topic D


Key Concepts



  • The “Central Dogma” describes the flow of genetic information: DNA → RNA → Protein

  • It encompasses three major processes: Replication, Transcription, and Translation

  • DNA holds the genetic blueprint; proteins carry out cellular functions






1. DNA Structure & Function

  • DNA is a double helix composed of nucleotides (A, T, C, G)

  • Complementary base pairing: A with T, C with G

  • DNA strands are antiparallel (5′ → 3′ direction vs 3′ → 5′)

  • Genetic code stored in sequences (genes)


2. Replication

  • Purpose: make identical copies of DNA prior to cell division

  • Enzyme: DNA polymerase

  • Steps:

    1. Double helix is unwound (via helicase)

    2. Each original strand acts as a template

    3. Nucleotides added in 5′→3′ direction, complementary to template

    4. Result: two DNA molecules, each with one old and one new strand (semi-conservative)


3. Transcription

  • Purpose: produce messenger RNA (mRNA) from DNA template

  • Enzyme: RNA polymerase

  • Steps:

    1. DNA double helix unwinds at gene region

    2. RNA polymerase binds promoter region

    3. RNA polymerase assembles RNA nucleotides (with U replacing T) complementary to the DNA template

    4. Terminator sequence causes RNA polymerase to detach

    5. In eukaryotes, pre-mRNA is processed (5′ cap, poly-A tail, splicing out introns)


  • Products: mRNA, tRNA, rRNA (other RNAs too)

4. Translation

  • Purpose: convert mRNA sequence into a polypeptide (protein)

  • Occurs in ribosome (rRNA + proteins)

  • Transfer RNA (tRNA) molecules bring amino acids

  • Steps:

    1. Initiation: ribosome binds mRNA at start codon (AUG)

    2. Elongation: codons read (three-base sequences), tRNAs bring matching amino acids

    3. Peptide bonds form between amino acids

    4. Termination: ribosome reaches stop codon (UAA, UAG, UGA), release factors bind, chain released


  • The amino acid sequence folds into a functional protein






5. The Genetic Code



  • Triplet codons: sets of three mRNA nucleotides code for one amino acid

  • Redundant / degenerate code: multiple codons can code for the same amino acid

  • Some codons are start or stop signals






6. Exceptions & Extensions (if covered)



  • Reverse transcription (RNA → DNA) in retroviruses

  • Regulation of gene expression (e.g. promoters, enhancers, transcription factors)

  • Post-translational modifications

  • Non-coding RNAs