Nucleic acids – Definition, Structure and Applications

Definition:

Nucleic acids are polymers of nucleotides, linked by phosphodiester bond, they are therefore called polynucleotides.

The nucleic acids are of two main types:

• Deoxyribonucleic acid or DNA
• Ribonucleic acid or RNA.

DNA is present in nuclei and small amounts are likewise present in mitochondria, whereas 90% of the RNA exists in the cell cytoplasm and 10% in the nucleolus.

Nucleotide

Nucleotides are organic molecules including a nucleoside and a phosphate. A nucleotide is one of the structural parts, or foundation, of DNA and RNA.

Each nucleotide includes 3 components:

1. A nitrogenous base
2. A pentose sugar
3. A phosphate group.

Nitrogenous Bases of RNA and DNA

Two types of nitrogenous bases specifically purines and pyrimidines exist in RNA and DNA.

Purine Bases

Two primary purine bases present in DNAs, along with RNAs are:

1. Adenine (A).
2. Guanine (G).

Pyrimidine Bases

Three significant pyrimidine bases are:

• i.Cytosine (C).
• ii.Uracil (U).
• iii. Thymine (T)

Cytosine and uracil are present in RNA and cytosine and thymine in DNA. Both DNA and RNA consist of the pyrimidine cytosine but they vary in their second pyrimidine base. DNA consists of thymine whereas RNA contains uracil.

Pentose Sugars Present in RNA and DNA

The pentose sugar is either D-ribose or D-2-deoxyribose. DNA and RNA are distinguished on the basis of the pentose sugar present. DNA consists of D-2-deoxyribose and RNA include D-ribose. A pentose sugar (D-ribose or D-2-deoxyribose) is connected to a base (purine or pyrimidine) via covalent N-glycosidic bond. The term nucleoside is utilized for structures consisting of just sugar and nitrogen base.

This linkage signs up with nitrogen-9 of the purine base or nitrogen 1 of the pyrimidine base with carbon 1 of pentose sugar. The atoms of the base in nucleosides are offered cardinal numbers, whereas the carbon atoms of the sugars are provided primed numbers to differentiate sugar atoms from those of the nitrogen base. The nucleosides of A, G, C, T and U are called adenosine, guanosine, cytidine, thymidine and uridine respectively. If the sugar is ribose, ribonucleoside is produced; if the sugar is 2-deoxyribose, a deoxyribonucleoside is produced.

Structure of nucleotides

Nucleotides are phosphorylated nucleosides. Nucleosides are nitrogen bases including pentose sugar. The phosphate group is attached to the nucleoside by an ester linkage to the hydroxyl group of the pentose sugar. The nucleotides are of 2 types depending upon the type of pentose sugar present.

1. Deoxyribonucleotides: These nucleotides consist of a pentose sugar, deoxyribose and are monomeric units of DNA.
2. Ribonucleotides: These nucleotides contain pentose sugar, D-ribose and are monomeric units of RNA.

Mononucleotides are nucleosides in which the single phosphate group is attached to the hydroxyl group of the pentose sugar. For instance, AMP (adenosine monophosphate) is adenine + ribose + phosphate. If an additional phosphate group is connected to the pre-existing phosphate of mononucleotide.

— A nucleoside diphosphate, e.g. ADP.

— A nucleoside triphosphate, e.g. ATP are formed.

Biologically Important Nucleotides

Besides being the structural parts of nucleic acids, a number of nucleotides such as ATP, ADP, c-AMP, GTP, GDP, c-GMP, UDP, CTP, CDP, and so on take part in several biochemical and physiological functions. Such nucleotides are called biologically important nucleotides. Nucleotides involved in different biochemical processes are given below.

ATP

ATP works as the primary biological source of energy in the cell. ATP is needed as a source of energy in numerous metabolic pathways, e.g. fatty acid synthesis, glycolysis, cholesterol synthesis, protein synthesis, gluconeogenesis, and in physiologic functions such as muscle contraction, nerve impulse transmission, etc.

AMP

AMP is the part of many coenzymes such as NAD⁺, NADP⁺, coenzyme A, etc. These coenzymes are important for the metabolism of carbohydrates, lipid and protein.

C-AMP (Cyclic adenosine 3′, 5′- monophosphate)

C-AMP is formed from ATP by the action of adenylate cyclase. C-AMP functions as a 2nd messenger for lots of hormones, e.g. epinephrine, glucagon, etc. And affects a vast array of cellular processes by acting as a second messenger. It enhances the deterioration of storage fuels like fat and glycogen by stimulating lipolysis, glycogenolysis. It hinders the aggregation of blood platelets. C-AMP increases the secretion of acid by the stomach mucosa.

GDP and GTP

These guanosine nucleotides take part in the conversion of succinyl-CoA to succinate, a reaction which is paired to the substrate-level phosphorylation of GDP to GTP in the citric acid cycle. GTP is needed for activation of adenylate cyclase by some hormones. GTP serves as an energy source for protein synthesis.

C-GMP (Cyclic guanosine 3′, 5′- monophosphate)

C-GMP is formed from GTP by guanylyl cyclase. C-GMP is an intracellular signal or second messenger that can act antagonistically to c-AMP. C-GMP is involved in the relaxation of smooth muscle and vasodilation.

UDP (Uridine diphosphate)

UDP takes part in glycogenesis-glucose and UDP-galactose takes part in galactose metabolism and required for the synthesis of lactose and cerebrosides. UDP-glucuronic acid is needed in detoxification processes and for the biosynthesis of mucopolysaccharides such as heparin, hyaluronic acid, and so on.

CTP (Cytidine triphosphate) and CDP (Cytidine diphosphate)

CTP and CDP are required for the biosynthesis of some phospholipids. CDP-choline is associated with the synthesis of sphingomyelin.

Applications of Synthetic Nucleotides or Antimetabolites

Chemically, manufactured analogues of purines and pyrimidines, their nucleosides and their nucleotides have restorative applications in medicine. An analogue is prepared either by modifying the heterocyclic ring or sugar moiety. These are used chemotherapeutically (treatment of illness by the use of chemical compounds)to manage cancer or infections. Analogues of purines and pyrimidines utilized in the treatment of infections or in cancer chemotherapy are:

The nucleoside cytarabine (arabinosyl cytosine; ara-c) in which arabinose replaces ribose, is used in the chemotherapy of cancer and viral infections.

• The purine analogue allopurinol used in the treatment of hyperuricemia and gout.
• Azidothymidine (AZT) is a structural analogue of thymidine utilized in the treatment of Acquired immunodeficiency syndrome (AIDS), a disease brought on by the human immunodeficiency infection (HIV).
• Azathioprine, which is used during organ transplantation to suppress events involved in immunologic rejection.
• 5-iododeoxyuridine, a nucleoside analogue has antiviral activity and is used in the treatment of herpetic keratitis, an infection of the cornea by the Herpes virus.

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