i. The small ribosomal subunit attaches to the mRNA.
ii. Initiation tRNA attaches to the mRNA at the start codon, AUG. This codon corresponds to the amino acid, methionine.
iii. The large ribosomal subunit binds to the small subunit, placing the initiator tRNA at the P-site.
7.1 Essential ideas
- Translation is the synthesis of polypeptides on ribosomes.
- Ribosomes consist of a large and a small sub-unit, each made from rRNA and proteins. They are assembled at the initiation of translation.
- The structure of ribosomes allows for the complementary base pairing of the mRNA codon with the anti-codons on tRNA molecules.
- There are many variations of tRNA molecules but all have the following common structures:
tRNA-activating enzymes and the role of phosphorylation
- tRNA molecules need to be charged and recharged with the appropriate amino acid so they can participate repeatedly in the process of translation.
- Aminoacyl-tRNA synthetases are enzymes that activate amino acids and use energy from phosphorylation to attach amino acids to tRNA molecules.
- There are 20 different tRNA-activating enzymes, each having a specifically shaped active site that works on one amino acid and its corresponding tRNA molecule.
- mRNA is translated by ribosomes in the 5’ to 3’ direction.
- The process occurs in three phases: initiation, elongation and termination.
- Initiation involves the assembly of ribosomes.
- Synthesis of the polypeptide involves a repeated cycle of events during the elongation phase.
i. A second tRNA molecule is recruited to the A-site, based on the complementary pairing of codon to anticodon.
ii. A peptide bond is formed between the two adjacent amino acids.
iii. The ribosome moves along the mRNA molecule, three bases at a time, moving the tRNA molecule from the A-site to the P-site.
iv. The tRNA molecule in the P-site moves to the E-site, from which it exits the ribosome.
v. A new tRNA molecule is recruited to the A-site. A peptide bond is formed between the two amino acids.
vi. The process is repeated until a ‘stop’ codon is reached.
- Disassembly of the components follows termination of translation.
i. When the stop codon (UAG, UAA or UGA) is reached, the corresponding tRNA molecule enters the P-site.
ii. The stop codon is not associated with an amino acid, so no new peptide bond is formed.
iii. A release factor enters the A-site. The polypeptide is released.
iv. The tRNA molecule exits from the E-site.
v. The ribosome disassembles.
Translation in prokaryotes and eukaryotes
- In prokaryotes:
- In eukaryotes:
- transcription occurs in the nucleus and translation in the cytoplasm
- translation occurs on free ribosomes in the cytoplasm or on bound ribosomes (attached to the endoplasmic reticulum)
- free ribosomes synthesise proteins for use within the cell
- proteins destined for secretion or for use in lysosomes are synthesised by bound ribosomes.
Skill: Identifying polysomes
- In the micrograph above, polysomes can be seen at multiple locations on mRNA branching from the DNA.
- Many ribosomes translate mRNA simultaneously in order to synthesise multiple copies of the same protein. A polysome is a complex of multiple ribosomes and mRNA that is formed during translation.
- Prokaryotic ribosomes are slightly smaller than eukaryotic ribosomes.
- Some textbooks do not include an E-site on the structure of ribosomes. In these instances, the tRNA molecule exits the ribosome after leaving the P-site.
Predict whether the following proteins are synthesised by free ribosomes or bound ribosomes: immunoglobins, transport proteins, rhodopsin, collagen.
- Review the relationship between genetic code and amino acids in 2.1.7.