Nucleic acid -Structure and function of dna

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Structure & Function of DNA
Dr. Sonia Bajaj, (Head of Department)

2. Introduction
Nucleic acid were first reported by friendrich Meischer from nuclei of pus cells. He called it “nuclein”, which
was later identified as DNA.
Nucleic acid are macromolecules formed by polymerization of nucleotides.
=Nitrogen base+ pentose sugar + phosphate
Two major type of nucleic acid are DNA and RNA.
DNA is an important chemical compound fine in each and every organism except some viruses.
This is mainly found on chromosome inside nucleus and also in some cell organelles like mitochondria and
chloroplast.
DNA is a genetic material which carry information from generation to generation.

3. Chemical composition of DNA
DNA is a high molecular weight compound whose molecular weight range from106 to
109 Dalton.
It is a polymer of nucleotides.
Each nucleotides in turn consists of three components.
1. Deoxyribose sugar- This is a pentose sugar which contains one less oxygen atom
than ribose sugar. It together with phosphate group from a backbone of DNA.
2. Phosphoric acid-In DNA, phosphoric acid is found in the form of phosphate
.phosphate is joined with 3rd and 5 th atom of carbon of “C” sugar molecules.
3. Nitrogenous base- This is the nitrogen containing compounds which joins with 1st
“C” atom of sugar by a glycoside bond. It is following to types-
A) Purines -These are the bases containing two benzene rings and maybe the following
of two types-1. Adenine 2. Guanine.
B) Pyrimidine- These are one benzene ring containing compound and may be of three
type-1. Cytosine 2.Thymine 3.Uracil
Uracil is completely absent in DNA. Present only RNA.

4. The Structure of DNA
In 1953, James Watson and Francis Crick discovered the structure of DNA.
The works of Rosalind Franklin lead to Watson and Crick’s discovery. Franklin first had pointed out that the DNA
is made up of two spirals.
Most of the DNA is in the classic Watson-Crick model simply called as B-DNA or B-form DNA.
The structure of DNA is a double helix structure because it looks like a twisted ladder.
1um of DNA contain 3000 base pairs.
DNA is double stranded but in Bacteriophages & S-13viruses, the DNA is single.
The DNA is composed of deoxyribonucleotides.
The sides of the ladder are made of alternating sugar (deoxyribose) and phosphate molecules.
The ladder are made up of a pair of nitrogen bases.
There are 4 types of nitrogen bases Adenine (A) Thymine (T) Guanine (G) Cytosine (C).

5.  Each strand has a 5′end (with a phosphate group) and a 3′end (with a hydroxyl
group).
 The deoxyribonucleotides are linked together by 3′ – 5′phosphodiester bonds.
 The strands are antiparallel, meaning that one strand runs in a 5′to 3′direction,
while the other strand runs in a 3′ to 5′ direction.
 The two strands are held together by hydrogen bonds and are complimentary
to each other.
 DNA helix has a diameter of 20A0
 Length of one tern (3600) is 34 A0.distance between sugars of two strands is
11.1 A0.
 The diameter of double helix is 2nm and the double helical structure repeats at
an interval of 3.4nm which corresponds to ten base pairs.
 The complimentary of the strands are due to the nature of the nitrogenous
bases. The base adenine always interacts with a thymine (A-T) on the opposite
strand via two hydrogen bonds and cytosine always interacts with guanine (C-
G) via three hydrogen bonds on the opposite strand.
 The shape of the helix is stabilized by hydrogen bonding and hydrophobic
interactions between bases.
 In 1950 Erwin Chargaff found that in any DNA molecule- The amount of Purines
and Pyrimidine’s are equal i.e. A+G=T+C.

6. Based on the location DNA may be:
Nuclear DNA
 Located within the nucleus of eukaryote cells.
 Usually has two copies per cell.
• The structure of nuclear DNA chromosomes is linear with open ends and includes
46 chromosomes containing 3 billion nucleotides.
 Nuclear DNA is diploid, ordinarily inheriting the DNA from two parents. The mutation rate for nuclear DNA is
less than 0.3%.
Mitochondrial DNA
 Mitochondrial DNA is located in the mitochondria.
 Contains 100-1,000 copies per cell.
 Mitochondrial DNA chromosomes usually have closed, circular structures, and contain for example 16,569
nucleotides in human.
 Mitochondrial DNA is haploid, coming only from the mother.
 The mutation rate for mitochondrial DNA is generally higher than nuclear DNA.
 This deviation in forms are based on their structural diversity.

7. DNA Helix length No of base pairs Distance
between two
pairs
Diameter
A 28 A0 11 pairs 2.56 A0 23 A0
B 34 A0 10 pairs 3.4 A0 20 A0
C 31 A0 9.33 pairs 3.32 A0 19 A0
D 24.24 A0 8 pairs 3.03 A0 19 A0
Types of DNA
Right Handed DNA - Clockwise twisting - BDNA
Left Handed DNA - Anticlockwise twisting - ZDNA

8. Functions of DNA
DNA has a crucial role as genetic material in most living organisms. It carries genetic information from cell to
cell and from generation to generation.
Thus its major functions include:
 Storing genetic information.
 Directing protein synthesis.
 Determining genetic coding.
 Directly responsible for metabolic activities, evolution, heredity, and differentiation.
• It is a stable molecule and holds more complex information for longer periods of time.