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How do cellular processes work?
the expression of a gene to form a functional protein in a eukaryotic cell, including transcription, RNA processing (5' capping, polyadenylation and splicing) and translation, and the role of mRNA, tRNA and ribosomes
A focused answer to the VCE Biology Unit 3 dot point on gene expression. Covers transcription, the three RNA processing steps (5' cap, poly-A tail, splicing), and translation at the ribosome with mRNA and tRNA.
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What this dot point is asking
VCAA wants the eukaryotic pathway from a gene to a functional protein, named in order: transcription, RNA processing (5' cap, poly-A tail, splicing) and translation. You also need the role of mRNA, tRNA and ribosomes.
The answer
Gene expression is the process by which the information in a gene is used to make a functional product (usually a protein). In eukaryotes it occurs in two main locations: transcription and processing in the nucleus, translation in the cytosol.
Transcription (nucleus)
Initiation. Transcription factors and RNA polymerase II assemble at the promoter region of the gene (a TATA box sits upstream of the start site). The DNA double helix unwinds, exposing the template strand.
Elongation. RNA polymerase moves along the template 3' to 5', adding complementary RNA nucleotides to the growing pre-mRNA in the 5' to 3' direction. Pairing follows A with U, T with A, G with C. The newly built pre-mRNA carries the same sequence as the non-template (coding) strand, with U in place of T.
Termination. RNA polymerase reaches a terminator sequence, releases the pre-mRNA, and the DNA re-zips.
RNA processing (nucleus)
Pre-mRNA undergoes three modifications before it can leave the nucleus.
- 5' capping. A 7-methylguanosine cap is added to the 5' end. It protects against exonucleases and is recognised by ribosomes for translation initiation.
- 3' polyadenylation. A poly-A tail of roughly 100 to 250 adenine nucleotides is added to the 3' end. It increases mRNA stability and aids export through the nuclear pore.
- Splicing. The spliceosome removes introns (non-coding regions) and joins the exons (coding regions). Alternative splicing can join different combinations of exons, so one gene can code for several related proteins.
The mature mRNA exits the nucleus through a nuclear pore to the cytosol.
Translation (cytosol or rough ER)
Initiation. The small ribosomal subunit binds the 5' cap and scans for the first AUG start codon. An initiator tRNA carrying methionine binds with anticodon UAC. The large subunit then joins to form a complete ribosome with three sites: A (aminoacyl), P (peptidyl) and E (exit).
Elongation. A charged tRNA enters the A site, and its anticodon pairs with the mRNA codon. The ribosome (rRNA acting as a ribozyme) catalyses a peptide bond between the amino acid in the A site and the polypeptide in the P site. The ribosome translocates one codon along the mRNA; the empty tRNA shifts to the E site and exits.
Termination. When a stop codon (UAA, UAG or UGA) reaches the A site, a release factor binds, the polypeptide is freed, and the ribosome dissociates.
Roles at a glance
| Molecule | Role |
|---|---|
| DNA | Template; carries the gene |
| mRNA | Carries the codon sequence from gene to ribosome |
| tRNA | Reads codons via its anticodon; delivers the matching amino acid |
| rRNA | With ribosomal proteins, forms the ribosome and catalyses peptide bonds |
Worked example
Template DNA: 3' TACGGGATTACG 5'.
Transcription. Pre-mRNA: 5' AUGCCCUAAUGC 3'.
Processing. A 5' cap and 3' poly-A tail are added; introns (none shown here) would be spliced out.
Translation. AUG (Met, start), CCC (Pro), UAA (stop). The ribosome releases a two-amino-acid peptide (Met-Pro). Note that real proteins are much longer; this example just shows codon reading.
Why processing matters
Without the 5' cap, the ribosome cannot recognise the start. Without the poly-A tail, mRNA is rapidly degraded. Without splicing, introns would be translated as nonsense codons or premature stops, producing a non-functional protein.
Common traps
Saying transcription happens at the ribosome. It happens at the gene in the nucleus. Translation happens at the ribosome in the cytosol.
Mixing up codons and anticodons. Codons are on mRNA; anticodons are on tRNA. They are complementary and antiparallel.
Reading the template strand 5' to 3'. RNA polymerase reads the template 3' to 5' and synthesises RNA 5' to 3'.
Forgetting that processing is eukaryote-specific. Prokaryotes do not splice or cap mRNA; transcription and translation are coupled in the cytoplasm.
In one sentence
A eukaryotic gene is expressed when RNA polymerase transcribes its template strand into pre-mRNA, which is then capped, polyadenylated and spliced before being translated at a ribosome where tRNAs match codons to amino acids and build the polypeptide.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE4 marksDescribe the steps involved in producing a mature mRNA molecule from a eukaryotic gene.Show worked answer →
A 4-mark answer needs transcription plus the three processing steps.
- Transcription. RNA polymerase binds the promoter, unwinds the DNA, and synthesises a pre-mRNA strand in the 5' to 3' direction using one DNA strand as template. Free RNA nucleotides pair with the template (A with U, G with C) and are joined by phosphodiester bonds.
- 5' capping. A modified guanine (7-methylguanosine) is added to the 5' end. This protects the mRNA from exonucleases and is recognised by the ribosome.
- Polyadenylation. A tail of about 100 to 250 adenine nucleotides is added to the 3' end. This stabilises the mRNA and aids export.
- Splicing. Introns (non-coding sequences) are removed by the spliceosome, and exons (coding sequences) are joined to form the mature mRNA.
The mature mRNA is then exported through a nuclear pore to the cytosol. Markers reward correct order and naming each step.
2024 VCE3 marksExplain the role of tRNA in translation.Show worked answer →
A 3-mark answer needs structure, anticodon function, and amino acid delivery.
tRNA is a short folded RNA that has an anticodon at one end and an amino acid attachment site at the 3' end. Each tRNA is loaded with a specific amino acid by an aminoacyl-tRNA synthetase.
During translation at the ribosome, the anticodon of an incoming tRNA pairs with a complementary codon on the mRNA. The ribosome then catalyses a peptide bond between the new amino acid and the growing polypeptide. The empty tRNA exits, and the ribosome translocates one codon along the mRNA so the next tRNA can bind.
Markers reward explicit codon-anticodon pairing and the link between the genetic code and the amino acid sequence.
Related dot points
- nucleic acids as information molecules that encode instructions for the synthesis of proteins: the structure of DNA, including nucleotide composition and the role of complementary base pairing, the three main forms of RNA (mRNA, tRNA, rRNA)
A focused answer to the VCE Biology Unit 3 dot point on nucleic acids. Covers nucleotide composition, the antiparallel double helix, complementary base pairing, and how mRNA, tRNA and rRNA differ in structure and role.
- the structure of genes including exons, introns and promoters and the role of regulator genes, including the role of the trp operon as an example of a regulatory process in prokaryotes
A focused answer to the VCE Biology Unit 3 dot point on gene structure and regulation. Covers exons, introns, promoters, regulator genes, and the trp operon as a worked prokaryotic example.
- amino acids as the monomers of a polypeptide chain and the resultant hierarchical levels of structure that give rise to a functional protein
A focused answer to the VCE Biology Unit 3 dot point on protein structure. Covers amino acids, the four levels of protein structure (primary, secondary, tertiary, quaternary) and the link between structure and function.