DNA is a double-helical structure that carries all the genetic information. Its length is determined by the number of nucleotide pairs present in it. It is an acidic substance in the nucleus identified by Friedrich Meischer. Its double helical structure was given by Watson and Crick.

• DNA is a long polymer of Deoxyribonucleotides .
• It is an acidic substance present in nucleus .
• It was first identified by friedrich meischer in 1869. He name it as Nuclein
• Altmann found these substances to be acidic in nature hence he named it nucleic acid .
• The length of DNA is usually defined as numbers of nucleotides or a pair of Nucleotides referred to as base pair ( by ) present in it.


RNA Biology is the leading peer-reviewed scientific journal in the field of ribonucleic acid research. It is indexed for MEDLINE.

Ribonucleic acid
Single — stranded structure
RNA is a Polymer (Polynucleotide Monomer units : ribonucleotides

Nucleic acid

Nucleic acid, naturally occurring chemical compound that is capable of being broken down to yield phosphoric acid, sugars, and a mixture of organic bases (purines and pyrimidines).

Nucleic acid is of two types in all living system .
• Deoxyribonucleic acid (DNA
• Ribonucleic acid ( RNA) .
DNA Is a genetic material in all living organisms except viruses. RNA is a genetic material in riboviruses

Structure Of Nucleotide

A nucleotide has three components.

Pentose sugar
Phosphate group
Nitrogenous base

There are two types of  nitrogenous bases:

•Purine :- adenine (A) and guanine (G)
• Pyrimidines :- Cytosine (c) and Thymine

Cytosine is commoitin both DNA and RNA .
Thymine is present in DNA and uracil is present in RNA at the,pra ce of thymine



Types of Linkge or Bond

N- glycosidic linkage
A nitrogenous base is linked to pentose sugar through N- glycosidic linkage to form a nucleoside
Phosphoester linkage
Phosphate group is linked to S’OH of nucleoside through phosphester linkage to form nucleotide .

Two nucleotides are linked through 3′- 5′ Phosphodiester linkage to form a dinucleotide

James Watson And Francis Crick Model

They proposed a very simple but famous double helix model for the structure of DNA . This was based on the observation of Erwin chargaff .

Salient features of the double helix structure of DNA are :-

1.DNA consist of two polynucleotide chains . The backbone is constituted by sugar – phosphate and the bases project inside .
2.Two chains of DNA run in antiparallel fashion with 5′ —. 3′ polarity in other chain .

3′————————————————————— 5′


•Two chains of DNA run in Antiparallel fashion with 5′ 3′ polarity in one and 3′—————>5′ polarity in chain .
•These bases in two strands are paired through hydrogen bonds .


•Adenine forms 2H – bonds with thymine and vice versa .
•Similarly, Guanine is bonded with cytosine with 3H — bonds .
•As a result always a purine(A,G ) comes opposite to pyrimidine (C,T,U ) .
•Each turn of double helix or the pitch of helix is 14 nm . It has a proximately 10 by in each turn

Central Dogma

FRANCIS CRICK proposed the central dogma of molecular biology which explains the one way for the synthesis of RNA from DNA And the process is called transcription , while synthesis of protein from RNA called Translation .


Some viruses show reverse transcription the synthesis of DNA from an RNA template. A class of RNA viruses, called retroviruses, are characterized by the presence of an RNA-dependent DNA polymerase (reverse transcriptase).

Packaging of DNA helix

  • The DNA inside of a cell is organized so that it fits well within the small size of a cell.
  • Because if DNA wouldn’t be compact it wouldn’t fit in the cell. Human cells contains 2- 3 meters Ion D
  • Imagine how effective compaction is that it makes a molecule 2-3 meters long fit in a ce that its million times

Packaging of DNA in Prokaryotes

1.In prokaryotes, DNA is not scattered throughout the cell although they do not have a defined nucleus .
2.DNA is organized into loops held by proteins which have positive charge .

DNA Packaging in Eukarvotes

•In eukaryotes , the DNA occurs inside nucleus . There are set of basic and positively charge proteins Is called histones.
•Histones are rich in lysine and arginine , which carry positive charge .
•There are five types of histones proteins 1-11. 112 A 1-1213 1-13 1-14 Four of them ( H2A H2B H H4 ) occur in it to form Histone
Octamer or nobody .
•Due to all of those phosphate groups along DNA backbone results it has a negative charge and is attracted to positively charged molecules such as histones.
•The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called Nucleosome .

•The condensed DNA or the nucleosome constitute repeating unit of a structure in nucleus called chromatin .
•Nucleosomes in chromatin are seen as ‘beads – on – strings’ under Electron Microscope.
•Nucleosome further coils to form Solenoid/ chromatin fibre .
•The packaging of chromatin at higher level requires additional set of proteins that collectively are referred as Non histone chromosomal (NHC ) proteins .

Chromatin is differentiated into two regions on the basis of staining behavior in a typical nucleus


The Search For Genetic Material

•Mendel found that there are alternative forms of factors—now called genes—that account for variations in inherited characteristics.
•Thomas hunt morgan gave the chromosomal theory of inheritance . He spoke about chromosomes he said that the chromosomes play a very important role in the process of inheritance . There is something in chromosomes which gets transmitted from one generation to another generation .
•Boveri —Sutton propose that chromosomes bear hereditary factors .

Transforming Principle

•This experiment was conducted by Fredrick Griffith in 1928.
•Griffith perform his experiment with diplococcus Pneumoni which is responsilble for pneumonia in organisms.
•He selected two strains of the bacteria . The one is S- type (have smooth polysaccharid—second other R-type ( devoid of polysaccharide coat ) .•
•He injected S- type strain into mice body and found that the e r9k dead after. Some time

Bacteria — streptococcus Pneumoniae


•S type ( smooth ) polysaccharide coat present. responsible for smoothness .
•It is resistant to immune system .

•R type ( rough) polysaccharide coat absent
•Destroyed by immune system of the host .


1.Mice + inject ( S – type ) —– Mice dead
Conclusion- S-type is virulent (poisonous)
2.Mice + inject ( R – type ) —— Mice alive
conclusion – R – type is not virulent
3. Mice + inject ( heated s-type )———Mice alive (As heat destroys the polysaccharide coat )
conclusion – heated S – type Non virulent .
4. Mice + inject ( heated S type + R type ) — Mice dead .

Conclusion :-
•In autopsy he found — virulent ( s type ) and Non virulent ( R type ) .
•Griffith concluded erh material in heated S-strains bacteria that transformed R type into virulent
S- type . He was not sure about the nature of genetic material whether it was DNA , RNA , carbohydrate or protein

Biochemical Characterisation of Transforming Principal

Oswald avery ,colin macleod and Maclyn mccarty repeated the experiment to identified the biochemical nature of transforming
They purified biochemical from heated-S strain i:e, protein , RNA, DNA .


Hershey And Chase Experiment

Experiment With T2 Bacteriophages

Viruses that infect bacteria and replicate inside it .
•The function of DNA and protein could be found out by labelling them with radioactive tracers DNA contain phosphorus but not sulphur.
•Similarily protein of phage contains sulphur but not phosphorus
Now some bacteriophages were grown in radioactive phosphorus medium . So that there DNA will be radioactive but protein will not be radioaetive because-protein does not contain phosphorus .

Similarly some were grown in the radioactive sulphur medium . So that the protein coat will be radioactive but dna will not be the radioactive one .

Bacteriophage With Radioactive DNA Attached To Bacteria

Three steps were followed :

Infection : Both types of labellecisra41,s were allowed to infect normally cultured bacteria in separate experiments .
•Blending : These bacterial cells were agitated in a blender to break the contact between virus and bacteria.
•Centrifugation : The virus particles were separated from the bacteria by spinning them in a centrifuge .

Bacterial cells showed the presence of radioactivity , no radioactivity detected in supernatant


Bacteriophages With Radioactive Protein Attach To Bacteria

•Similarly , bacteria was infected .
•Agitated in a blender to separate phage particles from the E.coli bacteria cells
•Centrifugation seperated the phages particles as supernatent.

Result :
Bacteria that were infected with viruses that had radioactive proteins were not radioactive
This indicates that proteins did-not enter the bacteria from the virus

Therefore ,DNA is the genetic material that is Passed from Virus to bacteria