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Cells need translation to stay alive, and understanding how it works (so we can shut it down with antibiotics) can save us from bacterial infections. Let's take a closer look at how translation happens, from the first step to the final product.
The genetic code
In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons. Here are some key features of codons to keep in mind as we move forward:
There are 616161 different codons for amino acids
Three “stop” codons mark the polypeptide as finished
One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine)
These relationships between mRNA codons and amino acids are known as the genetic code (which you can explore further in the genetic code article).
In translation, the codons of an mRNA are read in order (from the 5' end to the 3' end) by molecules called transfer RNAs, or tRNAs.
Each tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing. The other end of the tRNA carries the amino acid that's specified by the codon.
Translation: Beginning, middle, and end
A book or movie has three basic parts: a beginning, middle, and end. Translation has pretty much the same three parts, but they have fancier names: initiation, elongation, and termination.
Initiation ("beginning"): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin.
Elongation ("middle"): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.
Termination ("end"): in the last stage, the finished polypeptide is released to go and do its job in the cell.
Our polypeptide now has all its amino acids-does that mean it's ready to to its job in the cell?
Not necessarily. Polypeptides often need some "edits." During and after translation, amino acids may be chemically altered or removed. The new polypeptide will also fold into a distinct 3D structure, and may join with other polypeptides to make a multi-part protein.
Many proteins are good at folding on their own, but some need helpers ("chaperones") to keep them from sticking together incorrectly during the complex process of folding.
Some proteins also contain special amino acid sequences that direct them to certain parts of the cell. These sequences, often found close to the N- or C-terminus, can be thought of as the protein’s “train ticket” to its final destination. For more about how this works, see the article on protein targeting.
30 сен 2024
