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Biology · 9. Molecular genetics and gene regulation

Central dogma: Transcription, translation and gene regulation

Explore the flow of genetic information DNA → RNA → Protein and genetic switches via the classic Lac operon model.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How does a linear sequence of only four nucleotides in DNA accurately direct the synthesis of thousands of distinct functional proteins?

Curriculum coverage

Vietnam 2018 Biology 12 (Molecular genetics); Cambridge A-Level Biology 9700 (Gene expression); AP Biology Unit 6.

Model scope and limitations

Covers prokaryotic and basic eukaryotic central dogma; omits complex mammalian epigenetic chromatin remodelling.

Learning objectives

  • Describe mRNA transcription stages from the 3′ → 5′ template strand following base pairing.
  • Explain ribosome translation: codon-anticodon recognition, peptide bond synthesis and termination.
  • Analyse negative regulation of the E. coli Lac operon in the presence and absence of lactose.

Prerequisite knowledge

  • Chemical structure of nucleotides and phosphodiester/hydrogen bonding.
  • Triplet genetic code principles.

Core knowledge

Transcription fidelity

RNA polymerase binds the promoter, unwinds DNA, and synthesises mRNA in the 5′ to 3′ direction using base complementarity (A-U, T-A, G-C, C-G).

Translation and genetic code

64 triplet codons include 1 start codon (AUG, coding Met/fMet) and 3 stop codons (UAA, UAG, UGA) that terminate peptide elongation.

The Lac operon

Structural genes lacZ-lacY-lacA share a promoter and operator. Without lactose, the repressor blocks operator; with lactose, allolactose derepresses the operon.

Worked example

A DNA template reads 3′-TAC GGC TTA ACT-5′. Transcription yields mRNA: 5′-AUG CCG AAU UGA-3′. Translation produces a tripeptide: Met – Pro – Asn (UGA is a stop codon terminating synthesis).

Misconceptions to avoid

  • The transcribed template strand is 3′ to 5′; mRNA elongation is strictly 5′ to 3′.
  • The regulatory gene (lacI) is not part of the operon itself; it possesses its own promoter and is transcribed constitutively.

Virtual experiment procedure: Transcription, translation and the Lac operon

  1. Start the simulation, entering the initial DNA template sequence.
  2. Observe RNA polymerase translocation, ribonucleotide pairing and mRNA release.
  3. Dock mRNA into the ribosome, observing aminoacyl-tRNA recruitment, peptidyl transfer and stop codon release.

Safety and cautions

  • In wet labs, Ethidium Bromide nucleic acid stain is mutagenic; always wear nitrile gloves and handle under safety hoods.

Evidence to collect

  • Comparative table matching original DNA template, transcribed mRNA, and translated amino acids.
  • State diagram of the Lac operon (repressed vs induced) under varying glucose and lactose levels.
Open the virtual experiment
📋 Embed this simulation in online lectures (For Teachers & LMS)

Copy the iframe code below to embed this interactive simulation into Canvas, Moodle, Google Sites or educational blogs:

<iframe src="https://visuallab.win/bio-lab/?experiment=transcriptionTranslation" width="100%" height="560" frameborder="0" allow="fullscreen" loading="lazy"></iframe>

Diagnostic check

1. Which codon acts as a translation stop signal?

  1. UGA
  2. AUG
  3. UGG
Show answer and explanation

Correct answer: UGA

The three canonical stop codons are UAA, UAG, and UGA.

2. What substance acts as the inducer inactivating the repressor protein in the Lac operon?

  1. Allolactose (lactose isomer)
  2. Glucose
  3. Galactose
Show answer and explanation

Correct answer: Allolactose (lactose isomer)

Allolactose binds allosterically to the repressor, inducing conformational release from the operator.

Application practice

1. An mRNA contains 300 codons including start and stop. How many amino acids are in the released polypeptide?

  1. 299 amino acids
  2. 300 amino acids
  3. 100 amino acids
Show answer and explanation

Correct answer: 299 amino acids

The stop codon specifies no amino acid; the nascent chain contains 300 − 1 = 299 residues.

2. A point mutation changing a sense codon into a premature stop codon is known as what?

  1. Nonsense mutation
  2. Missense mutation
  3. Silent mutation
Show answer and explanation

Correct answer: Nonsense mutation

A nonsense mutation introduces a premature stop codon, causing truncated non-functional proteins.

3. If the Lac operator mutates such that the repressor cannot bind, what occurs?

  1. Structural genes are transcribed constitutively even without lactose
  2. The operon is permanently locked shut
  3. The bacterial cell dies immediately
Show answer and explanation

Correct answer: Structural genes are transcribed constitutively even without lactose

Without repressor binding, RNA polymerase transcribes unobstructed, resulting in constitutive expression.

Academic references

  1. Cambridge IGCSE Biology 0610 — official syllabus and resources
  2. Cambridge AS & A Level Biology 9700 — official syllabus
  3. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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