In the last article, we ended transcription with a strand of messenger RNA (mRNA) ready to leave the nucleus and travel to the ribosomes. The ribosomes are where translation happens – the next step in protein synthesis.
Translation
Translation begins with the mRNA strand binding between the two subunits of the ribosome. Each three base codon is going to code for one amino acid. The amino acids are brought to the ribosomes by transfer RNA (tRNA) molecules. Remember that each tRNA has an anticodon which is complementary to a specific codon, so they bind together. There is only room for two tRNA molecules in the ribosome at once. When two are side by side, a peptide bond forms (using a condensation reaction) between the two amino acids that they are carrying. As the ribosome moves along the mRNA strand, the polypeptide chain gradually extends. Eventually the ribosome reaches a stop codon which is the signal that the polypeptide chain is complete. The polypeptide chain might then head to the rough endoplasmic reticulum and Golgi apparatus for folding and processing into the final protein.
Inhibition of translation by RNAi
RNA interference (RNAi) is one way that gene expression can be regulated in eukaryotic cells (and some prokaryotes). Basically, it is where the mRNA is intercepted and destroyed in the cytoplasm to stop it being translated into a polypeptide at the ribosomes. This can be called gene silencing. There a couple of different ways that this can happen.
siRNA
Small interfering RNA (siRNA) are short pieces of double stranded RNA that don’t code for a polypeptide. This is how they work:
- The siRNA is processed to become single stranded and binds to a protein to form a RISC complex.
- The single stranded siRNA binds to an mRNA which is in the cytoplasm on its way from the nucleus to the ribosome. They bind together with complementary base pairing because the mRNA sequence is complementary to the siRNA.
- The ribosome can no longer bind to the mRNA and the mRNA could be hydrolysed.
Because siRNA are fully complementary to one mRNA, they specifically silence one gene.
miRNA
MicroRNA (miRNA) work slightly differently to silence genes.
- The hairpin shaped pre-miRNA is processed to form a single stranded miRNA which binds to a protein to form a RISC complex.
- The miRNA binds to an mRNA with complementary base pairing, but this time the miRNA is not fully complementary to the mRNA. This means that sometimes one miRNA could bind to several different mRNAs if the complementary base sequence is found within them.
- The ribosome can no longer bind to the mRNA and the mRNA could be hydrolysed.
miRNA could silence several genes because they could bind to multiple mRNA molecules.
Summary
- Translation produces a polypeptide chain based on an mRNA base sequence. It takes place at the ribosomes.
- Anticodons on tRNA are complementary to codons on mRNA, and this determines the order that the amino acids are joined together with peptide bonds.
- mRNA can be intercepted by siRNA or miRNA to stop it from being translated. This is called RNA interference and is a method of regulating gene expression.
