CH 10 pt 3 Protein Synthesis – The Genetic Code & Central Dogma

Overview of the Topic

• Focus of this lecture: Protein synthesis (Central Dogma)
  • Steps: DNA (gene) → RNA (transcription) → Protein (translation)
  • Current lecture = understanding the genetic code; later lectures will detail transcription, translation, and mutations.
• Historical note
  • Quote from molecular biologist Sydney Brenner (1960s genetic-code pioneer; Nobel Laureate; proposed Caenorhabditis elegans as a model organism).

Central Dogma Recap

• Information flow in all cells:
  • DNA (gene) → RNA (copy) – occurs in nucleus.
  • RNA exits via nuclear pores → cytoplasm → translated on ribosomes.
• Key definitions
  • Gene: heritable DNA region encoding one specific protein.
  • Transcription: creation of complementary RNA from DNA template.
  • Translation: conversion of RNA nucleotide language to amino-acid language.

Gene & Chromosome Organization

• Chromosomes = long DNA molecules containing many genes.
  • Genes have start & stop sites; intergenic regions may span thousands of nucleotides.
  • Gene orientation can be bidirectional (both DNA strands used).
• Most human DNA does NOT encode proteins.
  • Approx. 99\% is non-coding: regulatory, structural, repetitive, etc. (details in Ch. 11).

Cookbook Analogy

• DNA = Cookbook (all recipes).
• Transcription = copying one recipe to an index card (RNA).
• Translation = cooking in the cytoplasmic “kitchen” using the card; RNA itself is not part of the final casserole (protein).
• DNA never leaves the nucleus → keeps the master cookbook safe.

Base-Pairing Rules (DNA ↔ RNA)

• Memorize for transcription; common student error = mixing A–U vs. A–T.

The Genetic Code

• Triplet (3-base) system: every set of three nucleotides → one amino acid.
• Terminology
  • Base triplet: 3-base sequence on DNA template.
  • Codon: complementary 3-base sequence on mRNA read by ribosome.
• Mathematical basis: 4^3 = 64 possible codons; 20 standard amino acids.

Code Tables

• Two common formats
  1. Circular chart (textbook)
  2. Rectangular chart (older/high-school version)
• Reading circular chart (inside → out):
  • Example \text{GGC} → Glycine.
  • Example \text{UUU} → Phenylalanine.

Key Properties

1. Redundant (degenerate)
   • Multiple codons per amino acid (e.g., Leucine = 6 codons).
2. Unambiguous
   • Each codon specifies only one amino acid (e.g., \text{UAC} is ALWAYS Tyrosine).
3. Universal
   • Same code in bacteria, plants, humans → enables biotech feats (e.g., bacterial production of human insulin).

Start & Stop Signals

• Start codon: \text{AUG} → Methionine → signals ribosome where to begin translation (small + large subunits assemble here).
• Stop codons: \text{UAA},\text{UAG},\text{UGA} → no amino acid inserted; translation terminates.

No Overlap / No Punctuation

• mRNA read sequentially, non-overlapping, continuous: …|codon1|codon2|codon3|…
• Mutations that shift the reading frame ⇒ dramatic effects (frame-shift mutations; discussed later).

Worked Example (From Slide)

DNA template (5’→3’):

TAC TTC AAA ATC

Step 1 – Transcription (base-pair rules) → mRNA (5’→3’):

AUG AAG UUU UAG

Step 2 – Translation (use code table):

Quantitative & Statistical References

• Genetic code math: 4^3 = 64 codons vs. 20 amino acids ⇒ redundancy.
• Evolutionary time frame: common ancestor ≈ 3.6\,\text{billion} years ago.
• Human genome composition: \sim 99\% non-protein-coding DNA.

Practical / Ethical Implications

• Universal code underlies recombinant DNA technology: mass-produce hormones (insulin), vaccines, enzymes.
• Mutation analysis: understanding codon changes → predict amino-acid substitutions vs. silent vs. nonsense mutations.
• Conservation of code opens debate on genetic engineering ethics (e.g., transgenic organisms, gene therapy).

Connections to Prior & Future Content

• Ch. 1: Mentioned “common genetic code” as evidence of shared ancestry.
• Ch. 4: Ribosome structure/function (protein factory).
• Ch. 11: Gene regulation — why most DNA is non-coding yet vital.
• Upcoming lectures: detailed mechanisms of transcription (RNA polymerase, promoters) → translation (tRNA, ribosomal sites) → mutation types (missense, nonsense, frameshift).

Key Vocabulary

• Central dogma, gene, transcription, translation, mRNA, tRNA, rRNA, codon, anticodon, base triplet, start codon (AUG), stop codon (UAA/UAG/UGA), redundancy, degeneracy, unambiguous, universal.

Quick Reference Cheat-Sheet

• Start = \text{AUG} (Met)
• Stops = \text{UAA}, \text{UAG}, \text{UGA}
• Base-pair rules (DNA→RNA): A↔U, T↔A, C↔G, G↔C
• Codon count math: 64 codons ↔ 20 amino acids + 3 stop signals.
• Percentage of non-coding human DNA ≈ 99\%.
• Evolutionary conservation: same code across all life forms.