Nucleic Acids
Nucleic acids are polymers of the ribonucleotides or deoxyribonucleotides and are associated with the nucleus of a cell. The two types of nucleic acids are deoxyribonucleic acid and ribonucleic acid. A nucleotide is an organic molecule containing a five carbon sugar, a nitrogen-containing base, and phosphate groups. The five carbon sugar is ribose or deoxyribose. The nitrogen-containing base is adenine, guanine, cytosine, thymine, or uracil. A ribonucleoside contains ribose and a nitrogen-containing base (cytosine, uracil, adenine, or guanine). A ribonucleotide contains ribose, a nitrogen-containing base (cytosine, uracil, adenine, or guanine), a phosphate group. A deoxyribonucleoside contains deoxyribose and a nitrogen-containing base (cytosine, thymine, adenine, or guanine). A deoxyribonucleotide contains deoxyribose, a nitrogen-containing base (cytosine, thymine, adenine, or guanine), and a phosphate group.
Deoxyribonucleic Acid
Deoxyribonucleic acid or DNA is a very long thread-like molecule made up of a very large number of deoxyribonucleotides joined together. It is linear, double stranded polymer. DNA is present in the nucleus of the cell. Polynucleotides are formed by the condensation of two or more nucleotides. The condensation most commonly occurs between the alcohol of a 5'-phosphate of one nucleotide and the 3'-hydroxyl of a second, with the elimination of water, forming a phosphodiester bond. The formation of phosphodiester bonds in DNA and RNA exhibits directionality. Single DNA strands are not stable, but associate with a second strand to form a double helix structure, where both strands intertwine around each other. At the center of the helix the four bases are H-bonded to each other and they do so in a very specific way. The three-dimensional structure of DNA the double helix arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside. Purine bases form hydrogen bonds with pyrimidines, in the crucial phenomenon of base pairing. A will only base-pair with T, and C with G. According to this pattern, known as Watson-Crick base-pairing, the base-pairs composed of G and C contain three H-bonds, whereas those of A and T contain two H-bonds. This makes G-C base-pairs more stable than A-T base-pairs. The primary structure of DNA and RNA (the linear arrangement of the nucleotides) proceeds in the 5' ----> 3' direction. The antiparallel nature of the helix stems from the orientation of the individual strands. From any fixed position in the helix, one strand is oriented in the 5' ---> 3' direction and the other in the 3' ---> 5' direction. On its exterior surface, the double helix of DNA contains two deep grooves between the ribose-phosphate chains. These two grooves are of unequal size and termed the major and minor grooves. The difference in their size is due to the asymmetry of the deoxyribose rings and the structurally distinct nature of the upper surface of a base-pair relative to the bottom surface.
Ribonucleic Acid
An RNA molecule is a single stranded polymer in composed of dexoyribonucleotides which are linked together by means of phosphodiester bridges, or bonds. Messenger RNA (m-RNA) is a linear polynucleotide strand which carries the message for the synthesis of a protein. Transfer RNA (t-RNA) is a cloverleaf structure polynucleotide which is responsible for the decoding the information from DNA. Ribosomal RNA (r-RNA) constitutes 50% of a ribosome which is a molecular assembly involved in protein synthesis. However it is possible for them to fold on themselves to form short regions of either double helix or a hair-pin loop. mRNA is the RNA molecule that corresponds to the gene or group of genes being expressed. The length of mRNA depends on the size of protein or group of proteins that are to be synthesized. It is the blueprint for construction of a protein. The mRNA is read in sequential groups of three nucleotides called codon. Nothing separates one triplet code from the next triplet code. It looks at the first three nucleotides, then the next three, etc. The linear order is conserved from DNA to mRNA to protein. tRNA molecules are small nucleic acids of 60-95 nucleotides, mostly 76, with a molecular weight 18-20kD. tRNA carries amino acids in an activated form to the ribosome for peptide bond formation in sequence determined by mRNA code. There are 61 different tRNAs, each having a different binding site for the amino acid and a different anticodon. The secondary structure resembles a clover leaf. rRNA is a major structural component of ribosomes (site of protein synthesis), and is probably involved in binding the mRNA to the ribosome to enable it to be translated. It is the construction site where the protein is made.
