You can have two start codons
Translation: synthesis of protein
An mRNA molecule can be used for translation several times in a row, i.e. several identical protein molecules can be synthesized from one mRNA molecule. The word translation comes from the Latin "transferre" and means to transfer. And that is exactly what the ribosomes do: they transfer the codon sequence on the mRNA, determined by the start codon AUG, into the amino acid sequence in the protein.
In addition to the mRNA and the ribosomes, of course you also need the building blocks of proteins, the amino acids. The cell can build up many amino acids in its metabolism, but some are essential, i.e. the cell cannot synthesize them itself. These essential amino acids must therefore be ingested with food so that the cell can build its proteins. The amino acids are transported to the ribosomes by means of a "connecting molecule" specific for each amino acid, the transfer RNA or tRNA for short. These connecting molecules get their name from the fact that they indirectly bring the amino acid to which they are connected at one end into contact with the mRNA. When viewed in two dimensions, it resembles a clover leaf.
The ribosome can be imagined as a bead through which the "mRNA string" is pulled. Inside the pearl, in the tunnel, the connecting molecules ("adapters") with the 20 different amino acids are waiting. On the opposite side of the amino acid binding site, the adapters have a structure that we call Anticodon denote: Anticodons consist of base triplets that are complementary to the mRNA codons. When threading the mRNA, the start codon AUG is the first to enter the tunnel. The connecting molecule, which carries the base sequence UAC as an anticodon, can now pair with the mRNA. On the other hand, this connecting molecule carries the amino acid methionine, which is encoded by the start codon AUG according to the genetic code. This defines the reading frame for the mRNA, i.e. it is now rasterized in triplets or codons (shifting this frame by just one place would result in a completely different - usually a functionless - protein).
In the next step, the mRNA is threaded a little further and the next matching connecting molecule, which in turn carries the amino acid that matches the codon on the mRNA, can take its place via the base pairing. The amino acids of the two connecting molecules are now right next to each other and are linked by an enzyme that is active in the ribosome. This process is repeated until all of the codons on the mRNA have been sequentially occupied by connecting molecules with their anticodons and the corresponding amino acids have been linked to form the protein. While the amino acid chain grows at one end, the connecting molecules detach from the other, already finished end. When the mRNA is threaded through the ribosome so far that a Termination codon reaches the tunnel at its end, translation is terminated because there are no connecting molecules for these codons. The finished amino acid chain folds up at the end to form a spatial structure, this is what emerges biologically active protein .
The mRNA copy produced during transcription travels to the protein synthesis factories, the ribosomes, where the second step in converting the DNA information into protein products takes place. The mRNA is threaded into the ribosomes and, starting from the start codon AUG, which defines the reading frame, forms base triplet pairings one after the other with suitable connecting molecules.
For all codons on the mRNA that define an amino acid according to the genetic code, there is a connecting molecule with a suitable anticodon structure. The connecting molecules with the matching, complementary anticodon are connected on the opposite side to the amino acid which, according to the genetic code, corresponds to the mRNA codon. The amino acids on the connecting molecules, which are lined up next to one another by the codon-anticodon base pairings of the RNA with the connecting molecules, are linked to one another by enzymes in the ribosome.
In this way, the resulting protein grows until a stop codon is reached: there is no connecting molecule with a suitable anticodon for this sequence of three, the translation stops. The finished protein is released from the factory (the ribosome). With the anticodon structure, the connecting molecule, transfer RNA, binds to the codons of the messenger RNA (mRNA). The amino acid that corresponds to the rRNA codon according to the genetic code is coupled to the amino acid binding site. The single-stranded tRNA has double-stranded areas that are created by base pairing of different parts of the single strand.
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