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 |
Examples in context
Example 1. Insulin production in a pancreatic beta cell. A human pancreatic beta cell continuously transcribes the INS gene on chromosome 11 in response to rising blood glucose. RNA polymerase II produces a pre-mRNA that is spliced (two introns removed, three exons joined), 5'-capped, polyadenylated, and exported to the cytoplasm. Ribosomes on the rough ER then translate the mature mRNA into preproinsulin, a 110-amino-acid polypeptide. Further processing in the Golgi cleaves it into the A and B chains of mature insulin, linked by disulfide bonds. A single beta cell can secrete roughly one million insulin molecules per minute after a meal, all dependent on the transcription-translation pathway running without errors.
Example 2. mRNA vaccines for COVID-19. The Pfizer-BioNTech and Moderna COVID-19 vaccines used in Australia from 2021 contain synthetic mRNA encoding the SARS-CoV-2 spike protein. After injection, the mRNA enters muscle and dendritic cells (no transcription required because the mRNA is supplied directly), and ribosomes translate it into spike protein. The host immune system then recognises the spike as foreign and develops antibodies. The vaccine cleverly skips transcription and exploits translation alone, which is why the mRNA must be lipid-nanoparticle protected to enter cells and contain modified uridines so it is not destroyed by host RNases before it can be translated.
Try this
Q1. A DNA template strand has the sequence 3'-TACGCATGAATT-5'. Determine (a) the mRNA sequence produced by transcription, and (b) the amino acid sequence produced by translation, given the codon table: AUG = Met, CGU = Arg, ACU = Thr, UAA = stop. [2+2 marks]
- Cue. (a) mRNA: 5'-AUGCGUACUUAA-3'. (b) Read codons left to right: Met-Arg-Thr-stop, so the peptide is Met-Arg-Thr.
Q2. A point mutation changes a single mRNA codon from CGU to UGU. Using the genetic code, predict the type of mutation and the effect on the resulting polypeptide. [3 marks]
- Cue. CGU codes for Arg; UGU codes for Cys; this is a missense (non-synonymous) mutation that swaps one amino acid for another and may alter protein folding.
Q3. Compare the locations and roles of mRNA, tRNA and rRNA in a eukaryotic cell. (a) State where each is made. (b) State the function of each in protein synthesis. (c) Explain why all three are required for translation. [2+3+2 marks]
- Cue. (a) mRNA in nucleus, tRNA in nucleus, rRNA in nucleolus. (b) mRNA carries the code, tRNA brings amino acids, rRNA forms the ribosome. (c) Translation requires the code (mRNA), the building blocks delivered (tRNA), and a catalytic platform (rRNA).
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2024 HSC3 marksCompare Process A with DNA replication. [Process A is transcription, occurring in the nucleus during polypeptide synthesis.]Show worked answer →
3 marks for a sound comparison of transcription and DNA replication; 2 for some understanding of both processes; 1 for some relevant information. A genuine comparison must address both a similarity and a difference of the same feature.
Sample answer (marking guidelines): Both Process A (transcription) and DNA replication involve unwinding of the DNA double strand to produce a complementary strand. In replication, DNA is copied to produce two identical strands of DNA, whereas in transcription, DNA is copied to produce a single strand of mRNA.
Markers noted students often failed to compare the same feature for both similarity and difference.
2024 HSC5 marksExplain the importance of mRNA and tRNA in polypeptide synthesis.Show worked answer →
Marks build from 1 (some relevant information) to 5 (a thorough explanation of the importance of mRNA AND tRNA), with 2 marks for showing understanding of just mRNA or tRNA.
Sample answer (marking guidelines): mRNA is synthesised from the DNA template in the nucleus during transcription. This mRNA carries the genetic instructions (codons) from the DNA in the nucleus to the ribosomes in the cytoplasm. Once the mRNA reaches the ribosome, translation begins. Each tRNA molecule has an anticodon that pairs with a complementary codon on the mRNA. The mRNA codons are read in sequence, and as each tRNA brings its specific amino acid to the ribosome, the polypeptide is assembled.
To earn top marks use the correct terminology (codon/anticodon) and clearly link mRNA, tRNA and the building of the polypeptide.
2021 HSC3 marksDescribe the role of mRNA in human cells.Show worked answer →
3 marks for a thorough description of the role of mRNA; 2 for describing a feature; 1 for some relevant information.
Sample answer (marking guidelines): mRNA carries a complementary copy of a section of DNA that codes for a polypeptide to the ribosomes. At the ribosomes, mRNA provides a template. Each codon (three nucleotides) on the mRNA results in the addition of the correct amino acid to form a polypeptide chain.
Markers advised distinguishing protein synthesis from DNA replication and using terms such as codon and complementary.
Related dot points
- Model the processes involved in cell replication, including but not limited to: mitosis and meiosis, DNA replication using the Watson and Crick DNA model, including nucleotide composition, pairing and bonding
A focused answer to the HSC Biology Module 5 dot point on DNA replication. The semi-conservative model, the enzymes involved (helicase, primase, DNA polymerase, ligase), the leading and lagging strands, and the standard worked exam example.
- Investigate the inheritance of patterns including but not limited to: predicting genotypic and phenotypic ratios using Punnett squares and probability rules
A focused answer to the HSC Biology Module 5 dot point on Mendelian inheritance. Mendel's laws, dominant vs recessive alleles, Punnett squares step by step, monohybrid and dihybrid crosses, the standard 3:1 and 9:3:3:1 ratios, and worked HSC past exam questions.