Nucleic acids were first isolated in 1869 by F. Miescher from the nuclei of pus cells. Due to their isolation from nuclei and their acidic nature, they were named nucleic acids. They have two types deoxyribonucleic acid(DNA) and ribonucleic acid(RNA). DNA occurs in chromosomes, in the nuclei of the cells and in much lesser amounts in mitochondria and chloroplasts. RNA is present in the nucleolus, in the ribosomes, in cytosol and in smaller amounts in other parts of cell. Nucleic acids are complex substances. They are polymers of units called nucleotides. DNA is made up of deoxyribonucleotides while RNA is composed of ribonucleotides. Each nucleotide is made of three subunits a 5-carbon monosaccharide, a nitrogen containing base and a phosphoric acid. Pentose sugar in ribonucleotide is ribose while in deoxyribonucleotide it is deoxyribose. Nitrogenous bases are of two types single ringed pyrimidines and double ringed purines. Pyrimidines are cytosine, thymine and uracil. Purines are adenine and guanine. Phosphoric acid has ability to develop ester linkage with OH group of pentose sugar. In a typical nucleotide the nitrogenous base is attached to position 1 of pentose sugar while phosphoric acid is attached to carbon at position 5 of pentose sugar. The compound formed by combination of a base and a pentose sugar is called nucleoside. A nucleoside and a phosphoric acid combine to form a nucleotide. ATP is also an important nucleotide used as an energy currency by the cell.
DNA(Deoxyribonucleic acid): DNA is the hereditary material. It controls the properties and potential activities of a cell. It is made of four kinds of nucleotides namely d-adenosine monophosphate, d-guanosine monophosphate, d-cytidine monophosphate and d-thymidine monophosphate. These nucleotides are united with one another through phosphodiester linkages in a specific sequence to form long chains known as polynucleotide chains. Two nucleotides join together to form dinucleotide whereas three join together to form trinucleotide. Nicotinamide adenine dinucleotide abbreviated as NAD is an example of dinucleotide. It is important coenzyme in several oxidation-reduction reactions in cell. In 1951 Erwin Chargaff provided data about ratios of different bases present in this molecule. This data suggested that adenine and thymine are equal in ratio and so are guanine and cytosine. Maurice Wilkins and Rosalind Franklin used technique of X-ray diffraction to determine the structure of DNA. At the same time James D. Watson and Francis Crick built the scale model of DNA. All data thus obtained strongly suggested that DNA is made of two polynucleotide chains or strands. The two strands are coiled around each other in the form of a double helix. Coiling of two strands is opposite, they are coiled antiparallel to each other. Two chains are held together by weak bonds(hydrogen bonds). Adenine is always opposite to thymine and guanine and cytosine are opposite to each other. There are two hydrogen bonds between adenine and thymine pair and three hydrogen bonds between guanine and cytosine pair. The two strands are wound around each other so that there are 10 base pairs in each turn of about 34 Angstrom units. The amount of DNA is fixed for a particular species as it depends upon the number of chromosomes. The amount of DNA in germ cells is one half to that of somatic cells.
RNA(Ribonucleic acid): Like DNA, RNA is a polymer of ribonucleotides. RNA molecule occur as single strand which may be folded back itself to give double helical characteristics. Nitrogenous bases form the usual complementary pairing viz. cytosine with guanine and uracil with adenine. RNA is synthesized by DNA in a process known as transcription.
Types: Three main types of RNA : messenger RNA, transfer RNA and ribosomal RNA are recognized. All these three types are synthesized from DNA in the nucleus and then are moved out in the cytoplasm to perform their specific functions.
Messenger RNA: As the name indicates it takes the genetic message from nucleus to ribosomes in the cytoplasm to for particular proteins. It carries the genetic information from DNA to ribosomes, where amino acids are arranged according to information in mRNA to form specific protein molecule. This type of RNA consists of a single strand of variable length. Its length depends upon the size of gene as well as protein for which it is taking message. For example for a protein molecule of 1000 amino acids mRNA will have length of 3000 nucleotides. Messenger RNA is about 3 to 4% of total RNA in the cell.
Transfer RNA: It comprises about 10 to 20% of cellular RNA. Transfer RNA molecules are small each with a chain length of 75 to 90 nucleotides. It transfers amino acid molecules to the site where peptide chains are being synthesized. There is one specific tRNA for each amino acid. So the cell will have at least 20 kinds of tRNA molecules. It picks up amino acids and transfer them to ribosomes where they are linked to each other to form proteins.
Ribosomal RNA: It is major portion of RNA in cell and may be up to 80% of total RNA. It is strongly associated with ribosomal protein where 40 to 50% of it is present. It acts as a machinery for synthesis of proteins. On surface of ribosome the mRNA and tRNA molecules interact to translate information from genes into specific protein.
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