Medical brain notes 25

  Pyrimidine bases   Structure of pyrimidine bases Pyrimidine bases are derived from the parent compound pyrimidine. The pyrimidine bases present in nucleotides are cytosine, uracil and thymine. It is 6 membered heterocyclic compound contains carbon, nitrogen, hydrogen and oxygen atoms (Fig 7.1). Properties of pyrimidine ·             Pyrimidine bases are soluble in water. ·             They absorb UV light at 260 nm. This property is used to detect and estimate pyrimidine nucleotides. ·             They are capable of forming hydrogen bonds with other purine bases ·             They exhibit keto-enol tautormerism.

  Purine bases Structure of purine bases Purine bases are derived from the parent compound purine. Purine contains the heterocyclic ring system. Fusion of the pyrimidine ring with imidazole yields the purine ring (Fig. 7.2). The purine bases present in nucleic acids are adenine and guanine. Other purine bases are hypoxanthine and xanthine. They are intermediates in the formation of adenine and guanine nucleotides. Properties of purine bases ·             Purine bases are sparingly soluble in water. ·             They absorb light in UV region at 260 nm. This property is used for the detection of and the quantification of purine nucleotides. ·             They are capable of forming hydrogen bonds. ·             They exhibit keto-enol tautomerism at body pH.

  Nucleic acids Two types of nucleic acids are present in all mammalian cells. They are DNA - deoxy ribonucleic acid and RNA- ribonucleic acid. DNA is present in the nucleus and mitochondria. RNA is present in the nucleus, risosome and cytoplasm. Nucleic acids are acidic sustances containing nitrogenous bases, pentose sugar and phosphoric acid. Both DNA and RNA are polynucleotides. They are polymers of mononucleotides. In nucleic acids, nucleotides are joined together by phosphodiester linkages. Nucleosides A nucleoside is composed of purine or pyrimidine base and a pentose sugar. Two types of pentose sugar are present in nucleoside, they are ribose and deoxy ribose (Fig. 7.3). In the case of purine nucleosides, the sugar is attached to N-9 of the purine ring, whereas in pyrimidine nucleosides, the sugar is attached to N-1 of the pyrimidine ring. The type of linkage is N-glycosidic linkage (Fig. 7.4). Nucleotides Nucleotides are phosphorylated nucleosides usually one or two of hydr...

  Nucleosides A nucleoside is composed of purine or pyrimidine base and a pentose sugar. Two types of pentose sugar are present in nucleoside, they are ribose and deoxy ribose (Fig. 7.3). In the case of purine nucleosides, the sugar is attached to N-9 of the purine ring, whereas in pyrimidine nucleosides, the sugar is attached to N-1 of the pyrimidine ring. The type of linkage is N-glycosidic linkage (Fig. 7.4). Nucleotides Nucleotides are phosphorylated nucleosides usually one or two of hydroxyl groups of ribose (or) deoxyribose are phosphorylated. Thus a nucleotide has three structural components. They are nitrogenous base, sugar and phosphate. Phosphate is attached to ribose (or) deoxy ribose through an ester linkage (Fig.7.5).

  Structure of DNA Primary structure Nucleotide sequence of a nucleic acid is known as its primary structure which confers individuality to the polynucleotide chain. Polynucleotide chain has direction. They are represented in 5’---> 3’ and 3’----> 5’ directions. Each polynucleotide chain has 2 ends. The 5’ end carrying a phosphate group and 3’ end carrying an unreacted hydroxyl group (Fig 7.6). In 1953, J.D. Watson and F.H.C. Crick proposed a precise three dimensional model of DNA structure based on model building studies, base composition and X-ray diffraction studies. This model is popularly known as the DNA double helix (Fig.7.7). The purine bases present in DNA are adenine and guanine and the pyrimidine bases present are thymine and cytosine. The purine and pyrimidine bases of DNA carry genetic information where as the sugar and phosphate groups perform the structural role. Salient features of double helix ·           ...

  Functions of DNA 1.          DNA is the genetic material of living organisms. It is the greatest super chip ever made by man. 2.          DNA contain all the information required for the information of an individual organism. 3.          The genetic information in DNA is converted to characteristic features of living organisms like colour of the skin and eye, height, intelligence, ability to metabolise particular substance, ability to withstand stress, susceptibility to desease and ability to produce or synthesise certain substances. 4.          DNA is the source of information for the synthesis of all cellular proteins. The segment of DNA that contain information for a protein is known as gene. 5.          DNA is transmitted from parents to offsprings and hence transmit genetic info...

  Structure of RNA RNAs are present in the nucleus, ribosomes and cytoplasm of eukaryolic cells. They are involved in the transfer and expression of genetic information. They act as primer for DNA formation. Some act as enzymes and as coenzymes. RNA also function as genetic material for viruses. RNAs are also polynucleotides. In RNA polymer, purine and pyrimidine nucleotides are linked together through phosphodiester linkages. The sugar present is ribose. The nitrogenous bases present in RNA are adenine and guanine (purine bases), uracil and cytosine (pyrimidine bases). The nucleotides present in RNA are adenylic acid, quanidylic acid, cytidylic acid and uridylic acid. Types of RNA There are mainly three types of RNAs in all prokaryotic and eukaryotic cells. They are (1) Messenger RNA (mRNA) 2) Transfer RNA (tRNA) 3) Ribosomal RNA (rRNA). They differ from each other by size, formation and stability. 1. Messenger RNA It accounts for 1-5% of cellular RNA. They have a primary structur...