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6.02 Structure of DNA and RNA

Posted10/11/2024
Updated21/12/2024
ByBMF
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Overview: DNA and RNA

  • DNA: Deoxyribonucleic Acid
  • RNA: Ribonucleic Acid
    • Both DNA and RNA are polynucleotides (long chains of nucleotides).
  • Nucleic Acids: Named because they were first identified in cell nuclei.
  • Polymers: Composed of repeating units called nucleotides.

Structure of Nucleotides

  • Components of a Nucleotide:
    • Nitrogen-containing Base: Four types in DNA (Adenine, Guanine, Cytosine, Thymine); RNA has Uracil instead of Thymine.
      • Purines: Adenine (A) and Guanine (G) (two-ring structures).
      • Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U) (one-ring structures).
  • Pentose Sugar:
    • DNA: Deoxyribose (missing one oxygen atom).
    • RNA: Ribose.
  • Phosphate Group: Contributes the acidic nature of nucleic acids.
  • Polynucleotides:
    • Nucleotides are linked by phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next.
    • Forms a sugar-phosphate backbone with nitrogenous bases sticking out.

A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups.

 Structure of DNA. Notice that adenine (a purine) and thymine (a pyrimidine) are connected together with 2 hydrogen bonds, while guanine (a purine) and cytosine (a pyrimidine) are connected by three hydrogen bonds. There is a 5′ and 3′ end to both chains of nucleotides, which are antiparallel to each other. 

Formation of a Polynucleotide:

  • Process:
    • Condensation Reaction joins nucleotides.
    • Covalent bond forms between:
      • Phosphate group of one nucleotide.
      • Sugar (3′ OH group) of the next nucleotide.
  • Type of Bond:
    • Phosphodiester bond:
      • Links 5′ phosphate of one nucleotide to the 3′ hydroxyl of the next.

Key Points:

  • By-product: Water (H₂O) is released.
  • Directionality:
    • Chain grows in the 5′ to 3′ direction (important for DNA/RNA synthesis).
  • Result:
    • Formation of a sugar-phosphate backbone.
    • Nitrogenous bases extend from the backbone.

Formation of a Polynucleotide:

  • Bases project outward for base pairing (in DNA/RNA).
  • Condensation Reaction:
    • 5′ Phosphate + 3′ OH → Phosphodiester Bond + H₂O.
  • Repeating Units:
    • Backbone: Sugar–Phosphate–Sugar–Phosphate.

Structure of DNA

  • Double Helix Model (Watson and Crick, 1953):
    • Two Polypeptide Chains: Form a right-handed double helix.
    • Antiparallel Strands: Chains run in opposite directions (5′ to 3′ and 3′ to 5′).
    • Complementary Base Pairing:
      • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
      • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
      • Purines always pair with pyrimidines, maintaining a consistent width (three rings) between backbones.
    • Helix Characteristics:
      • One complete turn of the helix contains 10 base pairs.
      • Sugar-phosphate backbones form the outer framework, with bases projecting inwards at right angles.
  • Importance of Structure:
    • Storage of Information: Sequence of bases along one strand serves as a genetic code.
    • Replication: DNA can “unzip” along hydrogen bonds, allowing each strand to serve as a template for a new complementary strand, ensuring accurate copying.

The double-helix model shows DNA as two parallel strands of intertwining molecules.

DNA has (a) a double helix structure and (b) phosphodiester bonds. The (c) major and minor grooves are binding sites for DNA binding proteins during processes such as transcription (the copying of RNA from DNA) and replication.

Structure of RNA

  • Single-Stranded Molecule:
    • RNA consists of a single polynucleotide chain.
    • Three types of RNA involved in protein synthesis:
      • mRNA (Messenger RNA): Carries genetic code from DNA to ribosomes.
      • tRNA (Transfer RNA): Transports amino acids to ribosomes during protein synthesis.
      • rRNA (Ribosomal RNA): Structural component of ribosomes.

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