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Biology A1.2.2 Structures of RNA and DNA

Learning Objectives

  1. Understand that DNA and RNA are polymers formed by condensation of nucleotides.
  2. Describe the structure and roles of RNA
  3. Describe DNA as a double helix made of two antiparallel strands of nucleotides, linked by hydrogen bonds between complementary base pairs.
  4. Compare the differences between DNA and RNA.

Part 1: Condensation of Nucleotides

  • When 2 nucleotides (for example 2 RNA nucleotides) are linked together,
    • The phosphate group of one nucleotide reacts with the hydroxyl (-OH) group on the 3′ carbon of another nucleotide’s ribose sugar. This forms a phosphodiester bond (a covalent bond)
    • A water molecule (H₂O) is released, making this a condensation reaction.
  • Repeating the Process:
    • This reaction continues, forming a sugar-phosphate backbone with nitrogenous bases extending from it.
    • The result is a single-stranded RNA molecule.
DNA and RNA. Source: technologynetworks.com

Part 2: Structure and Roles of RNA

  • RNA (ribonucleic acid) is a single-stranded, unbranched polymer of nucleotides.
  • Composed of ribose sugar, phosphate group, and nitrogenous bases (Adenine, Uracil, Cytosine, Guanine).
    • The number of nucleotides in an RNA molecule is unlimited.
  • Formed by condensation reactions, creating a sugar-phosphate backbone.
    • This reaction often required enzymes and protein complexes
  • Shorter than DNA and can fold into complex secondary structures due to complementary base pairing.
  • RNA plays a central role in gene expression, acting as an intermediary between DNA and proteins while also regulating cellular functions.

Roles of RNA

  1. Messenger RNA (mRNA)
    • Carries genetic information from DNA (in the nucleus) to ribosomes (in the cytoplasm) for protein synthesis.
    • Acts as a template for translation which is protein synthesis.
  2. Ribosomal RNA (rRNA)
    • A major component of ribosomes.
    • Helps catalyse peptide bond formation during translation.
  3. Transfer RNA (tRNA)
    • Transfers specific amino acids to ribosomes during protein synthesis according to the mRNA code (codons) by complementing it with its anticodon.
  4. Small Nuclear RNA (snRNA)
    • Involved in RNA splicing, removing introns from pre-mRNA.

Part 3: Structure of DNA

DNA Structure

Structure of DNA as a Double Helix

  • DNA is a polymer of nucleotides, each consisting of a deoxyribose sugar, phosphate group, and nitrogenous base.
  • The four bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).
  • DNA is composed of two strands of nucleotides linked by hydrogen bonds between complementary base pairs:
    • A pairs with T (via two hydrogen bonds).
    • G pairs with C (via three hydrogen bonds).
  • The two DNA strands are antiparallel—they run in opposite directions:
    • One strand ends with a phosphate group (5’ end).
    • The other strand ends with a deoxyribose sugar (3’ end).
Directionality of DNA
  • If the strands ran in the same direction, hydrogen bonding between bases would not be possible.
  • DNA has a double helix structure, with a constant diameter of 2 nanometers (2 nm).
  • The helical shape allows for compact storage and stability of genetic information.
  • Diagrams are often used to represent DNA structure but may not fully capture its 3D helical nature.

Part 4: Differences of DNA and RNA

Ribose sugar is the sugar of RNA

Deoxyribose sugar is the sugar of DNA

Differences between DNA and RNA

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