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Tuesday, August 24, 2010

Nucleic Acids

Tuesday, August 24, 2010 - 0 Comments


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).

Proteins

Proteins are organic compounds of living organisms making 55 – 85% of dry weight of the cell.
Protein molecule has many different levels of structure and may be coiled and folded or it may interact with other protein molecules to form unique three dimensional structure.
Different kinds of protein molecules have different shapes related to their particular functions in life processes.


They are as under:
(1)        Primary Structure: It is the linear sequence of amino acids in polypeptide chains comprising the molecule.
(2)        Secondary Structure: It is repeating pattern of bonds (often Hydrogen bonds) between amino acids and it commonly takes the shape of alpha relize or pleated sheet.
(3)        Tertiary Structure: It results from the helix folding into three dimensional shape.
(4)        Quaternary Structure: In some instances two Protein chains join to form larger protein which is called quaternary structure.

(1)        Primary Structure of Proteins:
It is the linear structure of amino acids in polypeptide chains comprising the molecule. In primary structure in addition to peptide bonds, disulphide bonds (S – S) may link into peptide chains as in insulin where two amino acids chains are linked with one another. In ribonuclease single chain is folded and has disulphide bond – Single peptide chain is formed as a result of polymerization of 100 – 1000 or more amino acids in straight chain. This chain will always be having a carboxyl group on one end which is known as C-termed and an amino group on other end thus known as N-terminal. New amino acids can be added both at C or N terminals. New amino acid can be added both at C or N terminals. Proteins have specific primary structure due to
(i) Number of amino acids in Polypeptide chain
(ii) Types of amino acids present in polypeptide chain
(iii) Sequence of amino acids in polypeptide chain.
DNA template determines the specificity in number, size, type and arrangement of amino acids in Protein.

(2)        Secondary Structure of Proteins:
Polypeptide chain that shows folding due to the formation of hydrogen bonds and forms a stabilized structure is called secondary structure of protein. It may be of following types:
(i) α-Helix: It is formed when a polypeptide chain is twisted in such a way that every amide and carboxyl carbon is involved in hydrogen bonding. α-helix look like a helical spring α-helix is controlled by number of amino acids residues per turn, the pitch of single residue. α-helix protein chain is the structural protein of hair, wood, nails, claws, beaks, feathers, horns and vertebrate skin.
Amino acids have the ability to form hydrogen bonds and participate in helix formation. Such amino acids are called helix formers like Glutamic acid, Alanine, Leucine, Histidine.
(ii) β-Pleated sheets: In some cases torsion angles in polypeptide chains are so irregular that hydro bonding does not take place in extended peptide chain. Two of three such extended chains interact with one another by hydrogen bonding A form β-Pleated sheets.

(3)        Tertiary Structure of Proteins:
Secondary proteins on extensive coiling or folding form compact structure called tertiary structure of protein. Tertiary structure determines the shape protein. Folding is held together due to interaction of R-group of different amino acids present in peptide chains. These interactions include ionic, hydrogen, disulphide bonds and hydrophobic interaction.
(4)        Quaternary Structure of Proteins: Sometimes more than two monomeric proteins are essential to make a structure or collectively perform a function; such bigger structures with more than two units form Quaternary structure of Protein. It is classified as monometric, dimeric, oligomeric depending upon the number of submits as one, two or many respectively. Different monomers are held together by ionic bonds, hydrogen bonds and hydrophobic interactions.

Functions of protein are
(1) Enzyme Catalysis: Chemical reaction occurring in enzymes, the specific proteins, controls the body.
(2) Coordinated Motion: Proteins like myosin, actin, troponin and tubulin are responsible for movement of animals, organs, chromosomes, flagella, cilia, cells etc.
(3) Mechanical Support: A fibrous protein, collagen provides mechanical support and strength to skin, bones, connective tissues, tendons and ligaments.
(4) Transport and storage: Haemoglobin and myoglobin present in RBC and muscles are responsible for transportation of oxygen to the cells and muscles. During starvation proteins present in body is used by process of autolysis for energy production.

Carbohydrates and their Classification

Carbohydrate literary means hydrated carbon. Carbohydrates are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Carbohydrates polyhydroxy aldehydes or ketones or complex substances that on hydrolysis yield polyhdroxy aldehydes or ketones subunits. Hydrolysis involve break down of large molecules into smaller ones utilizing water molecules. Carbohydrate occurs abundantly in living organisms. They are found in all parts of the cell, cellulose of wood, cotton and paper starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrate. The sources of carbohydrates are green plants. These are primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes. Carbohydrates are animal’s major source of energy. Most animal cells have chemical machinery to breakdown the energy rich carbon hydrogen (C – H) bonds in sugars and starches. Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids. Glycoproteins and glycolipids have structural role in extracellular matrix of animals and bacterial cell wall. Carbohydrates play structural and functional roles. Simple carbohydrates are main constituents of cell walls in plants and micro organisms.

Classification of carbohydrates:

They are also called saccharides which means sugar. They have three groups.
(1) Monosaccharides
(2) Disaccharides
(3) Polysaccharides

(1)        Monosacchrides:
They are single sugars and are simple sugars. They have following properties.
They are sweet in taste, they are easily soluble in water and they cannot be hydrolyzed into simple sugars. Chemically they are either polyhdroxy aldehydes or ketones. All carbon atoms in a monosaccharaide except one have a hydroxyl group. Remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called also sugar and with keto group as keto sugar e.g. Glucose, Fructose.



(2)        Disaccharides:
It is formed by removing a molecule of water from two monosaccharides. Disaccharides have the same molecular formula C12H22O11 e.g. Sucrose, lactose and maltose. A molecule of glucose combines with a molecule of fructose through 1 – 2 glycosidic bond to form sucrose, the table sugar. If a glucose molecule bonds to another monosaccharide galactose through 1 – 4 glycosidic bonds, the disaccharides formed is lactose, the milk sugar, when two glucose submits join together by 1 – 4 glycosidic bonds they form maltose that is present in seeds, especially it gives barley seeds a sweet taste. Beer brewers ferment barley into alcohol. Organic compounds having same molecular formula but different structural formulae are called isomers and each isomer has unique properties.

(3)        Polysaccharides:
They are most complex and abundant carbohydrates in nature. They are usually branched and tasteless. Several monosaccharide units linked by glycosidic bonds form polysaccharides. They are tasteless and insoluble in water and thus are responsible for making structural part of cell and organelles. On hydrolysis they produce large number of monosaccharides. Monosaccharides are bonded together with glycosidic bond. Polysaccharides may be in straight chain formed through 1 – 4 glycosidic bonds or have branched chain formed by 1 – 4 and 1 – 6 glycosidic bonds e.g. starch, glycogen, cellulose, dextrin, agar, pectin and chitin. Starch is main source of carbohydrate, for animals and on hydrolysis provides glucose molecules for animal. It is found in fruits, grains, seeds and tubers.
Glycogen is generally known as animal starch. It is stored in liver and muscles of animals. It is insoluble in water and converted into glucose on hydrolysis. Cellulose is most abundant carbohydrate in nature. Cotton is the pure form of cellulose. It is main constituent of cell walls of plants and is highly insoluble in water. On hydrolysis it also yields glucose molecules. In herbivores it is digested because of presence of micro organisms like bacteria, yeasts and protozoa in the digestive tract. It is not digested in human digestive tract.
Chitin is major component of exoskeleton of insects and crustaceans. The monomer of chitin is amino sugar comprising of glucose with a nitrogen containing appendage.

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