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From RNA to Proteins
In protein synthesis, the translation of RNA into a polypeptide, and the involvement of the ribosomes are the most important parts of the process.
Legend to the Diagrams
Initiation
In the first step in protein synthesis, the small 30S subunit
of the ribosome
binds to the mRNA molecule (Diagram 1): this contains triplet codon (AUG, or GUG) at which protein synthesis
starts.
In bacteria, the first AA-tRNA to initiate translation is always a formyl derivative of methionine called FMet-tRNA.
The ribosome is able to discriminate between an AUG within an RNA sequence and at the beginning of mRNA. When this codon appears in the middle of messenger RNA, a normal methionine is incorporated.
In eukaryotes, synthesis is started by a special initiation Met-tRNA, but the methionine is not formylated. However, the initial methionine is usually split off from the finished polypeptide.
The start of translation requires three protein factors, and the binding of an initiator Fmet-tRNA (or Met) to the first AUG codon found on mRNA.
Step 1 of Elongation
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 The reaction between the first amino acid and the second one leads to the formation of a peptide bond . A molecule of water is released ( it is
a condensation reaction) (Diagram 3b). This only happens after hydrolysis of a GTP into GDP which allows the elongation factor to leave. This delay allows for proof reading as a wrong tRNA would leave before the reaction takes place. |
Step 2 of Elongation
The second tRNA has now moved into place and the now free tRNA has been released. Translocation can take place: it is the transfer of the newly formed dipeptide into the peptidyl site (the first one, also called donor site) (Diagram 4) when the ribosome shifts 3 nucleotides. |
The third AA (R)-tRNA can then bind to the mRNA / ribosome complex, and a new peptide bond is formed (Diagram 5). |
Step 3 of Elongation
This step is a repeat of step two, to show that once the process starts, it is fast, and repetitive. Diagram 6 a and b
To summarize this, why not see the movie?
Termination
Termination of the polypeptide occurs when the ribosome reaches a "Stop" Codon.
Chain termination leads to the release of a polypeptide, and tRNA, and the dissociation of the ribosome into 30S and 50S subunits. Stop codons are triplets which are not recognized by any tRNA (UAA, UAG, UGA), but by two proteins: the releasing factors (R), (R1 recognizes UAG and UAA, R2 recognizes UAA and UGA).
The polypeptide released will be processed in different parts of the cell, depending on its role, and destination. All the processing involved depends on the polypeptide sequence, therefore on the mRNA sequence (and therefore on the original DNA base sequence).
Translation on Wikipedia
Related Links
-YouTube: Transcription and TranslationThis is another clip from the PBS production DNA: The Secret of Life. A Windfall Films Production fo