Structure of eukaryotic DNA or Watson & Crick Model of DNA.                                                              

1) DNA is nucleic acid containing deoxyribose sugar.

2) It is present in all organizing except some viruses (RNA Viruses).

3) It is Master molecule of cell which carries genetic information from one
            generation to the other.

4) It is located mainly over chromosomes, but is also found in chloroplast and mitochondria.

5) DNA is linear double standard helix in eukaryotes and is circular in prokaryotes.

6)  WATSON & CRICK MODEL OF DNA

The structure can be explained as follows:

A) Double stranded Helix:

1) DNA molecule is linear & double stranded i.e. consisting of two Polynucleotide chains or strands.

2) The backbone of strand is made up of alternately placed sugar phosphate (Phosphoric Acid and deoxyribose sugar) which are linked together with help of phosphodiester bonds.

3) The two strands are equidistant and run anti parallel (i.e. opposite direction) the 2 strands are connected with help of transverse steps similar to ladder.

4) Each strand is composed of large number of Basic units called as deoxy ribonucleotides in the form of polymer chain.

5) DNA is heteropolymer due to reason that four different types of nucleotides such as A, G, C, T are present repeatedly.

B) Double Helix:

1) The coiling of two strands is plectonemic i.e. cannot be easily separated from each other.

2) The two strands are spirally coiled around each other as well as around central imaginary axis to form right handed DNA molecule. (Twisted in clockwise direction)  (Wilkins & Franklin).

C) Base – Pairing

1) DNA shows, four nitrogen bases, namely Adenine, Guanine (purines) and cytosine, thymine (Pyrimidines)

2) N – Base occupies first position in sugar molecule and is perpendicular to sugar chain.

3) N – Bases of two strands are present in pairs and connected by 2 or 3 hydrogen bonds. This is called as base pairing.

4) The base pairing is very specific and complimentary.

5) Adenine always pairs with thyamine with two H – bonds while (G) pairs with (C) with three H – bonds.

6) Though weak in nature H – bonds give strength to DNA molecule due to their large number.

D) Complimentary Nature:

1) The two strands are not similar but complimentary to each other.

2) If the direction of one strand is known that of other can be easily identified.

3) If number nature and sequence of nucleotides of one strand of DNA is known that of other can be easily identified.

E) Purine – Pyrimidine ratio:-

1) Because of specificity and complementary nature of base pairing, purine pyrimidine ratio equals to one.

2) According to Chargaff, A + G = T + C i.e.

      But, base ratio (never) and is species specific.

F) C3 & C5 end OR Polarity:-

1) DNA is polarized molecule i.e. it shows marked C3 & C5 ends.

2) The direction of strand is decided by attachment of phosphoric acid molecule to the first nucleotide in the strand.

G) Anti parallel Nature:-

1) Two strands run parallel but in opposite direction i.e. if one strand is in direction 3’ – 5’ (Ascending) then other, strand will be in direction 5’ – 3’ (Descending).

H) Grooves:

The helical DNA strand shows two types of grooves such as shallow minor grooves and deep major grooves.

Thus, DNA is not an absolute cylinder.

I) Dimensions of DNA Molecule.

1) Distance between two strands -  20 A0 (2 nm)

2) Distance between two successive base pair or nucleotides – 3.4 A0 (0.34 nm)

3) Distance between two major or minor grooves – 34 A0 (3.4 nm)

4) Angle between two base pairs is 360 

5) Pitch angle two - 3600. 

                                                                        

J) Functions of DNA:-

1) It is genetic material which carriers genetic information.

2) It undergoes replication i.e. it can form its exact copies

3) DNA regulates metabolic activities through protein synthesis.

4) It controls different types of characters of individuals.

5) It produces all the types of RNAs.

6) DNA can undergo mutation which serves as tool in evolution and variation.