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6.4 Translation

Keywords

English Term 中文翻译 Definition & Explanation
Translation 翻译 The process by which the sequence of bases of an mRNA is converted into the sequence of amino acids of a protein.
Codon 密码子 A three-nucleotide sequence on mRNA that specifies a particular amino acid or termination signal.
Anticodon 反密码子 A nucleotide triplet on a tRNA molecule that base-pairs with a particular complementary codon on an mRNA molecule.
Polypeptide 多肽 A polymer of many amino acids linked together by peptide bonds.
Reverse Transcriptase 逆转录酶 An enzyme encoded by certain RNA viruses (retroviruses) that uses RNA as a template for DNA synthesis.

1. Location and Timing

The translation of mRNA into a polypeptide occurs on ribosomes. These are found in: * The cytoplasm of all cells (prokaryotic and eukaryotic). * The surface of the Rough Endoplasmic Reticulum (ER) in eukaryotic cells.

Prokaryotic vs. Eukaryotic Timing

  • Prokaryotes: Since they lack a nucleus, translation of the mRNA often begins while it is still being transcribed. This is known as "coupled transcription and translation."
  • Eukaryotes: Transcription occurs in the nucleus, and translation occurs in the cytoplasm. The two processes are spatially and temporally separated.

2. The Process of Translation

Translation follows a sequence of three main steps: Initiation, Elongation, and Termination.

  1. Initiation: The rRNA in the ribosome interacts with the mRNA at the start codon.
    • The universal start codon is AUG, which codes for the amino acid methionine.
  2. Elongation: The mRNA is read in triplets called codons.
    • tRNA molecules bring the correct amino acids to the ribosome.
    • The tRNA anticodon base-pairs with the mRNA codon.
    • The amino acid is transferred to the growing polypeptide chain.
  3. Termination: The process continues until a stop codon is reached on the mRNA. The newly synthesized protein is then released.

The Genetic Code: Evidence for Common Ancestry

Nearly all living organisms use the same genetic code (e.g., UUU always codes for Phenylalanine). This universality is powerful evidence that all life shares a common evolutionary ancestor.

3. Special Case: Retroviruses

Retroviruses (like HIV) challenge the "Central Dogma" of molecular biology (\(DNA \rightarrow RNA \rightarrow \text{Protein}\)). They follow an alternate flow of information:

  1. Reverse Transcription: The enzyme reverse transcriptase copies the viral RNA genome into DNA.
  2. Integration: This viral DNA integrates into the host cell's genome.
  3. Expression: The host's machinery then transcribes and translates this viral DNA to assemble new viral progeny.

Analogy: The Global Kitchen

  • mRNA is the recipe sent from the office (nucleus) to the kitchen.
  • Ribosomes are the chefs who read the recipe.
  • tRNA are the delivery drivers bringing the specific ingredients (amino acids) to the kitchen.
  • The Genetic Code is the universal language of cooking that every chef in every country understands.

AP Exam Note: Exclusion Statement

  • You do not need to memorize the names of specific initiation/elongation factors or enzymes beyond those listed (ribosomes, tRNA, reverse transcriptase).
  • You do not need to memorize the genetic code chart, except for the start codon AUG.

Quiz

Campbell Biology Chapter 17 Practice Test: From Gene to Protein

Click the link above to practice related multiple-choice questions (opens in a new tab).