Protein synthesis
Cells express their genes by converting the genetic message into protein. This process of protein synthesis occurs in two stages - transcription and translation.
Transcription
When a gene is to be expressed, the base sequence of DNA is copied or transcribed into mRNA (messenger RNA). This process takes place in the nucleus and occurs in a series of stages.
- RNA polymerase moves along the DNA unwinding the strand.
- Hydrogen bonds between base pairs break which allows the unzipping of the double helix.
- As RNA polymerase breaks the bonds, it synthesises a primary transcript of mRNA using RNA nucleotides. These form hydrogen bonds with the exposed DNA strand by complementary base pairing.
- The primary transcript of mRNA is processed to produce a mature transcript of mRNA.
- The mature mRNA transcript is now ready to leave the nucleus and travel to the ribosome.
Translation
- The mRNA molecule travels through the cytoplasm and attaches to the ribosome.
- tRNA molecules transport specific amino acids to the ribosome.
- Each mRNA codon codes for a specific amino acid.
- The first codon of an mRNA molecule is a start codon. This signals the beginning of translation.
- The anti-codons and codons match up and form complementary base pairs.
- Peptide bonds form between the adjacent amino acids to form the polypeptide (protein).
- Used tRNA molecules exit the ribosome and collect another specific amino acid.
- The last codon of an mRNA molecule is a stop codon which signals the end of translation.
After translation, the polypeptide chain can be changed by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein.