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6.4 Translation

Syllabus
2025
Topic
6.4
Level
—

Learning objectives

6.4A—Explain how the phenotype of an organism is determined by its genotypeExplain how the phenotype of an organism is determined by its genotype.• T ranslation of the mRNA to generate a polypeptide occurs on ribosomes that are present in the cytoplasm of both prokaryotic and eukaryotic cells, as well as the cytoplasmic surface of the rough ER of eukaryotic cells.• In prokaryotic organisms, translation of the mRNA molecule occurs while it is being transcribed.• T ranslation involves many sequential steps, including initiation, elongation, and termination. The salient features of translation include:- i. T ranslation is initiated when the rRNA in the ribosome int eracts with the mRNA at the start codon (AUG, coding for the amino acid methionine).- ii. The sequence of nucleotides on the mRNA is read in triplets, c alled codons.- iii. Each codon encodes a specific amino acid, which can be deduc ed by using a genetic code chart. Many amino acids are encoded by more than one codon.- iv. Nearly all living organisms use the same genetic code , which is evidence for the common ancestry of all living organisms.- Exclusion: The details and names of the enzymes and factors involved in each of these steps are beyond the scope of the AP Exam. Memorization of the genetic code, with the exception of the start codon AUG, is beyond the scope of the AP Exam.• Genetic information in retroviruses is a special case and has an alternate flow of information: from RNA to DNA, made possible by reverse transcriptase, an enzyme that copies the viral RNA genome into DNA. This DNA integrates into the host genome and is transcribed and translated for the assembly of new viral progeny.

How Translation Connects Genotype to Phenotype

A genotype specifies a DNA base sequence. After transcription, the corresponding mRNA codons specify an amino-acid sequence during translation. That sequence helps determine the structure and function of the resulting protein, and protein activity contributes to phenotype.

Genotype (DNA sequence) → mRNA codon sequence → polypeptide amino-acid sequence → protein structure and function → contribution to phenotype. A sequence change can alter a codon and may therefore alter the protein and its phenotypic effect.

Stage What happens
Initiation Ribosomal rRNA interacts with mRNA at AUG, the start codon for methionine
Elongation The ribosome reads mRNA in three-base codons; each tRNA anticodon pairs with the specified codon and delivers the corresponding amino acid to the growing polypeptide
Termination Translation reaches a stop codon, and the newly synthesized protein is released

Ribosomes translate mRNA in the cytoplasm of prokaryotic and eukaryotic cells and on the cytoplasmic surface of rough ER in eukaryotes. In prokaryotes, translation can begin while the same mRNA is still being transcribed. Many amino acids have more than one codon, and the genetic code is nearly universal—evidence of common ancestry.

Retroviruses are a special case: reverse transcriptase copies the viral RNA genome into DNA. That DNA integrates into the host genome and can then be transcribed and translated to support production of new viral progeny.

A codon is a three-base sequence on mRNA, whereas an anticodon is on tRNA. The AP Exam does not require memorizing the genetic code except for AUG, or the names and details of additional translation enzymes and factors.