Nucleic acids are polymers found in cell nuclei that can be broken down into monomers called nucleotides. The two main types of nucleic acids are DNA and RNA. DNA is the genetic material found in cells and organisms. It exists in various forms within cells and organisms. RNA also exists in several types that play important roles in gene expression and regulation. Nucleic acids have a primary structure defined by their sequence of nucleotides, and DNA exhibits a characteristic secondary structure as a double helix modelled by Watson and Crick.

1. Nucleic acid
Types and Structure
By Dr. Farooq Ahmad

2. Nucleic acids
 Nucleic acids, which are relatively strong acids
found in the nuclei of cells, were first isolated in
1869 by twenty four year old Swiss physician,
Friedrich Miescher.
 The nucleic acids are polymers with molecular
weights as high as 100,000,000 grams per mole.
 They can be broken down, or digested, to form
monomers known as nucleotides.
 Each nucleotide contains three units: a sugar, an

3.  Deoxyribonucleic acid (DNA) and Ribonucleic
acid (RNA). DNA is the genetic material found in
living organisms.
 DNA is found in the nucleus, and other types
of organelles (such as mitochondria and the
chloroplasts of plants).
 Central Dogma: DNA ⟶ RNA ⟶ Protein

5. Structural model of Nucleic acid

8. TYPES OF NUCLEIC ACIDS

9. Types of DNA
1. B-DNA:
Structure of B-form of DNA has been proposed by Watson and
Crick. It is present in every cell at a very high relative
humidity (92%) and low concentration of ions. It has
antiparallel double helix, rotating clockwise (right hand).
2. A-DNA:
The A-form of DNA is found at 75% relative humidity in the
presence of Na+, K+ or Cs+ ions. It contains eleven base
pairs as compared to ten base pairs of B-DNA.
3. C-DNA:
The C-form of DNA is found at 66% relative humidity in the
presence of lithium (Lit+) ions. As compared to A-and B-
DNA, in C-DNA the number of base pairs per turn is less

10. 4. The D-Form of DNA (D-DNA):
Total number of base pairs per turn of helix is eight. This form is
also called poly (dA-dT) and poly (dG-dC) form.
5. Z-DNA:
Rich and coworkers proposed a left handed (synistral) double
helix model with zig-zag sugar-phosphate back bone running
in antiparallel direction.
6. Single Stranded (ss) DNA:
Almost all the organisms contain double stranded DNA except a
few viruses such as bacteriophage φ × 174 consists of single
stranded circular DNA.
7. Circular and Super Helical DNA:
Almost in all the prokaryotes and a few viruses, the DNA is

11. Types of RNA
1. mRNA
2. rRNA
3. tRNA
4. siRNA
5. miRNA
siRNA, is a double-stranded RNA molecules. It is
sometimes known as short interfering RNA or silencing
RNA
miRNA: A microRNA is a small non-coding RNA molecule
(containing about 22 nucleotides) function in regulation
of gene expression.
 The major difference between siRNAs and miRNAs is
that the former inhibit the expression of one specific target

14. TYPES OF STRUCTURE OF
NUCLEIC ACIDS

15. Primary Structure of Nucleic Acids
 Like proteins, nucleic acids have a primary structure that
is defined as the sequence of their nucleotides.
 Unlike proteins, which have 20 different kinds of amino
acids, there are only 4 different kinds of nucleotides in
nucleic acids.
 For amino acid sequences in proteins, the convention is
to write the amino acids in order starting with the N-
terminal amino acid.
 In writing nucleotide sequences for nucleic acids, the
convention is to write the nucleotides 5′ end with
phosphate group

17. Secondary Structure of DNA
 At Cambridge University in 1953, James D. Watson
and Francis Crick announced that they had a model
for the secondary structure of DNA.
 DNA is composed of two nucleic acid chains running
antiparallel to one another—that is, side-by-side with
the 5′ end of one chain next to the 3′ end of the other.
 Moreover, as their model showed, the two chains are
twisted to form a double helix