DNA Structure, Transcription, RNA Processing, and Translation Overview

Structure of DNA

• DNA Composition
  • Made up of nucleotides that consist of a:
  • Phosphate group
  • Sugar (specifically deoxyribose)
  • Nitrogenous base (can be A, T, C, or G)
• Sugar-Phosphate Backbone
  • The nucleotides are linked together by covalent bonds between the sugar of one nucleotide and the phosphate of the next.
  • Formations like sequences of A T G C connected to create a double helix shape.

DNA as a Double Helix

• Structure
  • DNA exists as a double helix with two strands running in opposite directions (5' to 3' and 3' to 5').
  • Base Pairing:
  • A pairs with T (2 hydrogen bonds)
  • C pairs with G (3 hydrogen bonds)

RNA Synthesis (Transcription)

• Process of Transcription
  1. Initiation:
  • RNA polymerase binds to the promoter region of DNA.
  1. Elongation:
  • RNA polymerase adds complementary RNA nucleotides based on the DNA template strand.
  • The DNA strands separate, and RNA synthesis occurs in the 5' to 3' direction.
  1. Termination:
  • RNA synthesis ends when RNA polymerase reaches a terminator sequence on the DNA strand.

RNA Processing

• Post-Transcriptional Modifications:
  • Addition of a 5' cap and poly-A tail at the 3' end to protect mRNA.
  • Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons) to form mature mRNA.

Translation Process

• Steps in Translation:
  1. Initiation:
     • Starts when the small ribosomal subunit binds to mRNA; the initiator tRNA attaches to the start codon (AUG).
  2. Elongation:
     • Codons of mRNA direct the addition of amino acids to the polypeptide chain through tRNAs binding to mRNA.
  3. Peptide Bond Formation:
     • A peptide bond forms between the growing polypeptide chain and new amino acid brought by tRNA.
  4. Translocation:
     • Ribosome moves along the mRNA, shifting the RNA-tRNA complex.
  5. Termination:
     • Occurs when a stop codon on mRNA is reached, ending translation and releasing the completed polypeptide.

Genetic Code

• Codon Table Overview:
  • Sequence of three nucleotides (codons) that correspond to specific amino acids.
  • Examples of codon assignments:
  • UUU -> Phenylalanine (Phe)
  • AUG -> Methionine (Met, Start codon)
  • UAA, UAG, UGA -> Stop codons
• Examples of Mutations:
  • Substitution, deletion, or insertion of nucleotides can lead to changes in the resultant protein, possibly causing diseases like sickle-cell anemia due to a single nucleotide change.