Lesson 12.3_ Gene Expression_ RNA and the Genetic Code

Lesson Overview

  • Topic: Gene Expression: RNA and the Genetic Code

Learning Outcomes

  • Explain the role of mRNA nucleotides in determining sequences of amino acids in polypeptides.

  • Describe the functions of transcription and translation.

Historical Context

  • Inborn Errors of Metabolism: Sir Archibald Garrod first described inherited metabolic disorders.

  • One Gene, One Enzyme Hypothesis: Proposed by George Beadle and Edward Tatum using red bread mold, linking defective genes to defective enzymes.

Gene Expression Process

  • Definition: Information flow from DNA to RNA to proteins, termed gene expression.

  • Importance of RNA: Essential in carrying genetic information.

RNA Structure

  • Composition: RNA is a polymer of nucleotides containing ribose sugar and bases A, C, G, U (uracil replaces thymine found in DNA).

  • Single-Stranded: Unlike DNA, RNA does not form a double helix.

Comparison: RNA vs. DNA

Feature

RNA

DNA

Sugar

Ribose

Deoxyribose

Bases

A, G, U, C

A, G, T, C

Strands

Single-stranded

Double-stranded

Helix

No

Yes

Classes of RNA

  1. Messenger RNA (mRNA): Carries genetic messages from DNA to ribosomes.

  2. Transfer RNA (tRNA): Transfers specific amino acids to ribosomes during translation.

  3. Ribosomal RNA (rRNA): Combines with proteins to form ribosomes where polypeptides are synthesized.

Steps in Protein Synthesis

  1. Transcription: DNA is transcribed to form RNA (mRNA, tRNA, rRNA).

  2. Translation: mRNA is read by ribosomes to synthesize proteins; codons are decoded into amino acids.

Central Dogma of Molecular Biology

  • Flow of information: DNA → RNA → Protein.

Genetic Code

  • Triplet Codons: Genetic code comprises sets of three nucleotides (codons) corresponding to amino acids.

  • Experimental Confirmation: Marshall Nirenberg's experiments confirmed RNA codon assignments using synthetic RNA.

Properties of the Genetic Code

  • Degenerate: Multiple codons represent the same amino acid, protecting against mutations (except for Met and Trp).

  • Unambiguous: Each codon has a specific meaning.

  • Start/Stop Signals: One start codon (AUG) and three stop codons.

Practical Application

  1. Transcribe DNA to mRNA.

  2. Translate mRNA to Amino Acids.

Exercises

  • Practice transcription and translation with provided DNA strands to reinforce understanding.