Transcription & Translation (Edexcel International A Level 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 (messenger RNA) molecule is produced
  • Translation – mRNA is translated and an amino acid sequence is produced
    • mRNA is a single-stranded molecule made up of many RNA nucleotides joined together
    • The role of mRNA is to carry the information encoded in the DNA from the nucleus to the site of translation on ribosomes

The process of transcription

• This stage of protein synthesis occurs in the nucleus of the cell
• Part of a DNA molecule unwinds and the hydrogen bonds between the complementary base pairs break
• This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
• A complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA 
  • This reaction is catalysed by RNA polymerase
• Free activated RNA nucleotides pair up, via hydrogen bonds, with their complementary bases on the exposed strand of the ‘unzipped’ DNA molecule
• The sugar-phosphate groups of these RNA nucleotides are then bonded together in a reaction catalysed by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule
• When the gene has been transcribed and the mRNA molecule is complete, the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule reforms
• The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

The transcription stage of protein synthesis - DNA is transcribed and an mRNA molecule is produced

Anti-sense and sense strands

• In the transcription stage of protein synthesis, free RNA nucleotides pair up with the exposed bases on the DNA molecule
• RNA nucleotides only pair with the bases on one strand of the DNA molecule
  • This strand of the DNA molecule is known as the antisense or template strand (or the transcribed strand) and it is used to produce the mRNA molecule
  • The other strand is known as the sense or coding strand (or the non-template or non-transcribed strand)
• RNA polymerase moves along the template strand in the 3' to 5' direction
  • This means that the mRNA molecule grows in the 5' to 3' direction
• Because the mRNA is formed by complementary pairing with the DNA template strand, the mRNA molecule contains the exact same sequence of nucleotides as the DNA coding strand (although the mRNA will contain uracil instead of thymine)

The antisense strand of DNA is the one that is transcribed into mRNA

• Translation occurs in the cytoplasm of the cell
• After leaving the nucleus via a nuclear pore, the mRNA molecule attaches to a ribosome
• In the cytoplasm there are free molecules of tRNA (transfer RNA)
  • tRNA is a single stranded molecule of RNA that folds into a clover-like structure
  • tRNA molecules have a triplet of unpaired bases at one end, known as the anticodon, and a region at the other end where a specific amino acid can attach
  • There are about 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
• The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
• The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet on the mRNA molecule called the codon
  • Near the beginning of the mRNA is a triplet of bases called the start codon (AUG)
  • This is a signal to start off translation 
  • AUG codes for an amino acid called methionine
• Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
• A peptide bond is then formed, via a condensation reaction, between the two 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
• The amino acid chain then forms the final polypeptide

The process of translation