Medical brain notes 25

  Nucleic Acid Metabolism   Introduction Nucleic acids are the chemical basis of life and heredity. They serve as transmitters of genetic information. As the name implies, their location is mainly in nuclei. However, it is also found to be present in other intracellular organelles. Nucleic acids are present both in the free state as well as conjucated with proteins (Nucleoproteins). Like amino acids in proteins, nucleotides are the basic units of nucleic acids. There are two types of nucleic acids namely,   1.             Ribonucleic acid 2.             Deoxyribonucleic acid   Structural Components of Nucleic acids   Phosphoric acid The molecular formula of phosphoric acid is H 3 PO 4 . It contains 3 monovalent hydroxyl groups and a divalent oxygen atom, all linked to a pentavalent phosphorus atom.   Pentose Sugar The two types of nucleic acids are ...

  Biosynthesis of DNA   The process by which the new double helical DNA synthesized from the existing DNA is called as "Replication". The mechanism of biosynthesis of DNA has been largely clarified by the discovery of the enzyme DNA Polymerase or DNA Nucleotidyl transferase. This enzyme catalyses the polymerization of mononucleotides to polynucleotides, which needs the following for its action.   1.             A template strand dictates the synthesis of the new daughter strand and sequence of the template strand determines the addition of the nucleotides.   2.             RNA primer to which subsequent nucleotides can be added.   3.             Four nucleoside triphosphates namely, dGTP, dATP, dTTP and dCTP.   4.             Magnesium ions ...

  Replication   It is a process in which DNA copies itself to produce identical daughter molecules of DNA.   Models of Replication Three models of replication had been proposed. They are 1. Conservative replication 2.Dispersive replication and 3.Semi conservative replication.   1. Conservative replication According to this model, the parental DNA is conserved to one daughter cell and the newly synthesized DNA to another daughter cell.   2. Dispersive Replication   According to this model, the parental DNA is unequally distributed (randomly) to the daughter cells.   3. Semiconservative Replication Semiconseravative model of replication showing the daughter DNA having one parental strand and one daughter strand. This model was established to be the correct mode of replication by the experiment carried out by Messlson and Stahl in 1957.  E. coli  cells were grown in a medium containing 15NH 4 Cl, for many generations such that the E.coli cells h...

  Sequential Process of Replication Initiation of DNA replication   o       Initiation of DNA synthesis occurs at a site called origin of replication.   o       In prokaryotes, only one origin.   o       In eukaryotes, there are multiple origins.   o       The origin consists of a short sequence of A = T base pairs.   Replication bubbles   The two complementary strands of DNA separate at a site of replication to form a bubble. In eukaryotes many replication bubbles occur.   Unwinding of parental DNA   After the initiation point is located the unwinding of the DNA takes place once in every 10 nucleotide pairs. This allows the “strand separation”.   In prokaryotes the unwinding of the structure occurs with the help of an enzyme called helicase, which requires energy of hydrolysis of two ATP molecules per base pair broken. Another protein sin...

  Catabolism of DNA by Deoxyribonucleases   The nucleic acids exist mainly in the nucleus as nucleoproteins. These nucleoproteins in the diet are degraded by HCl to nucleic acids and proteins.   Nucleoproteins----------------(HCL)----------------->   Nucleic acids + proteins   Nucleic acids pass as such from the stomach and their catabolism starts in duodenum by several enzymes (nucleases, nucleotidases and nucleosidases) which degrade the nucleic acids to purines, pyrimidines and pentoses. Three different types of enzymes which degrade the nucleic acids are discussed one by one.   Nucleases   The enzymes which degrade the nucleic acids are known as nucleases. Some are specific for RNA and thus known as ribonucleases, and others for DNA and thus known as dexoyribonucleases, while still some others are capable of attacking DNA as well as RNA.   Deoxyribonucleases are further classified in to 2 categories.   1. Exonucleases  are the nuc...