DNA – The Wonder Molecule

The  DNA :-

• Ø  DNA is a long polymer of Deoxyribonucleotides.
• Ø  The length of DNA is usually defined as number of nucleotides  or base pairs (bp) present in it.
• Ø  The length of DNA (No. of base pairs ) is a unique characteristic of an organism.

Example :-        i) Bacteriophse  - ΦX174 has 5386 nucleotides / base pairs (bp)

                        ii) Bacteriophase – Lambda (λ) has 48502 bp

                        iii) Escherechia coli  has 4.6 X 10⁶ bp

                        iv) Haploid content of human DNA is 3.3 X 10⁹ bp

     STRUCTURE OF A POLYNUCLEOTIDE CHAIN

    Two types of Polynucleotide chains :- DNA &  RNA

   DNA : Deoxyribo Nucleic Acid ( Sugar is – Deoxyribose ; two  – H is present at 2’position of  Carbon)

   RNA : Ribo Nucleic Acid( Sugar is – Ribose ; one –H and one –OH group is present at 2’ position of        

              Carbon

Ø  Polynucleotide = Polymer of Nucleotide

Ø  Nucleotide = Nitrogenous Base +  Pentose Sugar (Deoxyribose / Ribose ) +  Phosphate Group

Ø  Nucleoside = Nitrogenous Base +  Pentose Sugar (Deoxyribose / Ribose )

Ø  Nitrogenous Bases : Two Types  PURINES   &  PYRIMIDINES

Ø  PURINES : Adenine (A ) and  Guanine ( G )

Ø  PYRIMIDINES : Thymine ( T ) , Cytosine ( C )  and  Uracil ( U  - present only in RNA )

Ø  Four Nitrogenous Bases :  A , T , G, & C are present in DNA

Ø  Four Nitrogenous Bases  : A , U , G  & C  are present in RNA

CHEMICAL BONDS IN A NUCLEOTIDE

Ø   A nitrogenous base is linked to a pentose sugar through a N-glycosidic linkage to form a nucleotide

Ø  A phosphate group is linked to 5’-OH of a nucleoside through Phosphoester linkage

Ø   Two nucleotides are linked through 3’- 5’ phosphodiester linkage to form a polynucleotide.Many nucleotides are joined together through phosphodiester linkages to form polynucleotides chain.

NITROGENOUS  BASE	PENTOSE SUGAR	NUCLEOSIDE	PHOSPHATE  GROUP	NUCLEOTIDE
ADENINE	DEOXYRIBOSE	DEOXYADENOSINE	PHOSPHATE	DEOXYADENOSINE MONOPHOSPHATE
GUANINE	DEOXYRIBOSE	DEOXYGUANOSINE	PHOSPHATE	DEOXYGUANOSINE MONOPHOSPHATE
THYMINE	DEOXYRIBOSE	DEOXYTHYMIDINE	PHOSPHATE	DEOXYTHYMIDINE MONOPHOSPHATE
CYTOSINE	DEOXYRIBOSE	DEOXYCYTIDINE	PHOSPHATE	DEOXYCYTIDINE MONOPHOSPHATE

SOME FACTS WHICH HELPED IN DISCOVERING THE DOUBLE HELICAL STRUCTURE OF DNA

1. Friedrich Meischer (1869) first identified DNA as an acidic substance present in the nucleus . He named it as nuclein.
2. Maurice Wilkins and Rosalind Franklin (   )  gave important data by  X-ray diffraction of DNA .
3. Erwin Chargaff proposed that  the ratios between Adenine and Thymine = 1  and

                                                            the ratios between Guanine and Cytosine = 1

Based on these facts James Watson and Francis Crick (1953) proposed famous Doble Helix Model for the structure of DNA.

SALIENT FEATURES OF THE DOUBLE – HELIX STRUCTURE OF DNA

It is made of two polynucleotide chains, where the backbone is constituted by sugar – phosphate and the bases project inside.

1.     
The two chains have antiparallel polarity . It means , if one chain has the polarity 

       5’ - 3’, the other has 3’  -  5’

2.      The bases in two strands are paired through hydrogen bond ( H-bonds) forming base pairs (bp).

Adenine is bonded with Thymine through a double bond; Guanine is bonded with Cytisine through a triple bond. Always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.

3.      The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently the distance between abp in a helix is approximately equal to 0.34 nm.

4.      The plane of one base pair stacks over the other in a  double helix. This in addition to H-bonds , confers stability of the helical structure.

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