Gene Expression: Transcription to Translation

Overview

• Lecture explains how DNA codes for organisms via transcription and translation.
• Focus on how genes produce proteins through mRNA and ribosomes.
• Describes key molecules, steps, and terminology in gene expression.

Genes and Chromosomes

• Chromosome: very long DNA molecule with millions of base pairs.
• Gene: specific DNA segment that codes for a protein.
• Human gene length: typically 10,000–50,000 base pairs; some up to 2.5 million.
• Gene expression results in production of a specific protein.

Transcription (DNA → mRNA)

• Definition: enzymes use one DNA strand as template to synthesize messenger RNA.
• Main enzyme: RNA polymerase, aided by transcription factors.
• Promoter: specific DNA sequence where RNA polymerase binds and pries strands apart.
• Template (antisense) strand: read 3' → 5' to make mRNA.
• Nontemplate (sense) strand: not used as template during transcription.
• Initiation: RNA polymerase begins RNA synthesis at start site; does not require a primer.
• Elongation: RNA polymerase moves downstream, adding RNA nucleotides to 3' end of mRNA.
• Differences from DNA replication:
  • Produces RNA (ribose sugar, uracil instead of thymine).
  • RNA polymerase leaves DNA mostly zipped; ~10–20 bases exposed.
• Termination: RNA polymerase detaches at gene end; DNA reforms double helix.
• RNA processing: modifications to pre-mRNA occur before export to cytoplasm.
• mRNA exits nucleus to find ribosome for translation.

Translation (mRNA → Protein)

• Occurs in the cytoplasm at ribosomes.
• Codon: set of three mRNA bases that specifies an amino acid.
• Reading frame: grouping of codons along mRNA that determines amino acid sequence.
• Number of codons: 4^3 = 64 possible codons.
• Genetic code properties:
  • Redundant (multiple codons can code for same amino acid).
  • Unambiguous (each codon corresponds to one amino acid).
  • Special codons: AUG = start (codes for methionine); three stop codons terminate translation.
• tRNA: carries specific anticodon and is covalently linked to a particular amino acid.
• Ribosome assembly:
  • Small ribosomal subunit binds mRNA and initiator tRNA at start codon.
  • Large subunit joins to form complete initiation complex.
• Elongation cycle:
  • tRNA matching next codon enters ribosome carrying its amino acid.
  • Peptide bond forms between incoming amino acid and growing chain.
  • Initiator tRNA detaches after bond formation; ribosome translocates.
  • Cycle repeats, lengthening polypeptide.
• Termination: ribosome reaches stop codon; polypeptide released.
• Post-translation: polypeptide often enters organelles for folding and modification.

Key Terms and Definitions

• Chromosome: long DNA molecule made of base pairs.
• Gene: DNA region that codes for a protein.
• RNA polymerase: enzyme that synthesizes RNA from DNA template.
• Transcription factors: proteins that help RNA polymerase bind promoter.
• Promoter: DNA sequence where transcription begins.
• Template (antisense) strand: DNA strand read to make mRNA.
• Sense (nontemplate) strand: DNA strand not read for mRNA synthesis.
• mRNA (messenger RNA): RNA copy of gene used for translation.
• Codon: three-base mRNA sequence specifying an amino acid.
• tRNA (transfer RNA): adaptor molecule with anticodon and attached amino acid.
• Anticodon: three-base sequence on tRNA complementary to codon.
• Ribosome: molecular machine that translates mRNA into protein.
• Start codon: AUG, initiates translation and codes for methionine.
• Stop codon: any of three codons that terminate translation.
• Reading frame: the correct sequential grouping of codons on mRNA.

Summary Table: Key Steps, Molecules, and Directions

Connection to Organismal Traits

• Every gene codes for a protein; proteins determine structure and function of cells.
• Proteins include enzymes, receptors, structural components, and more.
• Differential gene expression leads to diverse cell types and organismal form.

Action Items / Next Steps (If Studying)

• Review codon table to memorize start/stop codons and redundancy patterns.
• Practice tracing transcription and translation directions (3' vs 5').
• Study tRNA structure and anticodon-codon pairing rules.
• Work through sample mRNA sequences to translate into amino acid chains.