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Nucleic acid molecules is a long chain polymer (polynucleotide) composed of monomeric units called nucleotides. The nucleic acids found in organism can be of two sub-class Deoxynucleic acid (DNA) and Ribonucleic acid (RNA). Each nucleotide consists of a nucleoside and a phosphate group and each nucleoside in its turn consists of a sugar molecule and a base .DNA is the prime genetic molecule, carrying all the hereditary information within chromosome, immediately following attention on its structure. The exact replication of the information in the DNA in any species assures its genetic continuity from generation to generation and is critical to the normal development of an individual. The information stored in DNA is arranged in hereditary units known as genes that control identifiable traits of an organism .In the process of transcription, the information stored in DNA is copied into ribonucleic acid, which has distinct roles in protein synthesis. (Strickberger, 2006)
Structure of DNA
DNA as a chemical, is quite simple and contains 3 types of chemical components;1) phosphate, 2)a sugar called deoxyribose and 3) four nitrogenous bases : adenine, guanine, cytosine and thymine. The carbon atoms in the sugar group also are assigned number in this, the number is followed by a prime (1', 2' and so forth). The sugar in DNA is called deoxyribose because it has only a hydrogen atom (H) at the 2'-carbon atom, unlike ribose (a chemical compound of RNA) which has a hydroxyl (OH) group at that position. Two of the bases adenine and guanine have a double structure called as purines. The other two bases cytosine and thymine have a single structure called pyrimidine. The chemical components of DNA are arranged into groups called nucleotide, each composed of a phosphate group, a deoxyribose sugar molecule, and any one of the four bases. (Griffiths et al, 2008)
(Source; Structure of Deoxyribose sugar, n.d.)
Adenine A Guanine G
Thymine T Cytosine C
Fig.Structure of DNA bases.
(DNA and RNA Structures,n.d)
DNA is a double helix structure in which they consists of two complementary polymeric chains twisted about each other. The two nucleotide strands twisted are held together by hydrogen bonds between the bases of each strand forming a structure like a spiral staircase. The backbone of each strand is formed alternating phosphate and deoxyribose sugar units that are connected by phosphodiester linkages. Each phosphate backbone is said to have 5' to 3' polarity or direction. In the double stranded DNA molecule, the two backbones are in opposite, or anti parallel, orientation. Each base is attached to the 1'-carbon atom of a deoxyribose sugar in backbone of each strand and faces inward toward a base on the other strand. The base pairs, which are flat planar structures, stack on top of one another at the center of the double helix. Stacking adds to the stability of the DNA molecules by excluding water molecules from the spaces between the base pairs. The most stable form that results from base stacking is a double helix with two distinct sizes of grooves, the helical shape of DNA depends entirely on the pairing and stacking of the bases in the anti parallel strands. Each purine base pairs with pyrimidine base G≡C, A=T. (Nelson et al, 2000)
Fig.structure of DNA
(Source; Structure o f DNA, n.d.)
Functions of DNA
DNA primarily serves as the storage material for genetic information. It is a double stranded structure which has a number of advantages the most important being when strand breaks,the entire molecule does not fall apart. If error is made in one strand on one base the other strand is still there in the original order to help maintain the original sequence on the opposite strand when a correction enzyme comes along to clip out the mismatch. Since the backbone of each strand is made of sugar-phosphate maintains its stability and also double helical hides the base inside them. The base pair complementarity is a consequence of size, shape and chemical composition of the bases. (Watson e tal, 1965)
The presence of thousands of hydrogen bonds in a DNA molecule contributes greatly to the stability of the double helix. Hydrophobic and Vanderwaals interaction between the stack adjacent base pairs further stabilize the double helical structure. The two grooves (minor and major grooves) which is formed by the intertwined strands also has some functions like, the DNA binding proteins can 'read' the sequence of bases in duplex DNA by contacting atoms in either the major or the minor grooves. In DNA Thymine is used as a base instead of uracil because thymine is a methylated form of DNA and thus methylation protects the DNA and makes unrecognizable to many nucleases. Also methyl restricts the thymine to pair only with adenine which improves the efficiency of DNA replication by reducing the rate of mismatches and thus reducing mutations. (Lodish e tal, 1986)
Structures of RNA
Ribonucleic acid (RNA) is a usually a single strandard nucleotides chain.RNA has ribose sugar in its nucleotides rather than the deoxyribose found in DNA. The two sugars differ in the presence or absence of just one oxygen atom. The RNA sugar contains hydroxyl group (OH) bound to the2'-carbon atom. A strand of RNA is formed of a sugar-phosphate backbone, with a base covalently linked at the1'position of each ribose. The sugar phosphate linkages are made of each ribose. The sugar phosphate linkages are made at the 5' and 3'positions of the sugar.RNA nucleotides (called ribonucleotides) contain the bases adenine, guanine and cytosine but the pyrimidine base uracil (U) is present instead of thymine. Uracil form hydrogen bonds with adenine and is capable of base pairing with G. The hydrogen bonds that can form between U and G are weaker than two that form between U and A.
The ability of U to pair with both A and G is major reason why RNA can form extensive and complicated structures. All the structure of RNA sugar is similar to that except presence of OH group at 2'carbon. (Herskowtz, 1973) (Source; Structure of Ribose, n.d.)
Adenine A Guanine G
Uracil U Cytosine C
Fig. Structures of RNA bases.
(RNA and DNA Structures,n.d)
Classes of RNA
RNA can be grouped into general classes; messenger RNA and functional RNA (which include tRNA and rRNA. Each of this different RNAs has a specific function. Different RNAs are explained in the following paragraphs;
Messenger RNA (mRNA)
Messenger RNA (mRNA) carries the genetic information transcribed from DNA in the form of series of 3 nucleotides sequences called codons, each of which specifies a particular amino acid. In prokaryotic cells, which have no nuclei, translation of an mRNA into protein can begin from the 5' end of mRNA, even while 3'end is still being synthesized by RNA polymers. But in eukaryotic cell, all eukaryotic precursor mRNAs (pre-mRNAs) initially modified at two ends, and this modification are retained in mRNAs. As the 5' end of a nascent RNA chains emerges from the surface of RNA polymerase II, it is immediately acted on by several enzyme that together synthesis the 5' cap, a 7-methylguanylate that is connected to the terminal nucleotide of the RNA by unusual 5', 5' triphosphate linkage, Processing at the 3' end of a pre-mRNA involves cleavage by an endonuclease to yield a free 3'- hydroxyl group to a which a string of adenylic acid residues is added one at the time poly (A) tail contains 100 to 250 bases.
The functional eukaryotic of mRNAs produces by RNA processing retain non coding regions, referred to as 5' and 3' untranslated regions (UTRS) at each end. (Lodish e tal, 1986)
(Source; Structure of mRNA,n.d.)
Transfer RNA (tRNA)
The mRNA carries genetic information transcribed from DNA in the form of codons but Transfer RNA (tRNA) is the key to deciphering the codons in mRNA .Each type of amino acid has its own subsets of tRNAs which bind the amino acid and carry it to the growing end of a polypeptide chain if the next codon in mRNA calls for it. All tRNA have the same general shape, sort of resembling a clover leaf. Parts of the molecule folds back in characteristic loops, which are held in shape by nucleotide-pairing between different areas of the molecule .There are two parts of the tRNA that are of particular importance: the aminoacyl attachment site and the anticodon. The aminoacyl attachment site is the site at which the amino acid is attached to the tRNA molecule. Each type of tRNA specifically binds only one type of aminoacid. The anticodon (three bases) of the tRNA base pairs with the appropriate mRNA codon at the mRNA-ribose complex .Because of nonstandard interactions, a tRNA may base Pair with more than one mRNA codon; conversely, a particular codon may base pair with multiple tRNAs.This temporarily binds the RNA to the mRNA, allowing the amino acid carried by the tRNA to be incorporated into the 5' →3' direction, anticodons are oriented and written in the 3'→5'.The tRNA with an attached amino acid is said to be charged. The flattened carbon is not the normal confirmations of tRNA molecule; a tRNA normally exists as an L-shaped folded clover leaf. The dimensional structure of tRNA was determined with the use of X-ray crystallography. (Russell, 2006)
Fig;structure of tRNA
(Source; Structure of tRNA, n.d)
Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) associates with a set of proteins to form ribosomes. These complex structures which physically move along an mRNA molecules catalyze the assembly of amino acids into polypeptide chains. The rRNA also helps to speed up or catalyze protein synthesis by interactions between the tRNA and proteins synthesis machines The various accessory proteins are composed of a large and a small subunit each of which contain its own rRNA molecule or molecular Ribosomal RNA (rRNA) molecules come in 3 basic sizes, in prokaryotes 23s,5s and 16s. Eukaryotes have 28s, 5s and 18s.All 3 RNA are single stranded and have unequal amounts of guanine and cytosine and of adenine and uracil. In prokaryotes, ribosome binds to the mRNA close to the translation start site. This ribosome binding site is referred to as Shine-Dalgamo sequence or as the ribosome recognition element. In eukaryotes, ribosomes bind at the 5'-end of the mRNA and soon down the mRNA until they encounter a suitable start codon. (Lodish et al, 1986)
Functions of RNA
RNA is usually a single-stranded nucleotide chain, not a double helix like DNA and as a result RNA is more flexible and can form a much greater variety of complex three dimensional molecular shapes than can a double stranded DNA. An RNA strand can bend in such a way that some of its own base pairs with each other. Such intra molecular base pairing is an important determination of RNA shape.RNA has a ribose as a sugar component and has hydroxyl group at the 2'position.Thus the OH group on C2 of ribose makes RNA more chemically labile than DNA and provides a chemically reactive group that takes part in RNA-mediated catalysis .As a result of this labiality, RNA is cleaved into mononucleotides by alkaline solution, whereas DNA is not and also the presence of OH group facilitates the action of RNA in many important cellular processes. .The folded domains of RNA molecules not only are structurally analogous to the alpha helices and beta strands found in proteins, but in some cases also have catalytic capacities. Uracil forms hydrogen bonds with adenine and is capable of base pairing with guanine. The ability of uracil to pair with adenine and guanine is a major reason for forming extensive and complicated structure, many of which are important in biological process. And the presence of ribose sugar in its nucleotides maintains its stability. (Griffiths et al, 2008)
DNA and RNA are chemically similar. The primary structures of both are linear composed of monomers called nucleotides.Cellular RNAs range in length from less than one hundred to many thousands of nucleotides.Cellular molecules can be as long as several hundred millions nucleotide.DNA and RNA each consists of only four different nucleotides.Nucleotides consist of an organic base linked to five-carbon sugar that has a phosphate group attached to carbon 5.In RNA sugar is ribose and in DNA ,deoxyribose.Both RNA and DNA contain three of the bases adenine,guanine and cytosine.However thymine in DNA and uracil in RNA.A single nucleic acid strand has a backbone composed of repeating pentose-phosphate units from which purines and pyrimidines bases extend as side groups.The chemical linkages between adjacent nucleotides,is called phosphodiester.
Although the primary structure of DNA and RNA are generally similar, their three dimensional conformations are quite different.These structural differences are critical to the different functions of the two types of nucleic acid.(Lodish et al, 1986)