3.18B Stages of Protein Synthesis (Edexcel IGCSE Biology)

• A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
• This process of protein synthesis occurs in two stages:
  • Transcription – DNA is transcribed and an mRNA molecule is produced
  • Translation – mRNA (messenger RNA) is translated and an amino acid sequence (protein) is produced

Transcription

• This stage of protein synthesis occurs in the nucleus of the cell, and its role is to produce an mRNA copy of a section of DNA known as a gene
• The sequence of events is as follows:
  • Part of the DNA molecule unwinds when hydrogen bonds between the complementary base pairs break
  • This exposes the template strand of the gene that codes for the protein being synthesised
  • Free mRNA nucleotides that are present in the nucleus bind to complementary nucleotides on the template strand
  • The mRNA nucleotides are joined to neighbouring nucleotides, forming a single strand of mRNA
  • The mRNA molecule leaves the nucleus via a pore in the nuclear envelope
• The new strand of mRNA is a complementary copy of the DNA code from the original gene

Transcription diagram

Transcription occurs in the nucleus and produces a molecule of single-stranded mRNA

Translation

• This stage of protein synthesis occurs in the cytoplasm of the cell, and results in the production of a chain of amino acids that will go on to form a protein
• The process of translation is as follows:
  • After leaving the nucleus, the mRNA molecule attaches to a ribosome
  • In the cytoplasm there are free molecules of tRNA (transfer RNA); these tRNA molecules have a triplet of unpaired bases at one end known as the anticodon, and an amino acid at the other
    • Each specific anticodon corresponds to a specific amino acid
  • The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule, bringing its specific amino acid along with it
  • A second tRNA molecules attaches to its complementary codon, and a peptide bond is formed between the two neighbouring amino acids
  • This process continues until a ‘stop’ codon on the mRNA molecule is reached; this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
• This amino acid chain is then folded and modified to form the final protein molecule, e.g. an enzyme or antibody

Translation occurs on the ribosomes in the cytoplasm, and results in the production of a chain of amino acids. Note that you don't need to know the details about start codons shown here.