What is DNA – How it was proved DNA is Genetic Material?

What does DNA Stands for?

DNA stands for Deoxyribonucleic Acid.

DNA is found in all eukaryotic and prokaryotic cells.

DNA is referred to as a group of molecules that are responsible for carrying and transmitting the hereditary materials or the genetic instructions to the offspring from their parents.

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Types and Structure

3 Steps of DNA replication

What is DNA?

DNA is the building block of genetic material.

Structurally, DNA is a double helical structure that is composed of polynucleotides.

The length of DNA is defined as the number of nucleotide pairs that are base pairs in it.

For example, human DNA is 3.3 × 10⁹ bp while Escherichia coli has 4.6 × 10⁶ bp.

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What is a Cell?

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What nucleotide is made up of?

It has three components: –

• Nitrogenous base

• Pentose sugar

•  Phosphate group

Nitrogenous bases

These are of two types i.e. purines and pyrimidines

Purines– these are double-ringed nitrogenous bases. Adenine (A) and Guanine (G) are included in them.

Pyrimidines – these are single-ringed nitrogenous bases. Cytosine (C), Thymine (T), and Uracil (U) are included in this category.

Uracil is present in RNA only (Ribo-Nucleic Acid).

Nitrogenous bases are linked to the OH of the 1′ C pentose sugar group by N-glycosidic linkages

Pentose sugar

DNA contains pentose type sugar while RNA consists of ribose type sugar. The main difference is due to the presence of an extra -OH group in ribose-type sugar.

Nitrogenous bases are linked to the OH of the 1′ C pentose sugar group by N-glycosidic linkages to form a nucleoside.

Adenine+pentose sugar = adenosine or deoxyadenosine

Guanine+pentose sugar= guanosine or deoxyguanosine

Cytosine+pentose sugar= cytidine or deoxycytidine

Thymine+pentose sugar = uridine or deoxythymidine.

Phosphate group

Nucleosides are linked to the phosphate group by their OH of 5’ C through phosphoester linkage resulting in the formation of nucleotides i.e., deoxynucleotide (DNA) and ribonucleotide (RNA).

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

Further, two nucleotides are linked to form dinucleotides through 3′-5′ phosphodiester linkage and then multiple nucleotides are added to form a polynucleotide.

Final polymer or polynucleotides has polarity at both ends. One end having phosphate group at 5’ end of sugar referred to as 5’-end while another end has free OH of 3’C group referred to as 3′ -end of the polynucleotide chain.

Discovery of DNA

DNA was found as an acidic substance present in the nucleus, was first identified by Friedrich Meischer in 1869, and named as ‘Nuclein’.

Francis Crick and James Watson Discovery

Double Helix Model of DNA given by James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin.

Erwin Chargaff discovery

DNA structure is further clearly understood by Erwin Chargaff’s rule, which states that the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.

Double helical structure of DNA

The salient features of the Double-helix structure of DNA are as follows:-

• There are two polynucleotide chains with a sugar-phosphate backbone with bases projecting inside.

• Both the polynucleotide chains run anti-parallel to each other. If one is in 5′ to 3′ direction then the other runs in 3′ to 5′.

• There are hydrogen bonds among bases of two strands resulting in base pairs (bp). Adenine is bonded to thymine by two hydrogen bonds while Guanine to cytosine by three hydrogen bonds.

• The two polynucleotide chains are coiled in a right-handed direction (B-type DNA).

• The pitch of the helix is 3.4 nm and, in each turn, there are 10 bp bases present. Therefore, the distance between a bp is approximately 0.34 nm in a helix.

• In a DNA double helix, one base pair stack over the other that provides extra stability to the helical structure.

Francis Crick proposed the Central dogma in molecular biology, which states that genetic information flows from DNA to RNA and finally to protein.

Packaging of DNA Helix

How DNA Helix is packaged inside a cell?

The total length of DNA equals (6.6 × 109 bp ×0.34 × 10-9m/bp) making approximately 2.2 meters so how it gets packaged sounds interesting?

Salient features on packaging of DNA

• DNA packaging is done with a set of positively charged, basic proteins called histones.

• Histones are rich in the basic amino acid residues lysine (K) and arginine (R).

• Histones are organized to form a unit of eight molecules (H2A, H2B, H3 & H4) called histone octamer linked by H1A.

• The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called a nucleosome.

• A typical nucleosome contains 200 bp of DNA helix.

• Nucleosomes constitute the repeating unit of a structure in the nucleus called chromatin, threadlike stained (coloured) bodies seen in the nucleus.

• The nucleosomes in chromatin are seen as ‘beads-on-string’ structures when viewed under the electron microscope (EM).

DNA as genetic material

Transforming Principle

Frederick Griffith in 1928 performs a series of experiments on Streptococcus pneumoniae.

S.pneumoniae produces two types of colonies i.e.smooth shiny colonies (S) and rough colonies (R) based on the presence of a polysaccharide coat.

S strain (virulent) causes the death of mice from pneumonia infection while R strain (non-virulent) does not cause any pneumonia and mice survive.

Griffith then injected heat-killed S strain and it does not cause any pneumonia.

Then, he injected mixed heat-killed S strain and live R strain and this caused pneumonia in mice and mice died.

From these experiments, he concluded that R strain has been transformed by heat-killed S strain by transfer of some Transforming Principle.

This Transforming Principle is the genetic material that transformed the R strain to form a smooth polysaccharide coat and becoming virulent.

However, the biochemical nature of this transforming principle was not understood by this experiment.

Biochemical Characterisation of Transforming Principle

Oswald Avery, Colin MacLeod, and Maclyn McCarty worked to determine the biochemical properties of genetic material.

They purified biochemicals from heat-killed strain and treated it with three different enzymes i.e. proteases, RNases, and DNases separately.

They found digestion with DNases inhibited the transformation and hence concluded that DNA is the genetic material.

Transforming Principle    digestion with Protease           Micedead

Transforming Principle   digestion with RNase               Mice dead

Transforming Principle    digestion with DNAase Mice live

Hershey and Chase experiment or Blender’s experiment

• In 1952, Alfred Hershey and Martha Chase worked on bacteriophages (virus infecting bacteria) to prove that DNA is the genetic material.

• When bacteriophages infect bacteria, they transmit their genetic material to bacteria.

• These bacteria replicate viral genetic material as their own and help viruses to multiply.

• Hershey and Chase performed an experiment to found out whether protein or DNA is the genetic material in these bacteriophages.

• For that, they grew viruses in separate mediums, one containing radioactive phosphorus (32P) and the other containing radioactive sulfur (³⁵S).

• The viruses grown in radioactive phosphorus (³²P) contained radioactive DNA as DNA contains phosphorus.

• The viruses grown in medium containing radioactive sulfur (³²S) contained radioactive protein as protein contains sulfur.

•  The bacteriophages were allowed to infect E.colibacteria separately and post infection coats were removed by agitating them in a blender.

• Then, virus particles were removed by centrifugation.

• Radioactivity was found only in those bacteria which were infected by ³²P labelled DNA not in the ³²S labelled protein.

• This indicated that DNA was transferred to the bacteria from the bacteriophages. hence, it was proved that DNA is the genetic material.

Author:- Supriya Bhatt