Nucleotides and Nucleic Acids

Nucleotides and Nucleic Acids

General Properties and Structure of Purine and Pyrimidine Bases

Key Features of Nucleotides and Nucleic Acids
  • Nucleotides are essential biomolecules that store and transfer genetic information.
  • Fundamental components of nucleotides:
    • Nitrogenous Bases: categorized into purines and pyrimidines.
    • Pentose Sugars: either ribose or deoxyribose.
    • Phosphate Groups: one or more can be present.

Purine and Pyrimidine Bases

Types of Nitrogenous Bases
  1. Purines: Double-ring structures.
    • Examples: Adenine (A) and Guanine (G) (consist of fused imidazole and pyrimidine rings).
  2. Pyrimidines: Single-ring structures.
    • Examples: Cytosine (C), Thymine (T), and Uracil (U).
Base Pairing Rules
  • Adenine (A) pairs with Thymine (T) in DNA (or Uracil (U) in RNA) via two hydrogen bonds.
  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Properties of Purines and Pyrimidines

  1. Aromatic and Planar: This property allows for stacking interactions, stabilizing nucleic acid structures.
  2. Hydrogen Bonding: Essential for complementary base pairing in both DNA and RNA.
  3. Tautomeric Shifts: Can alter base pairing properties, potentially leading to mutations.

Nucleosides, Nucleotides, and Nucleic Acids

Nucleosides
  • Composed of a sugar (ribose or deoxyribose) and a nitrogenous base.
  • Examples:
    • Adenosine (A + Ribose)
    • Guanosine (G + Ribose)
    • Cytidine (C + Ribose)
    • Uridine (U + Ribose)
    • Thymidine (T + Deoxyribose)
Nucleotides
  • Phosphate esters of nucleosides; consist of a nitrogenous base, a sugar, and one or more phosphate groups.
  • Examples: ATP, GTP, dATP, cAMP.
Nucleic Acids
  • Long polymers of nucleotides linked via 3' to 5' phosphodiester bonds.
  • Two main types:
    • DNA (Deoxyribonucleic Acid)
    • RNA (Ribonucleic Acid)

Deoxyribonucleic Acid (DNA)

  • Fundamental molecule that stores genetic information in living organisms.
  • Watson and Crick Model (1953):
    • Explains the double-helix structure of DNA, how genetic information is stored and replicated.
Key Features of DNA Structure
  1. Double Helix Structure: Two strands twisted around each other, forming a right-handed helix.
  2. Backbone Composition: Made of sugar (deoxyribose) and phosphate groups linked by phosphodiester bonds.
  3. Base Pairing:
    • Adenine (A) with Thymine (T) (via two hydrogen bonds).
    • Cytosine (C) with Guanine (G) (via three hydrogen bonds).
  4. Antiparallel Strands: One strand runs 5′ to 3' and the other runs 3′ to 5'.
  5. Major and Minor Grooves: Crucial for protein-DNA interactions.
  6. Replication Mechanism: Semi-conservative; each original strand serves as a template for the formation of a new complementary strand.
Forms of DNA
  • B-DNA: Most common form, right-handed helix found under normal conditions.
  • A-DNA: Compact form observed in dehydrated conditions.
  • Z-DNA: Left-handed helix associated with gene regulation.
DNA Replication
  • Semi-conservative mechanism where each new DNA retains one parental strand and synthesizes one new strand.
  • Enzymes involved:
    • Helicase: Unwinds the DNA strands.
    • DNA Polymerase: Synthesizes the new strand by adding nucleotides.
    • Ligase: Seals gaps between Okazaki fragments.
Importance of DNA
  • Stores genetic instructions for growth and function.
  • Transmits hereditary information across generations.
  • Undergoes replication, transcription, and translation for protein synthesis.

Ribonucleic Acid (RNA)

  • Crucial biomolecule involved in gene expression, protein synthesis, and regulation of cellular processes.
Structure and Function of RNA
  • Generally single-stranded but can form secondary structures (hairpins, loops) influencing its function.
  • Contains ribose, making it more reactive and less stable than DNA due to the 2'-OH group.
  • Versatile roles in gene regulation, enzymatic functions (ribozymes), and cellular signaling.
Types of RNA

  1. Messenger RNA (mRNA):

    • Template for protein synthesis, carrying genetic instructions from DNA to ribosomes.
    • Formed through transcription and processed via splicing in eukaryotes to remove introns.

  2. Ribosomal RNA (rRNA):

    • Structural and catalytic component of ribosomes.
    • Facilitates proper alignment of mRNA and tRNA during translation.

  3. Transfer RNA (tRNA):

    • Delivers specific amino acids to ribosomes during translation.
    • Contains an anticodon region complementary to a corresponding mRNA codon.
Anticodon in tRNA
  • A trinucleotide sequence located at one end of a tRNA molecule, complementary to a corresponding codon in an mRNA sequence.