Inquiry Question 3: How does genetic information flow from DNA to functional proteins?
Construct appropriate representations to model and compare the processes of transcription and translation, including but not limited to: the roles of mRNA, tRNA, rRNA and ribosomes in polypeptide synthesis
A focused answer to the HSC Biology Module 5 dot point on protein synthesis. Transcription in the nucleus (DNA to mRNA), translation at the ribosome (mRNA to polypeptide), the roles of mRNA, tRNA, rRNA, the codon-anticodon match, and the standard worked exam example.
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What this dot point is asking
NESA wants you to model both transcription and translation, naming the roles of all three RNAs (mRNA, tRNA, rRNA) and the ribosome. This is the standard "central dogma" question and appears almost every year in some form.
The answer
Protein synthesis happens in two stages: transcription (in the nucleus) and translation (at the ribosome in the cytoplasm).
Transcription
The DNA gene is copied into a complementary messenger RNA (mRNA) molecule.
- Initiation. RNA polymerase binds to the gene's promoter region and unwinds the DNA double helix.
- Elongation. RNA polymerase reads the template strand in the 3' to 5' direction and synthesises mRNA in the 5' to 3' direction. Base pairing rules: A pairs with U (in RNA), T pairs with A, G pairs with C.
- Processing. In eukaryotes, the pre-mRNA is processed: introns (non-coding) are spliced out, exons are joined. A 5' cap and poly-A tail are added for stability.
- Export. The mature mRNA exits the nucleus through a nuclear pore.
Translation
The mRNA sequence is decoded to build a polypeptide.
- Initiation. The mRNA binds to a ribosome (made of rRNA plus protein). The ribosome positions itself at the start codon AUG.
- Elongation. The ribosome reads the mRNA in codons (3-nucleotide units). Each codon specifies one of 20 amino acids. tRNA molecules each carry a specific amino acid and have an anticodon that base-pairs with the mRNA codon. The ribosome catalyses peptide bond formation between adjacent amino acids.
- Termination. When the ribosome reaches a stop codon (UAA, UAG, UGA), the polypeptide is released.
The three RNAs at a glance
| RNA | Made from | Role |
|---|---|---|
| mRNA | Transcribed from DNA | Carries the genetic code from the nucleus to the ribosome |
| tRNA | Made in the nucleus | Brings the correct amino acid to the ribosome based on codon-anticodon pairing |
| rRNA | Made in the nucleolus | Combines with protein to form the ribosome itself |
Worked example
A short DNA template strand reads: 3' TACGGCTAA 5'.
Transcription. The complementary mRNA is 5' AUGCCGAUU 3'.
Translation. Reading the mRNA in codons: AUG-CCG-AUU.
- AUG = Methionine (start)
- CCG = Proline
- AUU = Isoleucine
The polypeptide produced is Met-Pro-Ile.
Common traps
Confusing the strands. RNA polymerase reads the template (antisense) strand, not the coding (sense) strand. The mRNA sequence matches the coding strand (with U replacing T).
Forgetting that A pairs with U in RNA. In DNA, A pairs with T. In RNA, A pairs with U.
Mixing up codons and anticodons. The codon is on the mRNA. The anticodon is on the tRNA. They are complementary.
Skipping the start and stop codons. AUG is always the start. UAA, UAG, UGA are always stops. Including them shows you understand initiation and termination.
In one sentence
Transcription copies DNA into mRNA inside the nucleus using RNA polymerase, then translation at the ribosome (made of rRNA) reads mRNA in 3-base codons matched by tRNA anticodons to build a polypeptide one amino acid at a time.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2022 HSC5 marksOutline the processes of transcription and translation, including the role of mRNA, tRNA and ribosomes.Show worked answer →
A 5-mark answer needs both processes outlined with all three named RNAs.
Transcription (in the nucleus).
- RNA polymerase binds to a promoter region on the DNA and unwinds the double helix at the gene to be expressed.
- RNA polymerase reads the template (antisense) strand in the 3' to 5' direction and synthesises a complementary mRNA strand in the 5' to 3' direction, using A-U, T-A, G-C, C-G base pairing.
- The pre-mRNA is processed (introns spliced out, exons retained, 5' cap and poly-A tail added) to produce mature mRNA, which exits the nucleus through a nuclear pore.
Translation (in the cytoplasm at the ribosome).
- The mature mRNA binds to a ribosome (made of rRNA and protein). The ribosome reads the mRNA in 3-nucleotide units called codons, starting at the AUG start codon.
- tRNA molecules, each carrying a specific amino acid, have an anticodon that pairs with the matching mRNA codon. The ribosome catalyses the formation of peptide bonds between successive amino acids.
- Translation terminates at a stop codon (UAA, UAG, or UGA), releasing the completed polypeptide.
Markers reward naming the three RNAs (mRNA, tRNA, rRNA), the codon-anticodon pairing, and the AUG start.
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