Chromosomes, the vehicles of heriditary characters ,are composed of chromatin which mainly consists of nucleoproteins though other substances are known to be present in it .The nucleoprotein as the name suggests , is a complex of nucleic acid ,specially deoxy ribo nucleic acid , specially deoxy ribo nucleic acid[DNA] and histone protein The nuceic acid besides being responsible for the physiological and m olecular activities in the body also acts as genetic material . NUCLEIC ACIDS ; The nucleic acid are mainly found in the nucleous .They are long chain molecules , built of repeating small molecules , called nucleotides , joined end to end in a chian . They are not found in free state but complexed with the protein to form nucleo protein , the chromatin .The nucleic acid obtained earlier from yeast cells was named zymonucleic acid and that from the thymus , the thymo nucleic acid . The nucleic acid are of two types ,the deoxyribose nucleic acid , abbreviated as dna ,and ribose nucleic acid , abbreviated as rna .The former is found in the nucleus, mitochondria and chloroplasts, whereas the lattewr is found un nuccleolus and cyto plasm. DEOXYRIBOSE NUCLEIC ACID The dna is biolgically most important and stable molecule . It exercises extremely superior role in cellular in heritence ,growth an development in the cell division and synthesis of protiens and other chemical consituents of organisms .In fact it is a reference library that contains infomation neede by the cell for performing its activities ; it is associated with all biological processes . Miescher [1869] was the first scientist to seperate the DNA from the nucleus of the pus cells and named in nuclein.However ,its role in heredity was suggested by hertwig [1884] .The name nucleic acid was proposed by altman [1889] due to its acidic nature and occurrence in the nucleus. Scientists got more intersted in DNA after fourth decade of the present century and many scientists such as Boivin ,Roger ,Vendrelly,Ris ,McCarthy , investigated it and suggests its role in heridity.In 1953 Watson and Crick proposed an acceptable structural model of the DNA ,commonly known as double helix model, which can explain the specificity and replication of genes. Chemical composition of DNA.The DNA is a polymer and its buiding units are called nucleotide is made of three different molecules ,a pentose sugar [5-csugar] ,a nitrogenous base and phosphate In DNA,the sugar is deoxyribose . Four different nitrogennous bases occurs in the DNA .They includre adenine ,guanine ,cytosine and thymine.The adenine and guanine are purines ,whereas ,the cyto sine and thymine are pyrimidines .In their structure , the purines [adenine and guanine] have double rings , whereas pyrimidines hav er only single ring .The bases show the presence of an amino group ; in cytosine and adwenine this group is present at number 4 position of the first ring but in thymine and gua nine, these positions are occupied by a keto group [co]. At this c1b position ,the deoxy ribose[pentose sugar] joins a nitrogenous base to form a nucleoside ; the four bases form four free hydroxyl group of the pentose in a nucleoside to form a nucleotide .Deprnding on the four different bases , ther are four kind s of nucleotides in DNA.., deoxycytidilic acid or deoxy cyti dilate ,containing cytosine deoxy thymidyclic acid or deoxy guanilate , containing guanine .The linkage between the pentous sugar and phos phoric acid is called ester linkage . Thus a nucleotide is a phosphate diester linkages , one at 3' hydroxyl position and other at 5' hydro xyl position.Infact,a phosphoric acid molecule helps in joining two nucleosides ;it is joined to carbon atom 3 ' of one deoxyribose and carbon 5' of other.The four nucleotides join to form a single DNA strand ,i.e., DNA poynuceotide
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