Constructing Polypeptides: Transcription & Translation
- 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
Examiner Tip
Make sure you learn both stages of protein synthesis fully. Don’t forget – transcription occurs in the nucleus but translation occurs in the cytoplasm! Be careful – DNA polymerase is the enzyme involved in DNA replication; RNA polymerase is the enzyme involved in transcription – don’t get these confused.
