• 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?is produced

Transcription

• This stage of protein synthesis occurs?in the nucleus?of the cell
• Part of a DNA molecule?unwinds?(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?(messenger RNA)
• Free activated RNA nucleotides?pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule
• The sugar-phosphate groups of these RNA nucleotides are then?bonded?together by the enzyme?RNA polymerase?to form the sugar-phosphate backbone of the mRNA molecule
• When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the?double-stranded DNA molecule re-forms
• 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

Translation

• This stage of protein synthesis occurs?in the cytoplasm?of the cell
• 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 a region where a specific?amino acid can attach?at the other
• There are at least 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 (codon) on the mRNA molecule
• 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 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
• This amino acid chain then forms the final polypeptide

The translation stage of protein synthesis – tRNA molecules bind with their specific amino acids

The translation stage of protein synthesis – an amino acid chain is formed