Building Blocks Of Nucleic Acids

The most fascinating feature of Nucleic acids is self-replication which is not seen in other biomolecules. The monomer of nucleic acid is nucleotide. A nucleotide is made up of the following components:-

• Pentose sugar

• Nitrogenous bases Purines
  Pyrimidines
• Phosphoric acid

Based on the type of pentose sugar present and a difference in a kind of pyrimidine present, nucleotides are of 2 types:-

• Ribonucleotide
• 2’ Deoxyribonucleotide

RIBONUCLEOTIDES

1.Ribose Sugar

Molecular Formula:- C₅ H₁₀ O₅

Structural Formula:-

Ribose sugar is a kind of pentose sugar containing 5 carbon atoms. It is present in beta form. In beta ribose, the hydroxyl group of first carbon and the hydroxyl group of 5^th carbon are present in CIS form. Both the second and 3^rd carbon carry Hydroxyl group.

The nitrogenous base is added to the first carbon to produce a a ribonucleoside first. To the 5^th carbon, a phosphate group is attached to Produce ribonucleotide.

2.Nitrogenous Bases

A.PURINES

• Purines are heterocyclic Nitrogen containing aromatic compounds that  consist of 2 rings.
• 2 types of purines found  in both deoxyribonucleotides and ribonucleotides.
• They are adenine and guanine.
• Addition of base to first carbon of ribose sugar produces a ribonucleoside.

B.PYRIMIDINES

• Pyrimidines are heterocyclic Nitrogen containing aromatic compounds that  consist of 1 ring.
• 2 types of Pyrimidines present in ribonucleotide..

• They are cytosine and uracil.
• 3.PHOSPHATE GROUP
• When a Phosphate group is attached to the 5’ carbon of ribonucleoside, a ribonucleotide is formed. The bond through which it is joined with the 5’ carbon of ribonucleoside is phosphoester bond.

Addition of one phosphate group to the above ribonucleoside monophosphate produce ribonucleoside diphosphate and one more phosphate group to ribonucleoside diphosphate produces ribonucleoside triphosphate.

2’-DEOXYRIBONUCLEOTIDES

1.2’-deoxyribose Sugar

Molecular Formula:- C₅ H₁₀ O₄

Structural Formula:-

2’-deoxyribose sugar is also a pentose sugar containing 5 carbon atoms like ribose. It is also present in beta form in living organisms.In beta form, the hydroxyl group of first carbon and the hydroxyl group of 5^th carbon are present in CIS form. It is called 2’-deoxyribose due to the absence of one oxygen in  the second carbon. It lacks one oxygen as in place of hydroxyl group 2’ carbon carries one -H group. But the 3^rd carbon carry Hydroxyl group.

The nitrogenous base is added to the first carbon to produce a a 2’-deoxyribonucleoside first. To the 5^th carbon, a phosphate group is attached to Produce  2’-deoxyribonucleotide..

2.Nitrogenous Bases

A.PURINES

• 2’-DEOXYRIBONUCLEOTIDES also contain the purines, adenine and guanine just like ribonucleotides.
• Addition of base to first carbon of deoxyribose sugar produces a deoxyribonucleoside.

    B.PYRIMIDINES

• One of the Pyrimidine present in 2’-DEOXYRIBONUCLEOTIDES is same as that present in ribonucleotides that is cytosine.
• But the second Pyrimidine present in 2’-DEOXYRIBONUCLEOTIDES is different that is Thymine. A little modification occur in uracil to produce thymine,I.e, addition of one -CH₃ / methyl group in 5^th carbon. So thymine is also known as 5-methyl uracil.
• 3. Phosphate Group
• When a Phosphate group is attached to the 5’ carbon of 2’-DEOXYRIBONUCLEOSIDE, a 2’-DEOXYRIBONUCLEOTIDES is formed. The bond through which it is joined with the 5’ carbon of 2’-DEOXYRIBONUCLEOTIDES is phosphoester bond.

• Addition of one phosphate groups to the above 2’-DEOXYRIBONUCLEOSIDE monophosphate produce 2’-DEOXYRIBONUCLEOSIDE diphosphate and one more phosphate to 2’-DEOXYRIBONUCLEOSIDE diphosphate produce 2’-deoxyribonucleoside triphosphate.

Ribonucleotides are building blocks of RNA and 2’-deoxyribonucleotides are building blocks of DNA.

Question :- Why nature select DNA as the genetic material of living organisms but not select RNA?