Tuesday, August 24, 2010

Nucleic Acids


22 years old, Swiss physician and chemist Friedrich Miescher isolated a substance from nuclei of Pus cells which was quite different from other biomolecules and named it as ‘nucleic’. Later it was found that nuclei has acidic properties and hence it was renamed and nucleic acids. Nucleic acids are present in all organisms from virus to man. These micro-molecules are present either in Free State or bound to proteins as nucleoproteins. Like proteins nucleic acids are biopolymers of high molecular weight with mononucleotide or their sub units (monomers). Nucleic acids are long chain of polynucleotide in which mononucleotides are linked with each other. There are two kinds of nucleic acids, deoxyribose nucleic acid (DNA) and ribonucleic acid (RNA). DNA is found mainly in chromatin of cell nucleus where as most of RNA (90%) is present in the cell cytoplasm and little 10% in the nucleolus. Nucleic acids is polymer of nucleotide. 


Nucleotide is molecule which consists of following three parts.
(i) Pentose sugar (5 carbon)
(ii) Phosphoric acid (H2Po4)
(iii) A nitrogenous base
Pentose sugar found in nucleotides either ribose (C5H10O5) or Deoxyribose (C5H10O4).
Ribose is found in RNA nucleotides while Deoxyribose sugar is found DNA nucleotides, both of them are distinguished primarily or the basis of this Pentose Sugar. This Sugar behaves as basic Skeleton Phosphoric acid common in all nucleotides. It is attached with 5th carbon of Pentose Sugar in each nucleotide. There are two basic types of nitrogenous bases i.e. Purine and Pyrimidine. Purine includes two nitrogenous bases named Adenine (A) and Ginine (G) while Pyrimidine includes three nitrogenous bases Cytosine (C), Thymine (T) and Uracil (U).
The nucleotides differ on basis of their nitrogenous bases.
Formation of Nucleotide takes place in two steps. At first step nitrogenous base combines with Pentose Sugar at its first carbon to form a nucleoside. At the second step Phosphoric acid combines with 5th carbon of Pentose sugar to form a nucleotide. Nucleotides are of three kinds.

(1)        Mononucleotide:
Generally nucleotides are found in the nucleic acid as Polynucleotide but they are also found as mononucleotide and Di-nucleotide. Mononucleotides exist singly in the cell or as a part of other molecules. These are not the part of DNA or RNA. Some of these have extra phosphate groups e.g. ATP (Adenosine tri phosphate). It is most important among these nucleotides. It is an unstable molecule and carries energly from place to place within a cell. It is synthesized from ADP (Adenosine di-phosphate) and inorganic phosphate by capturing energy during photosynthesis. This energy is utilized to derive energy demanding reactions such as in synthesis of Proteins, Lipids, Carbohydrates, mechanical energy for Cyclosis, Contractility, Cell-division, movement of Flagella, active transport etc. ATP consists of Adenosine (Adenosine and ribose sugar) and three phosphate; among them two are energy rich phosphate bond. During conversion of ATP into ADP, free energy releases which is considerable large.

(2)        Di-nucleotide:
Sometimes two nucleotides are covalent by bounded together, these compounds are called di-nucleotide. One of the well known di-nucleotide is Nicotinamide adenine di-nucleotide (NAD). Nicotinanide is vitamin constituent. Two nucleotides are linked by phosphate of oen another. NAD is a coenzyme [Coenzymes are organic (molecule non protein) which bind to enzyme (Protein) and serve as a carrier for chemical groups or electrons] that carries electron and work with dehyrogenase enzyme. It removes tow hydrogen atom (2e- + 2H+) from its substrate, both electrons, but only one hydrogen ion is passed to NAD which reduces it to NADH.



(3)        Polypeptide:
Nucleic acids are informational macromolecules. They have a variety of role in living organisms. Inspite of all, unique and premiere service of nucleic acid is as repositories (store house) and transmitters of genetic information. They make it possible for cells to function according to specific patterns and give rise to new cells that either function similarly or develop new functions, according to plants encoded in nucleic acid molecule in a particular and simple fashion. Four different nucleotides make up each informational nucleic acid molecule. They are three letters in a genetic code.
Nucleic acid molecule is somewhat linear and the units (nucleotides) like letters on a printed page or digits magnetic signals on a computer tape. In the proper machinery these codes (nucleotides) can be interpreted. The cell interpretes the information present in many nucleic acid molecules as sequence of amino acid in protein and peptide molecules. The synthesis of protein with definite sequences of amino acids are controlled anounts of protein is observed as the expression of heredity of organisms which give physical appearance of that particular character.
DNA and RNA are basically similar structure because both of these are poly nucleotide chains but the nucleotide of both are different in following ways.
(i) DNA contains deoxyribose sugar (C5H10O4) while RNA contains ribose sugar (C5H10O5) in the nucleotides.
(ii) DNA contain Adenine Guanine, Cytosine and Thymine containing nucleotides where as RNA contains Adenine, Guanine, Cytosine and Uracil containing nucleotide.
(iii) DNA is double stranded helical structure while RNA is single stranded structure except RNA.
(iv) DNA is of just one kind while RNA is of three kinds rRNA, tRNA, and mRNA (r = ribosomal, t = transfer, m = messenger).

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