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.