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 begins with the mRNA strand settling 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. 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 chain might then head to the rough endoplasmic reticulum or Golgi apparatus for folding and processing into the final 3D 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 protein at the ribosomes. This can be called called gene silencing. There a couple of different ways that this can happen.
Small interfering RNA (siRNA) are short pieces of double stranded RNA that don’t code for a protein. This is how they work:
- The siRNA associates with an enzyme and breaks apart to leave a single strand.
- The single strand 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 part of the mRNA sequence is complementary to the siRNA.
- The enzyme (which is still associated with the siRNA) cuts the mRNA into small fragments (using hydrolysis reactions) which cannot be translated.
- The small mRNA fragments are taken up into a processing body where they get degraded (broken down).
MicroRNA (miRNA) is just another method of getting the same result. In plant cells, miRNA work in very much the same way as described above for siRNA. But in mammals, the process for miRNA is a bit different:
- The double stranded miRNA associates with an enzyme and breaks apart to leave a single strand.
- The single strand binds to an mRNA, but this time the miRNA is not fully complementary to the mRNA. This means that sometimes one miRNA could bind to several different mRNAs.
- The miRNA associated with the enzyme (the miRNA-enzyme complex) acts as a physical barrier to translation because it prevents the ribosome binding to mRNA.
- The mRNA is taken up into a processing body where it can be degraded or stored. So this method gives the option of the mRNA being returned to the cytoplasm for translation at a later time because it can remain intact.
- Translation produces a polypeptide chain (protein) based on on mRNA code. 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.