In 1869, Friedrich Miescher separated a cellular substance from the nuclei of pus cells and celled it ‘Nuclein’. Subsequent studies showed that this material has acidic properties and hence it was renamed as “nucleic acid”. Deoxyribo nucleic acid (DNA) and ribonucleic acid (RNA) are the two major types of nucleic acid found in cells.
Chemical components of nucleic acids :
Nucleotides are the structural units of nucleic acid. Each nucleotide has three components. These are as follows-
Sugar- It is pentose sugar called deoxyribose (C5H10O4) or ribose (C5H10O5). It is a five carbon compound and has pentagonal ring structure.
Phosphate group – It is actually phosphoric acid. (H3PO4). It has three active -OH groups of which two are involved in strand formation.
Nitrogen base – These are cyclic compounds made up of carbon, hydrogen, oxygen and nitrogen atoms. The bases are named adenine (A), thymine (T), cytosine (C), guanine (G), and uracil (U). These are classified into two groups – Purines and Pyrimidines.
Purines : Adenine and guanine are double ring compounds called purines.
Pyrimidines : Whereas thymine, cytosine and uracil are single ring compounds referred to as pyrimidines.
Uracil is absent in DNA and present in RNA.
The nucleotides are named according to their nitrogen base. Thus, a nucleotide containing thymine nucleotide and so on. The term nucleoside refers to the combination of a pentose sugar and a purine or pyrimidine base. It does not contain a phosphate group. A combination of nucleoside and phosphate is called nucleotide. In a nucleotide, nitrogen base is attached to 1 carbon of the sugar molecule by glycosidic bond. The phosphate group is joined with 5th carbon of the sugar by sugar-phosphate bond.
Pentose sugar + Nitrogen base = Nucleoside
Pentose sugar + Nitrogen base + phosphate = Nucleotide
Thus,
Nucleoside + Phosphate = Nucleotide
Structure of DNA:
In 1953, James Watson and Francis Crick proposed DNA structure based on X-ray crystallographic studies provided by Maurine Wilkins and Rosalind Franklin.
1. DNA molecule as a double helix : According to the model proposed by Watson and Crick, a DNA molecule consists of two long strands coiled around a common, imaginary, central axis to form a double helix. It looks like a twisted ladder.
2. Structure of each stand : Each strand of DNA consists of number of nucleotides. Each nucleotide is made up of deoxyribose sugar, phosphate group and a nitrogen base. The successive nucleotides of the same strand are linked by 3' - 5' phosphodiester linkages. i.e. phosphate attached to 5 carbon of a sugar of nucleotide is joined to 3 carbon of another.
3. Complementary base pairing : The nitrogen bases present on one strand, pair with the nitrogen bases of opposite strand. Purine base A always pairs with the pyrimidine base T by two hydrogen bonds and the purine base G always pairs with the pyrimidine base C with three hydrogen bonds. This pairing is termed as complementary base pairing.
Purines : Pyrimidines
Adenine = Thymine (A=T or T=A)
Guanine = Cytosine (G≡C or C≡G)
As the base sequence present on one strand of DNA decides the base sequence of the other strand, the strands are regarded as complementary strands. The nitrogen bases along with nitrogen bonds make up the rungs of the ladder. These are also referred to as transverse bars. The distance between two successive steps or rungs is 0.34 nm.
4. Purine: Pyrimidine ratio : Due to complementary base pairing total number of purine bases is always equal to the total number of pyrimidine bases (1:1 ratio). This is called Chargaff's rule. It may be represented as follows-
A + G = T + C or A + G = 1
T + C
5. Polarity of strands : Each strand of DNA has a specific polarity. One end is considered as 5' end whereas the other end is 3' end. This polarity is because of 3rd and 5th carbon atoms of the deoxyribose sugar. At 5' end there is a free phosphate group white at the 3' end there is a free -OH group. One of the strands runs in 5'- 3' direction while the other in 3' - 5' direction,
This arrangement of two strands of DNA is considered antiparallel due to which 5' end of one strand lies close to the 3' end of the other strand and vice versa.
6. Major and minor grooves : Normally the strands of DNA undergo right-handed coiling around a central imaginary axis. The coiling of the double helix results in formation of major or deep grooves and the twisting of two strands around one another forms the minor or shallow grooves. They are found in alternate manner.
7. Dimensions : The diameter of DNA molecule is 2 nm. The spiral ladder like arrangement of DNA molecule is due to deflection angle of 36° between two successive steps (rungs). DNA molecule makes one complete 360° turn after covering a distance of 3.4 nm (0.34 nm x 10 steps = 3.4 nm). There are 10 base pairs in one complete spiral. ( 1nm = 10A°)