What the exam asks
- Section I (Multiple Choice): Expect stimulus sets featuring a DNA/mRNA sequence and a codon chart, asking you to determine the resulting polypeptide sequence or predict the consequence of a point mutation. Questions also test diagrams of coupled transcription-translation in bacteria and life cycles of retroviruses.
- Section II (Free Response): Frequently appears in Question 4 (Conceptual Analysis) or Question 5 (Analyze a Model). You may be asked to describe the steps of translation, predict how a change in a nucleotide sequence alters protein structure, or explain how shared genetic mechanisms support common ancestry.
Core ideas
Location and cellular organization
Translation occurs on ribosomes (composed of rRNA and proteins) in all living cells, but cellular compartmentalization creates key differences between domains:
| Feature | Prokaryotes (Bacteria & Archaea) | Eukaryotes |
|---|---|---|
| Ribosome location | Free in the cytosol | Free in the cytosol OR bound to the rough endoplasmic reticulum (RER) |
| Timing relative to transcription | Coupled (simultaneous): Ribosomes bind the end of mRNA and begin translation while RNA polymerase is still transcribing the end. | Separated in space and time: Transcription and RNA processing occur in the nucleus; mature mRNA is exported through nuclear pores to the cytoplasm for translation. |
| Reason for difference | Lack of a nuclear envelope allows ribosomes immediate access to nascent mRNA; no intron splicing is required. | The nuclear envelope physically separates transcription and translation compartments. |
The steps of translation
Ribosomes read the mRNA template in the direction in non-overlapping three-base triplets called codons. Transfer RNA (tRNA) molecules act as adaptors: each carries a specific amino acid at its end and has a complementary three-base that base-pairs antiparallel to the mRNA codon.
- Initiation: The small ribosomal subunit binds to the mRNA and locates the start codon () near the end. An initiator tRNA carrying the amino acid methionine (Met) (with anticodon ) base-pairs with the codon. The large ribosomal subunit then joins, forming a functional ribosome.