The tRNA molecules are key to the translation process of the mRNA sequence into the amino acid sequence of proteins (at least one type of tRNA for every amino acid).
Prokaryotes, having no nucleus, transcribe the DNA sequence into mRNA, and the mRNA sequence is translated into the protein sequence with the intermediate tRNA molecules, which have the anti-codon information covalently linked to the corresponding amino acid.
This annealing of an RNA strand to its complementary DNA strand is called hybridization and plays a crucial role in the transcription and translation of genetic sequences into protein sequences.
Eventually, the ribosome will come to a stop codon. The three stop codons don't code for any amino acids, and so the process comes to a halt. The protein chain produced up to that point is then released from the ribosome, and then folds itself up into its secondary and tertiary structures.
We left the messenger RNA a little while back with part of a ribosome attached to it at the AUG start codon. The diagram shows this, together with a small part of the RNA base sequence downstream of the start codon needed to make an imaginary protein chain. The bases upstream of the start codon aren't relevant to us once the ribosome has found the place to start from.
The entire protein would actually require a much longer sequence of DNA bases.
Whenever a protein needs to be made, the correct DNA sequence for that protein is copied to a molecule called (mRNA).
You will remember that messenger RNA contains a sequence of bases which, read three at a time, code for the amino acids used to make protein chains. Each of the sets of three bases is known as a . The table below repeats one from the previous page:
There is a length of RNA upstream of the start codon which isn't actually used to build the protein chain. So how does the system know where to start? How does it find the right AUG codon from all the ones which are probably strung out along the RNA to code for the amino acid methionine?
and projection along helical axis
Incidentally, it was the structure of the -helix in proteins that led most structural biologist to look for an analogous structure in nucleic acids, i.e., helical or double helical structures with the bases sticking away from the helix center as do the residues in protein helices.
At the 3' end of every transfer RNA molecule, the chain ends with the sequence of bases C C A. Remember that the bases in RNA and DNA are attached to a backbone of alternating phosphate and sugar groups. At the very end of the chain is the -OH group on the 3' carbon of a ribose ring.
Since the DNA is a double helix formed by two complementary strands, the anti-sense strand is transcribed into mRNA resulting in the +sense on the mRNA level (with U instead of T).