3. SYNOPSIS Introduction. DNA. Occurrence. DNA as the genetic material. Chemical composition. Structure. Semi conservative method of replication. Monday, May 9, 2011
4. RNA Occurrence. Chemical composition. Brief account of structure and functions of RNA. rRNA, mRNA, and tRNA( clover leaf model ). Monday, May 9, 2011
5. Nucleic acids are linear mixed polymers of nucleotides. They are, therefore, poly nucleotides. The adjacent nucleotides are joined together by phosphodiester bonds. Monday, May 9, 2011
9. Pyrimidines Monocyclic or single ring nitrogen bases. Pyrimidines are of three main types-cytosine(C), thymine(T) and uracil(U). uracil occurs in only ribonucleotides. Monday, May 9, 2011
10. Phosphoric Acid It is chemically H3PO4. The bond linking two phosphate radicals carries a lot more energy than the ordinary covalent bond. Association of phosphoric acid in nucleotides also provides the latter with acidic trait. Monday, May 9, 2011
11. Nucleosides Nucleoside is a condensation product of a pentose sugar and a nitrogen base. Monday, May 9, 2011
13. Nucleotide Formation. Nucleotides are phosphorylated nucleosides. They are formed by condensation of pentose sugar, a nitrogen base and at least one phosphoric acid residue. Monday, May 9, 2011
16. Discovery of Nucleic Acids Nucleic acids are linear mixed polymers of purine and pyrimidine nucleotides. They are also called polynucleotides.The elements involved in their formation are C,H,N,O and P. Nucleic acids were first isolated by Swiss physician Friedrich Miescher (1868), from nuclei of pus cells and named nuclein. Fischer(1880), discovered purine and pyrimidine nitrogen bases while, Levine(1910), found phosphoric acid, deoxyribose sugar and four types of nucleotides to be component of nucleic acid. Monday, May 9, 2011
17. Chargaff(1949), found out that amounts of purines and pyrimidines were equal with adenine being to thymine and cytosine being equal to guanine. Frankin (1952) ,found DNA to be helix. Watson and Crick (1953) , proposed the double helix DNA model incorporating all the details. Watson and Crick along with Wilkins are awarded Noble Prize in 1962. Monday, May 9, 2011
18. DNA Deoxyribonucleic Acid. DNA or deoxyribonucleic acid is helically twisted double chain of deoxyribonucleotides which is the largest macromolecule and also represents genetic material of organisms and molecular basis of heredity. DNA present inside nucleus is called nuclear DNA. Many prokaryotes also possess extranuclear or extra-chromosomal small circular DNA segments called plasmids. Monday, May 9, 2011
20. Structure of DNA. DNA duplex has a diameter of 20A.The duplex is coiled plectonemically in a right handed manner just as a rope stair is twisted to form a spiral. This coiling produces alternate major(length 22A) and minor(length 12A) grooves. Each turn of spiral has a distance of 34 A. This length contains 10 deoxyribonucleotides in each chain so that the average distance between adjacent deoxyribonucleotides is 3.4A. The condition of other end of the duplex is reverse.The direction of nucleotide chain is 5’-3’ in one and 3’-5’ in the other chain. Nitrogen bases of the two chains lying opposite to each other are complementary between A and T and between C and G. Two types of forces stabilize the duplex and hold the two DNA chains together: (i) Hydrogen bonds between the complementary nitrogen bases of the two chains.(ii) Hydrophobic interactions between nitrogen bases. Monday, May 9, 2011
25. Linear DNA is the one in which the ends are free. In circular DNA, the two ends are covalently linked. Circular DNA occurs in prokaryotes(ex.bacteria) and semi-autonomous organelles(mitochondria and plastids).Linear DNA occurs in eukaryotic nuclei. Heredity: Cistron is that segment of DNA which specifies synthesis of a polypeptide.It is equivalent to a gene or hereditary unit. Cistrons containing introns or noncoding areas are called split genes. A change in the sequence of nitrogen bases changes the codons of a cistron. This produces a modified polypeptide or an in complete polypeptide. The phenomenon is called mutation. The smallest segment of DNA which can undergo change is called muton. DNA Functions 1.Hereditary Information. Cistrons. 2.Variations. Reshuffling of DNA molecules during meiosis. 3. Mutations. Changes in genetic material DNA. 4.Autocatalytic Function. Replication occurs at the time of cell division. 5.Heterocatalytic Function. transcription. Monday, May 9, 2011
26. RNA or Ribonucleic Acid. RNA or ribonucleic acid is a single stranded mixed polymer of four types of ribonucleotides. RNA is formed over the template or sense strand of DNA.The process of formation of RNA over DNA template is called transcription. In prokaryotes,the whole of RNA occurs in the cytoplasm,either free or inside ribosomes.In eucaryotes,RNA is found inside nucleolus(the site of ribosome formation), ribosomes,plastids,mitochondria and cytoplasm. These are three types of RNA-rRNA,tRNA,mRNAThey are differentiated by their sedimentation coefficient or Svedberg units RNA Functions. 1. RNA Primer. It is essential for DNA replication. 2. Ribosome’s. They are formed of ribosomal or rRNAs. 3. Expression of Genetic Code. RNAs are involved in expression of genetic code through transcription of messages. Monday, May 9, 2011
27. DNA Replication is Semi-Conservative : Replication is the formation of exact carbon copies of a substance. It occurs in case of DNA.Rather replication is an autocatalytic activity of DNA. The DNA structure proposed by Watson and Crick(1953) was based on its semiconservative replication with both the strands of DNA functioning as templates and producing new complementary chains.The new chain get permanently associated with the templates giving rise to two double stranded replicas. Replication proceeds is 5’----3’ direction in each strand. An RNA primer is required. Bases of exposed strands attract complementary bases. In one strand, DNA is formed in fragments called Okazaki fragments. They are sealed by DNA ligase. This strand is slow to be formed and is called lagging strand as opposed to other continuous or leading strand. Monday, May 9, 2011
