Nucleic Acids PDF

 NUCLEIC ACIDS By: Khalda Mahgoub Berair, MSc Assistant Professor of Biochemistry  Objectives By the end of this lecture, a student should be able to: Define nucleic acids. Describe the structure of DNA according to WATSON-CRICK MODEL. Explain the base pairing...

 NUCLEIC ACIDS By: Khalda Mahgoub Berair, MSc Assistant Professor of Biochemistry  Objectives By the end of this lecture, a student should be able to: Define nucleic acids. Describe the structure of DNA according to WATSON-CRICK MODEL. Explain the base pairing rules and the resulting complementarity of the two strands of DNA. Describe Chargaff’s rule & how it is used to identify nucleic acids. Explain what is meant by antiparallel strands and the significance of polarity & directions of each strand. Identify the various forms of DNA. Outline the cellular roles of DNA & how the function of DNA is dictated by its structure.  Nucleic Acids “Nucleic acids" literally means "acids from the nucleus.“ This name reflects their discovery and initial association with the cell nucleus, where DNA is found. Their acidic properties is due to the presence of phosphate groups.  Nucleic Acids There are two primary types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is composed of nucleotide units, each consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, and guanine). RNA is composed of nucleotides with a ribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, uracil, cytosine, and guanine).  NUCLEIC ACIDS DNA DNA is a polymer of deoxyribonucleotides. Precursors of DNA are 4 types of deoxyribonucleotides: dATP, dGTP, dCTP &dTTP. Monomer nucleotides are linked by a phosphodiester bond DNA Building Blocks  Nucleotide 1 Nucleotide 2 = one strand of DNA Note polarity Nucleotide 3 Nucleotide 4 So, what’s so special about DNA? The answer lies in DNA’s ability to store and copy information. DNA structure In 1953 two scientists James Watson and Francis Crick derived structural model for DNA.  WATSON-CRICK MODEL OF DNA Two helical polynucleotide chains coiled around a common axis Running in opposite directions. The sugar phosphate outside & the bases inside the helix. The diameter of the helix is20Ao.  The two strands are held by H-bonds between purines &Pyrimidines. Adenine is always paired with thymine(by two hydrogen bonds) and guanine is always paired with cytosine (via three hydrogen bonds). A =T, GΞC Thus, base pairing is specific, and the two strands are said to be complementary. The scientist Chargaff found that A/T and G/C were nearly 1 in all species studied : G+A=C+T Base pairing phenomena DNA is Made of Two Long Chains of Nucleotides Joined by Hydrogen Bonds A Nucleotide G and C are complementary as are A and T  The two strands are antiparallel i.e. one strand is in the direction 5`to3`,the other strand is in 3` to 5`. Nucleotide 1 one strand of DNA Nucleotide 2 Nucleotide 3 Nucleotide 4 5’ 3’ Two strands of DNA Double-stranded DNA Anti-parallel Hydrogen bonds AT or GC pairs 3’ 5’  Double- stranded DNA exist in at least 6 forms (A-E and Z). B-form is usually found under physiologic condition. Nucleic Acid Structure The double helix Minor Groove Major Groove  There are major and minor grooves on the surface of the double helix. In these grooves regulatory protein can interact with nucleotides and control the expression of specific genes.  Functions for DNA: 1-replicates itself for cell division 2-controls cell activity through control of protein synthesis dna Nucleus Cytoplasm DNA Replication – Something Old and Something New In Each Daughter Molecule Complementary Base Pairing Allows Each Strand of DNA to Serve as a Template for DNA Replication DNA is a perfect illustration of function following form (structure dictates function). Thank You