purines and pyrimidines
in DNA structure
20. Single stranded DNA and RNA do not obeys the Chargaff’s
rule except double stranded RNA in the viruses
The important features of Chargaff’s rule are following
• The % age of Adenine residues is equal the %age
of thymine residues (%A=%T)
• the %age of guanine residues is equal the cytosine
residues
21. Adenine always pairs with thymine and guanine pairs with
cytosine
Adenine makes double hydrogen bond with thymine and
guanine makes triple hydrogen bond with cytosine
The concentration of purines (A+G) is equal to the
concentration of pyrimidines (T+C)
22. Double Helical structure of DNA
The double helical structure of DNA was
proposed by James Watson and Francis Crick in
1953.
The salient features of Watson-Crick
model of DNA are:
• The DNA is a right handed double helix
• It consists of two poly
deoxyribonucleotide chains (strands)
twisted around each other on a common axis
23. The two strands are antiparallel, i.e., one strand runs
in the 5' to 3' direction while the other in 3’
to 5'direction
The width (or diameter) of a double helix is 20 Ao (2 nm)
Each turn (pitch) of the helix is
(3.4 nm) with 10 pairs of nucleotides, each pair placed at
a distance of about 3.4 A0
24. Each strand of DNA has a hydrophilic
deoxyribose phosphate backbone on the
outside of the molecule while the hydrophobic bases
are stacked inside
The two polynucleotide chains are not identical but
complementary to each other due to base pairing
The two strands are held together by
hydrogen bonds formed by complementary base pairs
25. The A-T pair has 2 hydrogen bonds while G-
C pair has 3 hydrogen bonds.
The G = C is stronger by about 50% than A=T
The hydrogen bonds are formed between a purine
and a pyrimidine
26.
27.
28. Triple-stranded DNA
Triple stranded DNA is formed by the formation of
Hoogsteen hydrogen bond
A thymine can selectively form two Hoogsteen
hydrogen bonds to the adenine of A-T pair
to form T-A-T
A cytosine can also form two
hydrogen bonds with guanine of G
C pairs that results in C-G-C
29.
30. Four-stranded DNA
Polynucleotides with very high contents of
guanine can form a novel tetrameric structure
called G-quartets
These structures are planar
and are connected by Hoogsteen hydrogen
bond
Antiparallel four-stranded DNA structures,
referred to as G-tetraplexes
31.
32. Tertiary Structure of DNA
Tertiary structure refers to the overall three-
dimensional shape
Tertiary structure arises from supercoiling
DNA is “packaged” by coiling around a core of
proteins known as histones
The DNA-histone assembly is called a nucleosome
Histones are rich in lysine and arginine residues
