After RNA processing, the mRNA leaves the nucleus and searched for a ribosome. The mRNA molecule carries the message from DNA in the form of codons. Codons are a group of three bases (letters) that correspond to one of 20 amino acids. Certain amino acids are specified by more than one codon.
The mRNA attaches to the ribosome and waits for the appropriate amino acids to arrive at the ribosome. The amino acid is attached to tRNA (transfer RNA). A tRNA molecule has a three-dimensional structure that resembles a four-leaf clover.
One end of the tRNA carries an amino acid. The other end, called an anticodon, has three bases that pairs with the codon in the mRNA.
Each tRNA becomes charged and attaches to an amino acid via enzymes. The charging enzymes involved in forming the bond between the amino acid and tRNA require ATP.
The translation process involves three phases: initiation, elongation, and termination.
Ribosomes contain three binding sites: A site, P site, and E site. An initiator tRNA activates translation and occupies the P site. In all organisms, the codon for the initiation of protein synthesis is AUG, which codes for the amino acid methionine. The tRNA with the complementary anticodon, UAC, brings methionine to the mRNA. Once the methionine tRNA is attached to the P site, the A site can be filled by the appropriate tRNA that corresponds to the next codon. The E site binds a free tRNA before the tRNA exits the ribosome.
The addition of amino acids is called elongation. As each amino acid is brought to the mRNA, it is linked to its neighboring amino acid by a peptide bond. A chain of multiple amino acids is a polypeptide.
The polypeptide synthesis is ended by stop codons. A codon doesn’t always code for an amino acids. There are three codons that serve as a stop codon: UAA, UAG, and UGA. Termination occurs when the ribosome reaches one of these three stop codon.
The polypeptide chain undergoes several changes before it forms a protein. Proteins can have four levels of structure.
- primary structure of a protein – the linear sequence of amino acids -
- secondary structure – the polypeptide twists and either forms a coil (alpha helix) or zig-zag patterns (bête-pleated sheets)
- tertiary structure – polypeptide folds in a three-dimensional pattern
- quaternary structure – two or more polypeptides from a three-dimensional structure
In some cases, chaperon proteins (chaperonins) assist in the folding process to ensure that the proteins fold correctly and efficiently.
AP Biology Exam Study Tips
Make a flow diagram showing the process of translation. This will help you remember the sequence of events.