Skills you’ll gain in this topic:
- Describe translation and the roles of mRNA, tRNA, and ribosomes.
- Explain how the genetic code specifies amino acid sequences.
- Predict how mRNA sequence mutations affect protein structure.
- Relate translation stages to protein synthesis efficiency.
- Analyze translation errors’ effects on cellular function.
RNA to Polypeptide Chain
Translation is the process by which the genetic information encoded in mRNA is used to synthesize a polypeptide, which is a chain of amino acids that form a protein. This process occurs on ribosomes, which are complex molecular machines made up of proteins and RNA. 🎰
Ribosomes are present in the cytoplasm of BOTH prokaryotic and eukaryotic cells. In prokaryotic cells, ribosomes are found floating freely in the cytoplasm. In eukaryotic cells, translation occurs on ribosomes in BOTH the cytoplasm AND the rough endoplasmic reticulum (RER). Free ribosomes in the cytoplasm produce proteins that function within the cell, while ribosomes on the RER synthesize proteins destined for secretion or membrane insertion.
During translation, the mRNA is first bound to the ribosome and then read in groups of three nucleotides, called codons. Each codon specifies a specific amino acid – the start codon, AUG, codes for methionine and initiates the process. The ribosome adds the corresponding amino acid to the growing polypeptide chain. This process continues until a stop codon is reached, at which point the polypeptide is complete. Translation terminates with the release of the newly synthesized protein.
It's worth noting that in eukaryotic cells, the process of protein synthesis begins in the nucleus, where the mRNA is transcribed from DNA. The mRNA then moves to the cytoplasm, where it is translated into a protein. This is in contrast to prokaryotic cells, where transcription and translation occur in the same location, the cytoplasm.
Additionally, the relationship between genotype and phenotype is emphasized, as genetic expression through translation determines an organism’s traits.
In Prokaryotes
In prokaryotic organisms, the process of transcription and translation occurs simultaneously. This is in contrast to eukaryotic cells, where transcription occurs in the nucleus, and the mRNA must be transported to the cytoplasm for translation. 🚲
During transcription in prokaryotes, the DNA double helix is unwound and one strand of the DNA, known as the template strand, is used as a template to synthesize a complementary RNA molecule. As the RNA polymerase moves along the template strand, it adds nucleotides to the growing RNA molecule. At the same time, ribosomes bind to the mRNA and begin translation.
This simultaneous transcription and translation allow prokaryotes to quickly respond to changes in their environment and produce the necessary proteins in a timely manner. This also allows prokaryotes to produce multiple copies of the same protein at once, increasing efficiency and allowing for rapid growth and reproduction.
The process of simultaneous transcription and translation in prokaryotes is known as co-transcriptional translation. This process is possible because the mRNA is synthesized in a continuous fashion, and the ribosomes bind to the mRNA as soon as it is synthesized, thus there is no need for the mRNA to be transported to the cytoplasm.
Translation is a complex process that involves multiple sequential steps and requires energy in order to be completed. The three main (sequential) steps of translation are initiation, elongation, and termination.
Initiation
Initiation occurs when the rRNA in the ribosome interacts with the mRNA at the start codon.
Initiation is the first step in translation and involves the binding of a specific initiator tRNA and the small and large ribosomal subunit to the mRNA. This forms the initiation complex, which recognizes the start codon AUG on the mRNA. The start codon AUG codes for the amino acid methionine, which is the first amino acid in nearly all proteins. The initiator tRNA carrying methionine is positioned at the P site (peptidyl site) on the ribosome.
Elongation
tRNA brings the amino acid as specified by the mRNA codons.
Elongation is the second step in translation, and it involves the addition of amino acids to the growing polypeptide chain. This is done by the transfer RNAs (tRNAs) that carry specific amino acids. The tRNA in the A site (aminoacyl site) brings in the next amino acid, and the ribosome catalyzes the formation of a peptide bond between the amino acid in the P site and the incoming amino acid in the A site. The ribosome then moves along the mRNA, bringing the next codon in the A site, and the process repeats.
Termination
Termination is the final step in translation and it involves the release of the polypeptide from the ribosome. This is done by the recognition of one of the three stop codons (UAG, UGA, or UAA) by a release factor. The release factor binds to the stop codon and causes the polypeptide to be released from the ribosome. The ribosome, mRNA, and tRNAs are then free to be recycled for another round of translation. The energy required for translation comes from the hydrolysis of adenosine triphosphate (ATP) and guanosine triphosphate (GTP). The energy from these molecules is used to drive the conformational changes in the ribosome and tRNAs that are necessary for peptide bond formation and for the movement of the ribosome along the mRNA. 🏆
Features of Translation
To be more specific, translation is a fundamental process that converts the genetic information encoded in mRNA into a functional protein. The salient features of translation include:
a. Translation is initiated when the rRNA in the ribosome interacts with the mRNA at the start codon. The start codon is usually AUG and the binding of the ribosome to the start codon initiates the translation process.
b. The sequence of nucleotides on the mRNA is read in triplets called codons. Each codon is a sequence of three nucleotides that specifies a specific amino acid.
c. Each codon encodes a specific amino acid, which can be deduced by using a genetic code chart. The genetic code is the set of rules by which the sequence of nucleotides in DNA and RNA is translated into the sequence of amino acids in proteins. The genetic code is nearly universal, meaning that most amino acids are encoded by more than one codon.
d. Nearly all living organisms use the same genetic code, which is evidence for the common ancestry of all living organisms. This suggests that the genetic code is a product of evolution and has been conserved across different species.
e. tRNA brings the correct amino acid to the correct place specified by the codon on the mRNA. The tRNA is a small RNA molecule that carries an amino acid at one end and a specific sequence at the other end called the anticodon. The anticodon is complementary to the codon on the mRNA and this allows the correct tRNA to bind to the correct codon.
f. The amino acid is transferred to the growing polypeptide chain. The ribosome catalyzes the formation of a peptide bond between the amino acid in the P site and the incoming amino acid in the A site.
g. The process continues along the mRNA until a stop codon is reached. The stop codon is a sequence of three nucleotides that signals the end of translation.
h. The process terminates by release of the newly synthesized polypeptide/protein. The release of the protein from the ribosome is mediated by release factors that recognize the stop codon and promote the release of the protein from the ribosome.
A Special Case: Retroviruses
Retroviruses are a unique class of viruses that possess the ability to reverse the flow of genetic information. Unlike other viruses, which use DNA as the genetic material and replicate through transcription and translation, retroviruses use RNA as the genetic material and replicate through a process called reverse transcription. This process is catalyzed by an enzyme called reverse transcriptase, which converts the viral RNA genome into DNA.
The process of reverse transcription begins when the viral RNA genome enters the host cell. Once inside, the viral RNA genome binds to the reverse transcriptase enzyme, which begins to copy the viral RNA genome into DNA. This process is error-prone and can lead to mutations in the viral DNA genome. The newly synthesized viral DNA genome then enters the host cell's nucleus, where it can integrate into the host genome.
Once the viral DNA integrates into the host genome, it is transcribed and translated for the assembly of new viral progeny. The integrated viral DNA is transcribed into viral RNA (including both genomic RNA and mRNAs), and the viral mRNAs are translated into viral proteins. These viral proteins, along with the newly synthesized viral RNA genomes and reverse transcriptase, assemble to form new viral particles (progeny), which can then exit the host cell and infect other cells. This process demonstrates how retroviruses hijack the host cell's transcription and translation machinery to produce new viral progeny.
This alternate flow of genetic information in retroviruses is a special case, and it is also what makes them unique and challenging to control. The ability of the viral DNA genome to integrate into the host genome can lead to long-term viral persistence and the development of viral-associated diseases, such as AIDS caused by HIV.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| amino acid | Organic molecules that serve as the building blocks of proteins, each composed of a central carbon atom bonded to a hydrogen atom, a carboxyl group, an amine group, and a variable R group. |
| codon | A sequence of three nucleotides on mRNA that specifies a particular amino acid or stop signal during translation. |
| elongation | The stage of translation in which amino acids are sequentially added to the growing polypeptide chain. |
| eukaryotic | Referring to organisms that have a membrane-bound nucleus and organelles, such as animals, plants, and fungi. |
| genetic code | The set of rules by which nucleotide sequences in mRNA are translated into amino acid sequences in proteins. |
| genotype | The genetic makeup of an organism; the specific alleles present for each gene. |
| initiation | The first stage of translation in which the ribosome assembles on the mRNA at the start codon. |
| messenger RNA | The RNA molecule that carries genetic information from DNA and serves as the template for protein synthesis. |
| phenotype | The observable physical and biochemical characteristics of an organism, determined by both genetic and environmental factors. |
| polypeptide | A chain of amino acids linked together by peptide bonds. |
| prokaryotic | Referring to organisms that lack a membrane-bound nucleus and organelles, such as bacteria. |
| protein | Macromolecules composed of amino acids linked together, containing carbon, hydrogen, oxygen, nitrogen, and often sulfur, that perform diverse functions in cells. |
| retrovirus | A virus that uses reverse transcriptase to convert its RNA genome into DNA for integration into the host genome. |
| reverse transcriptase | An enzyme that synthesizes DNA from an RNA template, used by retroviruses to convert their RNA genome to DNA. |
| ribosomal RNA | The RNA component of the ribosome that catalyzes peptide bond formation. |
| ribosome | The cellular structure composed of rRNA and proteins that catalyzes the synthesis of polypeptides during translation. |
| rough endoplasmic reticulum | Endoplasmic reticulum with attached ribosomes on its cytoplasmic surface; site of synthesis for proteins destined for secretion or membrane insertion. |
| start codon | The codon AUG where translation begins, coding for the amino acid methionine. |
| stop codon | A codon that signals the termination of translation and the release of the completed polypeptide chain. |
| termination | The final stage of translation in which the ribosome releases the completed polypeptide chain. |
| transcription | The process by which RNA polymerase synthesizes RNA molecules using a DNA template strand. |
| transfer RNA | An RNA molecule that binds specific amino acids and uses anticodon sequences to recognize and pair with mRNA codons during translation. |
| translation | The process by which mRNA is decoded by ribosomes to synthesize a polypeptide chain. |
Frequently Asked Questions
What is translation and how does it work in cells?
Translation is the process that reads an mRNA sequence and builds a polypeptide (protein) on ribosomes in the cytoplasm or on the rough ER (EK 6.4.A.1). It has three main stages: initiation (ribosomal rRNA pairs with the start codon AUG, which codes for methionine), elongation (ribosome reads mRNA codons three bases at a time; tRNAs with matching anticodons bring the correct amino acids and each amino acid is added to the growing chain), and termination (a stop codon halts translation and the finished protein is released) (EK 6.4.A.3 i–viii). In prokaryotes transcription and translation can be coupled (they happen at the same time) (EK 6.4.A.2). Remember you only need to know AUG as the start codon for the exam and not to memorize the whole code (CED exclusions). For a focused review, check the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh), the Unit 6 overview (https://library.fiveable.me/ap-biology/unit-6), and practice questions (https://library.fiveable.me/practice/ap-biology).
Why does translation happen in the cytoplasm and not the nucleus?
Translation happens in the cytoplasm (or on the rough ER) because ribosomes—the machines that build proteins—are located there (EK 6.4.A.1). In eukaryotes the nucleus is a membrane-bound compartment where transcription and RNA processing (capping, splicing, poly-A) occur; the mature mRNA must be exported to the cytoplasm before ribosomes can read the start codon (AUG) and recruit tRNAs to begin initiation, elongation, and termination (EK 6.4.A.3). Prokaryotes don’t have a nucleus, so transcription and translation can be coupled (EK 6.4.A.2). Practically, separating transcription (nucleus) from translation (cytoplasm/rough ER) lets the cell process and regulate mRNA before making protein—important for correct phenotype expression (LO 6.4.A). For a quick AP-aligned refresher, check the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and more unit review at (https://library.fiveable.me/ap-biology/unit-6). Practice questions: (https://library.fiveable.me/practice/ap-biology).
What's the difference between transcription and translation?
Transcription and translation are two separate steps in gene expression. Transcription copies a gene’s DNA sequence into messenger RNA (mRNA) in the nucleus (eukaryotes) or cytoplasm (prokaryotes). It’s about making an RNA “copy” of the coding information. Translation reads that mRNA sequence on ribosomes (cytoplasm or rough ER surface in eukaryotes) and builds a polypeptide by reading codons (three-nucleotide units). Key differences: transcription uses DNA → RNA and produces mRNA (plus tRNA, rRNA), while translation uses mRNA → amino acids to make protein; translation requires tRNA anticodons, the start codon AUG (methionine), and proceeds by initiation, elongation, termination until a stop codon. Note special cases: in prokaryotes transcription and translation can be coupled (happen at once), and retroviruses can go RNA → DNA via reverse transcriptase (outside AP detail). For AP review, focus on locations, the start codon AUG, ribosome role, codons/anticodons, and that memorizing the whole genetic code isn’t required (CED). More review: (topic study guide: https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh), unit overview (https://library.fiveable.me/ap-biology/unit-6) and lots of practice problems (https://library.fiveable.me/practice/ap-biology).
How do ribosomes know where to start translating mRNA?
Ribosomes start translation at the start codon (AUG)—that’s the key AP fact. In both prokaryotes and eukaryotes the ribosomal rRNA (part of the ribosome) interacts with the mRNA so the ribosome’s position lines up with that AUG (EK 6.4.A.3.i). A tRNA carrying methionine pairs its anticodon to AUG, and that sets the reading frame so codons are read in triplets (EK 6.4.A.3.ii–vi). In bacteria this can happen while the mRNA’s still being made (coupled transcription–translation, EK 6.4.A.2) and ribosomes often bind at a nearby ribosome-binding site to find the start quickly. In eukaryotes the ribosome typically assembles at the 5' end of the mRNA and scans along until it finds the first AUG. Remember: for the AP exam you only need to know AUG as the start codon and that ribosomal rRNA interacts with mRNA at initiation. For review, see the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and practice questions (https://library.fiveable.me/practice/ap-biology).
I'm confused about codons - what are they and how do they work?
A codon is a three-nucleotide “word” in mRNA that the ribosome reads during translation. Because nucleotides are read in triplets (EK 6.4.A.3.ii), each codon specifies one amino acid (or a stop signal). Translation starts at the start codon AUG (codes for methionine) and proceeds codon by codon as tRNAs with complementary anticodons bring the correct amino acids (EK 6.4.A.3.i, v–vi). Some amino acids have multiple codons (redundancy), and three codons are stop codons that end translation (EK 6.4.A.3.iii, vii). The nearly universal genetic code across life is evidence of common ancestry (EK 6.4.A.3.iv). For the AP exam you only need to memorize AUG as the start codon—don’t memorize the whole table (CED exclusion). For a quick Topic 6.4 review, see the Fiveable translation study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and practice questions (https://library.fiveable.me/practice/ap-biology).
Why is AUG always the start codon and what amino acid does it code for?
AUG is the start codon because ribosomes (via rRNA) recognize that specific triplet as the signal to begin translation—it sets the reading frame so codons are read in the right groups of three (initiation). The tRNA with the matching anticodon brings the first amino acid. AUG codes for methionine (in bacteria the first Met is often modified to formyl-Met, but for the AP Exam you only need to know AUG = methionine). Nearly all organisms use the same genetic code, so AUG is the universal start signal in most cases. The CED even highlights that AUG is the one codon you should memorize. If you want a quick topic review, check the Unit 6 translation study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and more unit resources (https://library.fiveable.me/ap-biology/unit-6). For extra practice, see Fiveable’s AP practice questions (https://library.fiveable.me/practice/ap-biology).
How does tRNA bring the right amino acid to the ribosome?
tRNA brings the right amino acid to the ribosome by matching its anticodon to the mRNA codon and being pre-loaded with the correct amino acid. Each tRNA has a 3-base anticodon that base-pairs with the mRNA codon in the ribosome’s A site, so the mRNA sequence determines which tRNA (and thus which amino acid) fits next. Before translation, specific enzymes attach the correct amino acid to each tRNA—that “charging” step gives each tRNA its identity. The genetic code (read in triplets) plus base-pairing rules (including wobble at the third codon position) ensure most codons bring the right amino acid. This is exactly what EK 6.4.A.3.v describes for translation (LO 6.4.A). For a quick Topic 6.4 review, see the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and try practice questions (https://library.fiveable.me/practice/ap-biology).
What happens during the three steps of translation - initiation, elongation, and termination?
Initiation: Ribosomes (in the cytoplasm or on rough ER) bind the mRNA and position the rRNA so the start codon AUG is in the P site. A tRNA with the anticodon for AUG brings methionine and pairs with the start codon—that sets the reading frame and starts the polypeptide. Elongation: The ribosome reads mRNA codons three bases at a time. A new tRNA with the matching anticodon delivers the correct amino acid to the A site, the amino acid is transferred to the growing polypeptide (peptide bond), and the ribosome translocates so the next codon is in the A site. This repeats, making the polypeptide longer. Multiple ribosomes can translate one mRNA (polyribosome), and in prokaryotes translation can start while transcription is still happening. Termination: When a stop codon (UAA, UAG, UGA) enters the A site, no tRNA matches it. Release factors promote release of the completed polypeptide and disassembly of the ribosome–mRNA complex. Remember AP rules: you only need to know AUG as the start codon and general steps (not all enzyme/factor names). For a quick review, see the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh). For extra practice, use the AP practice problems (https://library.fiveable.me/practice/ap-biology).
Why do most amino acids have more than one codon that codes for them?
Most amino acids are encoded by more than one codon because the genetic code is degenerate (redundant). There are 64 possible codons (3-base triplets) but only 20 standard amino acids, so several codons map to the same amino acid. Mechanistically, this works because tRNA anticodons can “wobble” at the third nucleotide—one tRNA can pair with multiple codons for the same amino acid, so the third base is often flexible. Biologically this helps: many single-base (point) mutations in the third position don’t change the amino acid (synonymous/silent mutations), so proteins are buffered against harmful changes. The AP CED expects you to know codons are read in triplets, tRNA brings the correct amino acid (anticodon pairing), and that many amino acids have multiple codons; you don’t have to memorize the whole code (only AUG). If you want a quick review, check the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) or practice questions (https://library.fiveable.me/practice/ap-biology).
How is translation different in prokaryotes vs eukaryotes?
Short answer: the core chemistry of translation is the same, but timing, location, and context differ. - Where it happens: both use ribosomes in the cytoplasm; eukaryotic translation also occurs on ribosomes bound to the rough ER for secreted/membrane proteins (CED EK 6.4.A.1). - Timing/coupling: in prokaryotes translation often begins while mRNA is still being transcribed (coupled transcription–translation), but in eukaryotes transcription (in nucleus) and translation (in cytoplasm/ER) are separated (CED EK 6.4.A.2). - Ribosome organization: both make polyribosomes (polysomes) that synthesize many copies from one mRNA. - Same basics: both read codons (AUG start → methionine), use tRNAs, and follow initiation → elongation → termination (CED EK 6.4.A.3). - AP note: you don’t need to memorize all initiation/elongation factors or the whole code—just know AUG and these big differences (see the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and Unit 6 overview (https://library.fiveable.me/ap-biology/unit-6)). For more practice, try the 1000+ problems (https://library.fiveable.me/practice/ap-biology).
What are stop codons and how does the ribosome know when to stop translating?
Stop codons are three specific mRNA codons (UAA, UAG, UGA) that don’t code for any amino acid. During elongation the ribosome reads mRNA in codons (EK 6.4.A.3.ii–vi); when it reaches a stop codon (EK 6.4.A.3.vii) no tRNA has a matching anticodon to add an amino acid. Instead, proteins called release factors bind the A site of the ribosome. That binding triggers hydrolysis of the final tRNA–polypeptide bond, releasing the newly made protein and causing the ribosome subunits to fall off (termination, EK 6.4.A.3.viii). This is how the ribosome “knows” to stop translating. For the AP exam, you only need to know AUG is the start codon and that translation ends at stop codons (no need to memorize all translation factors). Review Topic 6.4 on Fiveable (study guide: https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and practice more at https://library.fiveable.me/practice/ap-biology.
I don't understand how the genetic code chart works - can someone explain it?
Think of the genetic code chart as a translator that converts mRNA codons (three-nucleotide "words") into amino acids. Read it this way: the first base of the codon picks the row, the second picks the column, and the third narrows it down inside that box—that gives you the amino acid. For example, AUG: first base A (row), second U (column), third G → methionine and also the start codon (EK 6.4.A.3.i–iii). tRNA molecules have anticodons complementary to each codon and bring the correct amino acid to the ribosome (EK 6.4.A.3.v). The code is redundant (many amino acids have multiple codons) and nearly universal (EK 6.4.A.3.iv). For AP purposes you only need to know start AUG and the triplet reading frame concept—you don’t have to memorize the whole chart. If you want a quick visual and practice, check the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and try practice questions (https://library.fiveable.me/practice/ap-biology).
Why do scientists say the genetic code is evidence for common ancestry?
Because nearly every living organism uses the same codon-to-amino-acid mapping, the genetic code is a strong signal of shared ancestry. On the molecular level translation works the same way in prokaryotes and eukaryotes (ribosomes read mRNA triplet codons; start codon AUG → methionine; tRNAs bring amino acids)—EK 6.4.A.1 and EK 6.4.A.3 in the CED. If very different lineages had separate origins you’d expect very different codes; instead we see the same code conserved across bacteria, plants, animals, and fungi. That widespread universality is most parsimoniously explained by descent from a common ancestor that already used that code. (There are a few minor exceptions—mitochondria and some protists use slightly altered codes—but those are derived changes, not independent origins.) For AP review see the Topic 6.4 study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and practice questions (https://library.fiveable.me/practice/ap-biology).
What is reverse transcriptase and how do retroviruses like HIV use it?
Reverse transcriptase is an enzyme that makes a DNA copy from an RNA template—basically reversing the usual flow (RNA → DNA). Retroviruses like HIV carry reverse transcriptase in their virions. After the virus enters a host cell, RT copies the viral RNA genome into complementary DNA (cDNA). That viral DNA is then integrated into the host’s genome (a provirus). The host’s transcription and translation machinery then treat viral genes like normal genes: host RNA polymerase transcribes viral DNA into mRNA, and ribosomes translate that mRNA into viral polypeptides for new virus assembly (see EK 6.4.A.4). This alternate information flow (RNA → DNA → host genome → mRNA → protein) is a key exception you'll see on the AP exam. For a quick Topic 6.4 review, check the translation study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh). For broader unit review and extra practice, use (https://library.fiveable.me/ap-biology/unit-6) and (https://library.fiveable.me/practice/ap-biology).
How does the ribosome move along the mRNA during translation?
The ribosome “walks” along the mRNA three nucleotides (one codon) at a time during elongation. After a charged tRNA pairs its anticodon with the mRNA in the A site, the ribosome catalyzes a peptide bond and the growing chain is transferred onto the tRNA in the A site. Then the ribosome translocates—the tRNAs shift: the peptidyl tRNA moves from the A site to the P site, the empty tRNA moves to the E site and exits, and the mRNA advances so the next codon enters the A site. This cycle repeats codon-by-codon until a stop codon (no tRNA) is reached and translation terminates. Remember: the mRNA is read in triplets (AUG start codon → methionine) and translation occurs on cytoplasmic or rough ER ribosomes (CED EK 6.4.A.1–3). Want a quick topic review? Check the AP Bio translation study guide (https://library.fiveable.me/ap-biology/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh) and practice problems (https://library.fiveable.me/practice/ap-biology).