Nucleic Acids

nuCLEIC ACIDS Nucleotides Nitrogenous Bases The monomer of a nucleic acid is called a nucleotide Each nucleotide possesses one of five different nitrogenous bases • Adenine, Guanine, Cytosine, Thymine or Uracil Each nucleotide consists of three basic components: • A pentose sugar • A phosphate group • A nitrogenous base Bases may either be purines (A, G) or pyrimidines (C, T, U) • T is present in DNA, whereas U is present in RNA H2N Base O 5 4 Sugar 1 3 PURINES CH2 H O N H2N N N Adenine N H HN 2 N H N O H Uracil H3C N N Cytosine NH3 H H HN NH H N O NH H Guanine O O PYRIMIDINES Phosphate N Thymine O Nucleotides are linked together into a single strand via condensation reactions (between a 5’-phosphate and a 3’-hydroxyl group of adjacent nucleotides) C G A T 3’ ← 5’ This polynucleotide arrangement results in the formation of a sugar-phosphate backbone that is covalently linked together by phosphodiester bonds 3’ ← 5’ Polynucleotide Formation DNA Structure Two complementary strands line up in opposite directions (anti-parallel) with the bases facing inwards and connected by hydrogen bonds (G ≡ C and A = T) The double stranded molecule then twists in order to adopt a more stable energy configuration – a double helix RNA Structure The polynucleotide chain remains single stranded, but may fold upon itself to form double stranded motifs (e.g. the cloverleaf shape of a tRNA molecule) DNA Ladder Double Helix DNA versus RNA Watson and Crick DNA and RNA are both polymers of nucleotides, however they differ in a few key structural aspects The structure of DNA was elucidated by Watson and Crick in 1953 DNA RNA Sugar is deoxyribose Sugar is ribose Has thymine (T) Has uracil (U) (along with A, C and G) (along with A, C and G) Is double stranded (forms a double helix) Is single stranded Using data from previous scientific experiments (plus trial and error), Watson and Crick developed a DNA model that demonstrated: • A double helix structure composed of antiparallel DNA strands • Internally facing bases with complementary pairing (A=T, G≡C) James Watson Francis Crick Model