DNA- deoxyribonucleic acid A long molecule that looks like a twisted ladder made up of four types of simple units and the sequence of these units carries genetic information. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).

1. DNA STRUCTURE AND FUNCTION
Dr. M.Sankareswaran
Assistant professor, Department of
Microbiology.

2. Introduction
DNA- deoxyribonucleic acid
A long molecule that looks like a twisted ladder made up of four types of
simple units and the sequence of these units carries genetic information.
Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a
small amount of DNA can also be found in the mitochondria (where it is called
mitochondrial DNA or mtDNA).

3. History
Nucleic acids were first isolated by Friedrich Miescher (1869) from pus cells and named as nuclein.
Fisher (1880s) discovered the presence of purine and pyrimidine bases in nucleic acids.
Levene (1910) found deoxyribose nucleic acid to contain phosphoric acid as well as deoxyribose sugar.
He characterised four types of nucleotides present in DNA.
W.T. Astbury had found through X-ray diffraction that DNA is a polynucleotide with nucleotides
arranged perpendicular to the long axis of the molecule and separated from one another by a distance
of 0.34 nm.
In 1953, Wilkins and Franklin got very fine X-ray photographs of DNA.
The photographs showed that DNA was a helix with a width of 2.0 nm. One turn of the helix was 3.4
nm with 10 layers of bases stacked in it.
Wilkins, Watson and Crick were awarded Nobel Prize for the same in 1962.
Watson and Crick (1953) built a 3D, molecular model of DNA that satisfied all the details obtained from
X-ray photographs.

5. STRUCTURE OF DNA
The structure of DNA was found by Rosalind Franklin by x-ray crystallography.
The double helical structure of DNA was discovered by Watson and Crick in 1953.
The structure of DNA is a helical, double-stranded macromolecule with Two
polynucleotide chains coil around the same axis to form a right –handed double helix.
Backbone of each chain which consist of alternate sugar-phosphate residues,
(hydrophilic) are on the out side of the double helix, facing the surrounding.
The nitrogen bases are stacked perpendicular to the long axis of the double helix
The double-helix has a diameter of 10 Å.
Each adjacent base on one strand of the double-helix is 3.4 Å apart.
Every 10 base-pairs constitutes a 360° turn in the helix, and the length of the helix is

7. Nucleoside
They are derivatives of purines and pyrimidines that have a sugar linked
to a nitrogen ring.

8. Nucleotide
The building blocks of nucleic acids are nucleotides.
The nucleotide which contains a nitrogenous base
and deoxyribose
is called deoxyribonucleoside.
A nucleotide in the DNA chain consists of three
parts:
A nitrogenous base,
A phosphate group, and
A molecule of deoxyribose

9. Types of deoxyribonucleotides
There are four kinds of deoxyribonucleotides are,
dATP – deoxy Adenosine triphosphate
dGTP – deoxy Guanosine triphosphate
dCTP – deoxy Cytosine triphosphate
dTTP – deoxy Thymine triphosphate

10. Nitrogenous Bases
Nitrogenous Bases are the foundational structure of DNA polymers, the structure of
DNA polymers vary with the different attached nitrogenous bases.
The nitrogenous bases of each nucleotide chain are of two major types:
Purines and
Pyrimidines.
The purine and pyrimidine bases of both strands are stacked inside the double
helix and stabilized by
Van Der Waals interactions.

11. Purines
Purines have two fused rings of carbon and nitrogen atoms.
The two purine bases in DNA are
1. Adenine (A) and
2. Guanine (G).

12. Pyrimidines
Pyrimidines have one rings of carbon and nitrogen atoms.
The pyrimidines bases in DNA are
1.Cytosine (C) and
2.Thymine (T).

13. Phosphate Group
Phosphodiester linkages form the covalent backbone of DNA.
A phosphodiester bond is the linkage formed between the 3' carbon
atom and the 5' carbon of the sugar deoxyribose in DNA.
The phosphate groups in a phosphodiester bond are negatively-
charged.
The phosphate group of DNA is derived from a molecule of phosphoric
acid and connects the deoxyribose molecules to one another in the
nucleotide chain.

14. Interactions between bases
The two strands of double-stranded DNA are held together by a number
of weak interactions such as
Hydrogen bonds,
Stacking interactions, and
Hydrophobic effects.

15. Major and Minor Grooves
As a result of the double helical nature of DNA, the molecule has two asymmetric
grooves.
One groove is smaller than the other.
The larger groove is called the major groove, occurs when the backbones are far
apart; while the smaller one is called the minor groove, occurs when they are close
together.
These grooves are important binding sites for proteins that maintain DNA and
regulate gene activity

16. Antiparallel orientation
Double-stranded DNA is an antiparallel molecule.
There are two strands are always complementary in sequence.
One strand serves as a template for the formation of the other during DNA replication,
a major source of inheritance.
The two strands run in opposite directions, one going in a 3' to 5' direction and the
other going in a 5' to 3' direction.

17. Complementary base pairing
Adenine always stands opposite and binds to thymine.
Guanine always stands opposite and binds to cytosine.
Adenine and thymine are said to be complementary, as are guanine
and cytosine.
This is known as the principle of complementary base pairing.

18. Chargaff's rules
Austrian biochemist Erwin Chargaff analyzed the DNA of different species,
determining its composition of A, T, C, and G bases.
A, T, C, and G were not found in equal quantities.
The amounts of the bases varied among species, but not between individuals of the
same species.
The amount of A always equalled the amount of T, and the amount of C always
equalled the amount of G (A = T and G = C).
These findings, called Chargaff's rules.

19. Functions of DNA
DNA is the genetic material which carries all the hereditary information.
DNA is essential for equitable distribution of DNA during cell division.
Changes in sequence of nitrogen bases due to addition, deletion or wrong replication give rise to
mutation.
DNA gives rise to RNAs through the process of transcription.
It controls the metabolic reactions of the cells through the help of specific RNAs, synthesis of specific
proteins, enzymes and hormones.
DNA controls development of an organism through working of an internal genetic clock with or
without the help of extrinsic information.
DNA are used in identification of individuals and deciphering their relationships. The mechanism is
called DNA finger printing.
Defective heredity can be rectified by incorporating correct genes in place of defective ones.
Excess availability of anti-mRNA or antisense RNAs will not allow the pathogenic genes to express
themselves.

20. Different forms of DNA
On the basis of number of nucleoside residues, DNA is classified into 3
types,
A - DNA,
B - DNA,
Z- DNA.

22. A DNA
Right handed
Size is about 26 angstroms Sugar pucker C3'-endo
Shorter, wider helix than B.
11 base pairs per helical turn and vertical length of 2.6 nm so wider than B form.
Deep, narrow major groove not easily accessible to proteins
Wide, shallow minor groove accessible to proteins, but lower information content
than major groove.

23. B DNA
Right-handed form called the B-helix.
Double helical structure.
It is about 20 angstroms with a C-2' endo sugar pucker conformation.
The helix makes one complete turn approximately every 10 base pairs (= 34 A per
repeat/3.4 A per base).
B-DNA has two principal grooves, a wide major groove and a narrow minor
groove.

24. Z DNA
Left-handed helical rotation.
The Z form is about 18 angstroms and there are 12 base pairs per helical turn, and the
structure appears more slender and elongated.
The DNA backbone takes on a zigzag appearance.
To form the left-handed helix in Z-DNA, the purine residues flip to the syn conformation
alternating with pyrimidines in the anti conformation.
The major groove is barely apparent in Z-DNA, and the minor groove is narrow and
deep.
For pyrimidines, the sugar pucker conformation is C-2' endo and for purines, it is a C-3'
endo.