By the end of this lesson
- Describe transcription.
- Describe translation using codons and tRNA.
- Explain how sequence can affect protein function.
BIG QUESTION
How does a base sequence become a protein with a particular shape and function?
Information flows from DNA sequence to RNA codons to amino-acid sequence, followed by folding and modification.
Transcription produces messenger RNA
The DNA strands separate over a gene. RNA polymerase uses one strand as a template and joins complementary RNA nucleotides to form mRNA.
The transcript is processed in eukaryotic cells and exported from the nucleus.
Translation reads the sequence in triplets
A ribosome reads mRNA in three-base codonA sequence of three bases in mRNA (copied from DNA) that codes for one amino acid or a start or stop signal.. Transfer RNAs with complementary anticodons deliver specific amino acids.
The ribosome joins amino acids by peptide bonds in the order specified by the codons, producing a polypeptide. Start and stop signals define the translated region.
Amino-acid order influences function
The polypeptide folds, and may be modified or combined with other chains, to form a functional protein.
A DNA base change can alter an mRNA codon and amino-acid sequence, but the genetic code is redundant, so some substitutions are silent.
From gene to protein
Location, template and product change at each stage.
| Stage | Main event | Product/location |
|---|---|---|
| Transcription | RNA made from DNA template. | mRNA in nucleus. |
| mRNA export | Transcript passes through nuclear pore. | mRNA in cytoplasm. |
| Translation initiation | Ribosome begins at start codon. | Reading frame established. |
| Elongation | tRNAs deliver amino acids. | Polypeptide grows. |
| Folding/modification | Chain adopts functional structure. | Working protein. |
Language that earns precision
- codon
- Three-base mRNA sequence specifying an amino acid or start/stop signal.
- anticodon
- Three-base tRNA sequence complementary to an mRNA codon.
- translation
- Synthesis of a polypeptide using the codon sequence of mRNA.
- polypeptide
- Chain of amino acids joined by peptide bonds.
Common misconceptions
Claim: A codon is found on tRNA.
Correction: Codons are on mRNA; complementary anticodons are on tRNA.
Claim: Ribosomes make amino acids.
Correction: They join supplied amino acids into a polypeptide.
Claim: Every DNA mutation changes a protein.
Correction: Some lie outside coding regions or produce synonymous codons.
Match the answer to the command word
DNA → mRNA → ribosome → tRNA/amino acids → polypeptide.
Base change → codon change → possible amino-acid/folding/function change.
Recall, apply, then justify
Describe translation.
Hint
Ribosome, codon, tRNA, amino acid and bond.
Suggested marking points
- mRNA binds to a ribosome.
- The ribosome reads mRNA codons.
- tRNAs with complementary anticodons bind.
- Each tRNA delivers a specific amino acid.
- Amino acids are joined into a polypeptide in the coded order.
Explain how a gene can determine a protein’s function.
Hint
Base sequence through folding.
Suggested marking points
- The gene’s base sequence determines the mRNA codon sequence.
- Codons determine the amino-acid order.
- Amino-acid order affects folding and three-dimensional shape.
- Protein shape determines interactions and function.
Explain why a single-base substitution may have no effect, a small effect or a major effect on phenotype.
Planning hint
Code redundancy, amino-acid chemistry and protein position.
What a strong design includes
- The altered codon may specify the same amino acid because the code is redundant.
- It may specify a chemically similar amino acid with little structural effect.
- It may alter a critical active-site or structural amino acid.
- A stop codon could truncate the protein.
- Protein folding or stability may change.
- Phenotypic effect also depends on gene expression, allele context and environmental conditions.