In-Depth Notes on DNA Structure, Function, and Related Processes

DNA Composition

• Made of nucleotides, which consist of:
  • Phosphate group
  • 5-carbon deoxyribose sugar
  • Nitrogenous bases (4 types):
  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

DNA Replication and Repair

• Semi-Conservative Replication: Each new DNA molecule comprises one old strand and one newly formed strand.
• Process of Replication:
  • Unwinding: Helicase breaks hydrogen bonds between nitrogen bases.
  • Stabilization: Binding proteins maintain separation of the strands, forming a replication fork (Y-shaped).
  • Primer Addition: Primase synthesizes RNA primer on the parent strand.
  • Nucleotide Addition: DNA polymerase adds nucleotides to the primer, elongating the new strand.
  • Origins of Replication: Sites where replication initiates and forms replication bubbles that merge to create two new DNA strands.
  • DNA Repair: If an alteration occurs in the DNA sequence, damaged strands are excised, and DNA polymerase repairs by synthesizing new, complementary strands.

Central Dogma of Molecular Biology

• Flow of Genetic Information:
  • DNA -> mRNA (Transcription)
  • mRNA -> Protein (Translation)
• Each set of three bases on mRNA (codon) codes for one amino acid, totaling 64 possible codons.

Transcription Process

• Initiation:
  • Starts at a promoter sequence.
  • Proteins recruit RNA polymerase to bind at the promoter.
  • DNA unwinds to expose base sequences.
• Elongation:
  • RNA polymerase moves along the DNA, synthesizing the mRNA strand.
• Termination:
  • Process ends when RNA polymerase reaches a terminator sequence (ATT, ATC, ACT).

Types of RNA

• mRNA: Carries genetic information from DNA to ribosomes; undergoes modification (adding cap, tail, splicing).
• tRNA: Transfers amino acids to ribosomes for protein synthesis; contains anticodon for codon recognition on mRNA.
• rRNA: Forms the structure of ribosomes and facilitates the assembly of amino acids into proteins.

Translation Process

• Initiation:
  • mRNA binds to the smaller ribosomal subunit.
  • An amino acid-carrying tRNA recognizes the first codon on the mRNA.
  • The larger ribosomal subunit joins, positioning tRNA in the P site.
• Elongation:
  • Codon recognition and amino acid activation.
  • Formation of a peptide bond between amino acids; the ribosome shifts to make room for a new tRNA in the A site.
• Termination:
  • Occurs when a stop codon (UAA, UGA, UAG) is reached.
  • Release factors bind to the ribosome, leading to the disassembly of the components and the release of the new polypeptide.

Mutations

• Types of Mutations:
  • Base Substitutions: Replacement of one DNA base with another.
  • Silent Mutation: No change in protein.
  • Missense Mutation: Alters the protein's amino acid sequence.
  • Nonsense Mutation: Leads to premature termination of protein synthesis.
  • Insertions and Deletions: Cause shifts in the reading frame (frameshift mutations) leading to significant changes in the resulting protein.

Viral Infections

• Viruses Classification by Host:
  • Bacteriophages: Infect bacteria.
  • Plant Viruses: Infect plants.
  • Animal Viruses: Infect animals (e.g., RNA viruses like HIV).
• HIV: A retrovirus causing AIDS, targets T cells, with transmission routes including sexual contact and vertical transmission. No vaccine available.

Visual Aid:

An illustration representing the structure of DNA, highlighting the double helix formation, nucleotides, and nitrogenous bases involved in its structure.