·         DNA is a long polymer of deoxyribonucleotides.

·         The length of the DNA depends on, number of nucleotide pair present in it.

·         Characteristics of the organism depend on the length of the DNA.

·         Bacteriophage ø174 has 5386 nucleotides.

·         Bacteriophage lambda has 48502 base pairs.

·         Escherichia coli have 4.6 X 106 base pairs.

·         Human genome (haploid) is 3.3 X 109 bp.

Structure of polynucleotide chain:

·         A nucleotide has three component:-

o    A nitrogen base

o    A pentose sugar ( ribose in RNA and deoxyribose in DNA)

o    A phosphoric acid.

·         There are two types of nitrogen bases:

o    Purines ( Adenine and Guanine)

o    Pyrimidines ( Cytosine, Uracil and Thymine)

·         Adenine, Guanine and Cytosine is common in RNA and DNA.

·         Uracil is present in RNA and Thymine is present in DNA in place of Uracil.

·         Pentose sugar is ribose in RNA and Deoxyribose in DNA.

·         A nitrogen base attached to the pentose sugar at C1 of pentose sugar by

 N-glycosidic linkage to form a nucleoside.

·         According to the nature of pentose sugar, two types of nucleosides are formed ribonucleoside anddeoxyribonucleotides.

·         Ribonucleosides are:

o    Adenosine

o    Guanosine

o    Cytidine

o    Uridine

·         Deoxyribonucleosides are:

o    Deoxyadenosine

o    Deoxyguanosine

o    Deoxycytidine

o    Deoxythymidine.

·         Phosphoric acid attached to the 5’ OH of a nucleoside by Phosphodiester linkage a corresponding nucleotide is formed. (Ribonucleotide or deoxyribonucleotides depending on the sugar unit).

·         Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.

·         More than two nucleotides joined to form polynucleotide chain.

·         Polynucleotide chain has a free phosphate moiety at 5’ end of sugar, is referred to as 5’ end

·         In the other end of the polymer with 3’-OH group called 3’ end.

·         The backbone of the polynucleotide chain is sugar and phosphate.

·         Nitrogen bases linked to the sugar moiety project from the backbone.

·         In RNA every nucleotide has an additional –OH group at 2’ of ribose.

·         In RNA Uracil is found in place of thymine.

·         5-methyl uracil is the other name of thymine.

History of DNA:

·         DNA is an acidic substance in the nucleus was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein”

·         1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray defraction data produced Maurice Wilkins and Rosalind Franklin.

·         Hallmark of their proposition was base pairing between two strands of polynucleotide chains. This was based on observation of Erwin Chargaff.

·         Chargaff’s observation was that for a double stranded DNA, the ratio between Adenine and Thymine, and Guanine and Cytosine are constant and equal one.

Salient features of Double helix structure of DNA:

·         Made of two polynucleotide chains.

·         Sugar and phosphate forms the backbone and bases projected to inside.

·         Two chains have anti-parallel polarity.

·         Two strands are held together by hydrogen bond present in between bases.

·         Adenine of one strand pairs with Thymine of another strand by two hydrogen bonds and vice versa.

·         Guanine of one strand pairs with Cytosine of another strand by three hydrogen bonds and vice versa.

·         A purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.

·         The two chains are coiled in a right – handed fashion.

·         The pitch of the helix is 3.4 nm or 34 A0

·         There are roughly 10 bp in turn.

·         The distance between the bp in a helix is 0.34nm or 3.4 A0.

·         The plane of one base pair stacks over the other in double helix.

·         H-bond confers stability of the helical structure of the DNA.

·         Central dogma of flow of genetic information: DNA→RNA→Protein.



Criteria for genetic material:

  • It should be able to generate its replica (replication)
  • It should be chemically and structurally stable.
  • It should provide the scope for slow changes (mutation) that required for evolution.
  • It should be able to express itself in the form of ‘Mendelian Character’.
  • Protein dose not fulfill the criteria hence it is not the genetic material.
  • RNA and DNA fulfill the criteria.

 RNA is unstable:

  • 2’-OH group present at every nucleotide (ribose sugar) in RNA is a reactive group and makes RNA liable and easily degradable.
  • RNA is also now known as catalyst, hence reactive.
  • RNA is unstable and mutates faster. Consequently the viruses having RNA genome and having shorter life span mutate and evolve faster.

DNA is more stable:

  • Stability as one of the properties of genetic material was very evident in Griffith’s ‘transforming principle’ itself that heat, which killed the bacteria at least did not destroy some of the properties of genetic material.
  • Two strands being complementary if separated by heating come together, when appropriate conditions are provided.
  • Presence of Thymine in place of uracil confers additional stability to DNA
  • DNA is chemically less reactive and structurally more stable when compared to RNA.
  • Therefore among the two nucleic acids the DNA is a better genetic material.

Better genetic material (DNA or RNA)

  • Presence of thymine at the place of uracil confers more stability to DNA.
  • Both DNA and RNA are able to mutate.
  • In fact RNA being unstable mutate at a faster rate.
  • RNA can directly code for the synthesis of proteins, hence easily express.
  • DNA however depends on RNA for protein synthesis.
  • The protein synthesis machinery has evolved around RNA.
  • Both RNA and DNA can functions as genetic material, but DNA being more stable is preferred for storage of genetic information.
  • For the transmission of genetic information RNA is better.


  • RNA is the first genetic material.
  • Essential life processes evolved around RNA.
  • RNA used to act as a genetic material as well as catalyst.
  • But RNA being catalyst was reactive and hence unstable.
  • Hence DNA has evolved from RNA with chemical modifications that make it more stable.
  • DNA being double stranded and having complementary strand further resists changes by evolving a process of repair.

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