Transcription and Translation: The Central Dogma of Molecular Biology

The process of converting genetic information from DNA into functional proteins involves two key steps:

1. Transcription (DNA → mRNA)
2. Translation (mRNA → Protein)

1. Transcription: DNA to mRNA

Definition: Transcription is the process of synthesizing messenger RNA (mRNA) from a DNA template.
Location: Nucleus (in eukaryotes) / Cytoplasm (in prokaryotes)
Enzyme involved: RNA polymerase
Strands involved: Only one DNA strand (template strand) is transcribed

Steps of Transcription:

Initiation:

• RNA polymerase binds to the promoter region on DNA.
• DNA unwinds, exposing the template strand.

Elongation:

• RNA polymerase adds complementary RNA nucleotides (A → U, T → A, C → G, G → C).
• The mRNA strand grows in the 5’ to 3’ direction.

Termination:

• RNA polymerase reaches a terminator sequence and stops transcription.
• The mRNA strand detaches from DNA.

Post-Transcriptional Modifications (Eukaryotes only):

• 5’ Capping: A modified guanine cap is added to the 5’ end.
• Polyadenylation: A poly-A tail is added to the 3’ end.
• Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

2. Translation: mRNA to Protein

Definition: Translation is the process of decoding mRNA into a polypeptide (protein).
Location: Ribosome (in the cytoplasm or rough ER)
 Key players:

• mRNA (provides the genetic code)
• tRNA (carries amino acids)
• Ribosomes (site of protein synthesis)

Steps of Translation:

 Initiation:

• The ribosome binds to the start codon (AUG) on mRNA.
• The first tRNA carrying methionine (Met) binds to the start codon.

 Elongation:

• The ribosome moves along mRNA, reading codons (three-nucleotide sequences).
• Each codon is matched with a complementary tRNA anticodon, bringing the correct amino acid.
• Peptide bonds form between amino acids, creating a polypeptide chain.

 Termination:

• The ribosome reaches a stop codon (UAA, UAG, UGA).
• The completed polypeptide (protein) is released.

Key Differences Between Transcription and Translation

Significance of Transcription & Translation

Essential for protein synthesis

Determines gene expression
Errors can lead to genetic diseases