describe how the information in DNA is used during transcription and translation to construct polypeptides, including the roles of: RNA polymerase, messenger RNA (mRNA), codons, transfer RNA (tRNA), anticodons, ribosomes

Protein Synthesis

Learning Objective

Describe how the information in DNA is used during transcription and translation to construct polypeptides, including the roles of RNA polymerase, messenger RNA (mRNA), codons, transfer RNA (tRNA), anticodons, and ribosomes.

Overview

Protein synthesis occurs in two main stages:

1. Transcription – the synthesis of a complementary RNA strand from a DNA template.
2. Translation – the decoding of the mRNA sequence into a specific chain of amino acids (polypeptide) at the ribosome.

Transcription

Transcription takes place in the nucleus (or nucleoid in prokaryotes) and is catalysed by RNA polymerase. The main steps are:

• Initiation

  • RNA polymerase binds to the promoter region of a gene.
  • DNA strands unwind, exposing the template strand.

• Elongation

  • RNA polymerase moves along the template strand in the \$3' \rightarrow 5'\$ direction, adding ribonucleotides to the growing RNA chain in the \$5' \rightarrow 3'\$ direction.
  • Base‑pairing follows the rule: A–U, C–G (DNA‑RNA complementarity).

• Termination

  • When a termination signal is reached, RNA polymerase releases the newly formed pre‑mRNA.
  • In eukaryotes, the pre‑mRNA undergoes processing (5′‑cap, poly‑A tail, intron removal) to become mature mRNA.

Translation

Translation occurs on ribosomes in the cytoplasm (or on the rough endoplasmic reticulum). The process can be divided into three phases:

1. Initiation

   • The small ribosomal subunit binds to the 5′‑end of the mRNA (often at the 5′‑cap in eukaryotes).
   • The initiator tRNA carrying methionine pairs its anticodon with the start codon (AUG) on the mRNA.
   • The large ribosomal subunit joins, forming the complete ribosome with three sites: A (aminoacyl), P (peptidyl), and E (exit).

2. Elongation

   • A charged tRNA enters the A‑site, matching its anticodon with the next codon on the mRNA.
   • A peptide bond forms between the amino acid in the P‑site and the one in the A‑site.
   • The ribosome translocates one codon downstream; the tRNA in the P‑site moves to the E‑site and exits, while the tRNA in the A‑site shifts to the P‑site.

3. Termination

   • When a stop codon (UAA, UAG, or UGA) enters the A‑site, no tRNA can pair.
   • Release factors bind, prompting hydrolysis of the bond between the polypeptide and the tRNA in the P‑site.
   • The completed polypeptide is released, and the ribosomal subunits dissociate.

Key Molecular Players

Putting It All Together – Flow of Information

The central dogma for protein synthesis can be summarised as:

\$\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{pre‑mRNA} \xrightarrow{\text{processing}} \text{mRNA} \xrightarrow{\text{ribosome}} \text{polypeptide}\$

Suggested Diagram

Summary Checklist

• Identify the promoter and direction of transcription.
• Explain how RNA polymerase synthesises mRNA.
• Define codon and anticodon and their complementary relationship.
• Describe the three sites of the ribosome and their functions.
• Outline the steps of initiation, elongation, and termination in translation.
• State the role of each molecular component listed in the table.