Translation
Introduced| English |
|---|
| codon/ˈkəʊdɒn/ |
| translation/trænˈsleɪʃn/ |
| termination/ˌtɜːmɪˈneɪʃn/ |
| transfer RNA/ˈtrænsfɜː ˌɑː en ˈeɪ/ |
| anticodon/ˌæntɪˈkəʊdɒn/ |
| initiation/ɪˌnɪʃɪˈeɪʃn/ |
| elongation/iːˌlɒnˈɡeɪʃn/ |
| reverse transcriptase/rɪˈvɜːs trænˈskrɪpteɪs/ |
Read the stop sign without adding another amino acid
- A ribosome reads four codons 密码子 but releases a chain containing only three amino acids. The last codon was a stop instruction, not another ingredient.
- Translation 翻译 converts an RNA message into a polypeptide. Start in the correct reading frame, use the genetic code and distinguish chain-building codons from termination 终止.
Codons: reading in threes
- Translation reads the mRNA three bases at a time.
- Each group of three bases is a codon.
- Most codons specify an amino acid; stop codons end translation.
- So the order of codons sets the order of amino acids in the protein.
Translation builds a protein by reading…
Translation reads the mRNA message and builds the matching chain of amino acids — a protein.
How many mRNA bases make up one codon?
A codon contains three bases. An amino-acid codon specifies an amino acid; a stop codon ends translation.
The ribosome and tRNA
- A ribosome clamps onto the mRNA and moves along it.
- Transfer RNA 转运RNA molecules, called tRNA, bring the amino acids.
- Each tRNA has an anticodon 反密码子 that pairs with a matching codon.
- This ensures the right amino acid arrives at the right spot.
Translation at the ribosome
Step through translation - the ribosome reads mRNA in codons and tRNA brings the matching amino acids.
The molecule that carries an amino acid to the ribosome is called ____ RNA.
Transfer RNA (tRNA) carries an amino acid and has an anticodon that matches the mRNA codon.
A tRNA's anticodon pairs with the matching codon on the mRNA.
The anticodon base-pairs with the codon, so the right amino acid is added in the right place.
Building the chain
- As the ribosome reads each codon, the matching amino acid is added.
- The growing chain gets one link longer for each amino-acid codon.
- A special "stop" codon signals the end of the protein.
- The finished chain then folds into a working protein.
Select all true statements about translation.
Copying DNA into mRNA is transcription, not translation. The other three are correct.
Start, extend, release
- During initiation 起始, ribosomal RNA helps position the mRNA at the start codon AUG; the first amino acid is methionine. During elongation 延伸, matching tRNAs deliver amino acids and the ribosome joins them by peptide bonds.
- During termination, a stop codon triggers release of the chain. Stop codons do not encode amino acids. Many other codons can encode the same amino acid; use a supplied code chart rather than guessing.
Put the translation stages in order.
The ribosome starts in the correct frame, extends the chain, then releases it.
Where translation happens
- Ribosomes translate mRNA in the cytoplasm of both prokaryotic and eukaryotic cells. Eukaryotic ribosomes also work on the cytoplasmic surface of the rough ER; there are no ribosomes inside its lumen.
- Prokaryotes have no nuclear barrier, so a ribosome can begin translating an mRNA while it is still being transcribed. The nearly universal genetic code supports common ancestry.
In a prokaryote, translation may start before transcription of that mRNA finishes.
Without a nucleus separating the processes, ribosomes can access the growing mRNA.
Worked example: decode a short message
- Given this code key: AUG = methionine, GCU = alanine, UAA = stop. Read 5′–AUG GCU GCU UAA–3′ as methionine–alanine–alanine, then release the chain.
- There are four codons but only three amino acids. Changing GCU to GCC would leave alanine unchanged if the supplied code chart assigns both codons to alanine.
Using the supplied code key, how many amino acids come from AUG GCU GCU UAA?
AUG, GCU and GCU each specify an amino acid; UAA ends translation.
Retroviruses reverse one information step
- A retrovirus uses reverse transcriptase 逆转录酶 to copy viral RNA into DNA. That DNA can integrate into the host genome.
- The host then transcribes the integrated DNA into RNA and translates viral mRNA into proteins. This is RNA → DNA → RNA → protein, not direct translation of DNA.
Which enzyme allows information to flow from viral RNA to DNA?
Reverse transcriptase makes a DNA copy of the RNA genome.
The code is read in triplets: three bases make one codon; an amino-acid codon specifies one amino acid, while a stop codon ends the chain. Read them out of step — starting one base early or late — and every codon after is wrong. The reading frame must line up exactly.
From codon to amino acid:
- The mRNA codon $\text{A–U–G}$ codes for the amino acid methionine — and also says "start".
- The next codon adds the next amino acid, and so on.
- A stop codon (like $\text{U–A–A}$) has no amino acid; it ends the chain.
Translation builds a protein from mRNA. The ribosome reads the mRNA in codons (groups of three bases), and transfer RNA (tRNA) brings each amino acid, its anticodon pairing with the codon. Amino acids join in order until a stop codon ends the protein.