D1.2 Protein synthesis

Protein synthesis is the process by which cells use genetic information encoded in DNA to build proteins. It is one of the most fundamental processes in all living organisms and occurs in two major stages:

1. Transcription , the genetic information in DNA is copied into messenger RNA (mRNA), which takes place in the nucleus of eukaryotic cells.
2. Translation , the mRNA sequence is read by ribosomes to assemble a chain of amino acids (a polypeptide), which takes place in the cytoplasm.

This flow of information , DNA → RNA → Protein , is known as the central dogma of molecular biology.

Transcription: Transcription is the process by which the genetic information encoded in a DNA sequence is copied into a complementary RNA sequence, producing messenger RNA (mRNA).

The key enzyme responsible for transcription is RNA polymerase. It reads the DNA template strand and builds a complementary RNA strand using free ribonucleotides.

The Three Stages of Transcription

1. Initiation

• RNA polymerase binds to a specific DNA sequence called the promoter, located near the start of the gene.
• The promoter acts as a recognition site that tells RNA polymerase exactly where to begin.

2. Elongation

• RNA polymerase unwinds the DNA double helix, separating the two strands:
  • Template strand (3' → 5'): used as the guide to synthesise RNA.
  • Coding strand (5' → 3'): not directly read; it has the same sequence as the RNA (except RNA uses uracil instead of thymine).
• RNA polymerase moves along the template strand, adding RNA nucleotides one by one in the 5' to 3' direction through complementary base pairing.
• Covalent phosphodiester bonds form between successive nucleotides, creating a continuous RNA strand.

3. Termination

• Transcription stops when RNA polymerase reaches a termination sequence on the DNA.
• The newly synthesised mRNA molecule is released, and the DNA double helix reforms.