DNAunion: the last part from my personal notes on DNA/RNA
Translation
Translation is the synthesis of proteins based upon the informational content of mRNA. So for those products of transcription that are mRNA, the next step is translation (the other RNAs are also involved in translation rRNA is a component of ribosomes, and tRNA brings the correct amino acid to the ribosome, but these two types of RNA are not themselves translated). Since the informations language is actually being changed it begins as nucleotides and ends as amino acids the term translation is appropriate here (the term transcription is inappropriate as it does not express the change in languages).
Before continuing, both codons and the genetic code should be briefly discussed. As previously mentioned, three consecutive nucleotides on a mRNA molecule, which code for a particular amino acid, are called a codon. The genetic code is the association between mRNA codons (nucleotide triplets on mRNA) and the corresponding amino acids for which they code. To call this a code is quite appropriate since without the proper deciphering mechanism the information encoded in nucleotide sequences could not be discerned by either biologists or cells (biologists use a lookup table which shows the mappings between codons and amino acids; cells use the tRNA adaptor molecules to translate between the two code sets). As with any other code, a certain sequence of symbols in the encrypted message can be decrypted into the proper symbol(s) of the natural language once the code has been broken. In the genetic code, mRNA codons code for amino acids for example, the mRNA codons GGU, GGC, GGA, and GGG all code for the single amino acid glycine, always. Note that in many instances the third nucleotide in a codon is not that important for glycine, it can be U, C, A, or G, as long as the first two nucleotides are GG. This ability for the correct amino acid to be specified by codons with different nucleotides in the third position is termed wobble to indicate the codes looseness in the third position in such cases. There are 64 possible codons when taking three nucleotides at a time (43 = 64), and each of the 20 biological amino acids is specified by from 1 to 6 of these codons. In addition, codons also specify the two punctuation marks of the code: start and stop.
In cells, the association between codons and amino acids is made by tRNA. Each tRNA molecule has one site (acceptor stem) that will bind only a certain amino acid that is, the tRNA for methionine will bind the amino acid methionine only. In addition, each tRNA has on its opposite end a particular anticodon, which is a linear sequence of three nucleotides on a tRNA molecule that is complementary to a particular mRNA codon. Therefore, since a particular mRNAs codon specifies and particular tRNA (through complementary base pairing between codon and anticodon) , and that particular tRNA specifies a particular amino acid, tRNA acts to translate languages from nucleotide triplets to single amino acids.
A functional ribosome consists of two parts: an LSU (large subunit) and an SSU (small subunit). These two subunits are separate and do not join until initiation of translation occurs. A functional ribosome has two key sites: the P site (Peptidyl site so called because throughout most of translation, the growing polypeptide chain is attached to the tRNA bound to that site) and the A site (Aminoacyl site so called because throughout most of translation, this site is occupied by the tRNA that binds the next amino acid to be incorporated into the growing polypeptide) note that there is also an E site (Exit site), but it is not terribly important to a general understanding of translation. The ribosome also contains the ribozyme peptidyl transferase, which forms the peptide bond between the amino acids attached to the tRNAs in the P and A sites, and then transfers the growing polypeptide chain from the tRNA in the P site to the tRNA in the A site (this positioning of the growing polypeptide chain, passing it on to the tRNA in the A site, is only temporary - translocation of the tRNA in the A site to the P site will occur quickly such that the P site again contains the tRNA that possesses the growing polypeptide).
Translation consists of three stages: initiation, elongation, and termination. During initiation, mRNA and a particular tRNA join with the large and small subunits of the ribosome to form an initiation complex. The first mRNA codon is always the start codon, AUG, so its associated amino acid, methionine (or a modified form), is always the first amino acid in every polypeptide. However, this single translation-initiating methionine is often removed at some point after initiation once it has served its purpose and is no longer needed.
After the initiation stage, chain elongation begins. The second amino acid, which is coded for by the mRNA codon currently associated with the A site, is brought to the ribosome by the appropriate tRNA molecule. With one tRNA in the P site and one in the A site, both with an amino acid attached, the ribosomal enzyme peptidyl transferase creates a peptide bond between the two amino acids, detaches the amino acid from the tRNA in the P site, and transfers it to the amino acid associated with the tRNA docked in the A site. The tRNA in the P site, which is no longer charged, is released from the ribosome. The next phase of elongation is traslocation. During translocation, the ribosome moves exactly three nucleotides along the mRNA it is reading, which causes the mRNA-bound tRNA in the A site - with the dipeptide attached - to be moved to the just-vacated P site, and simultaneously positions the next mRNA codon into position to be read by the ribosome (that is, the next codon in sequence becomes associated with the A site). Now the process repeats. The tRNA specified by the mRNA now associated with the A site carries the correct amino acid to the ribosome and docks into the A site; peptidyl transferase peptide bonds the growing polypeptide (at this point, just a didpeptide) attached to the tRNA in the P site to the newly-arrived tRNAs amino acid and transfers the growing polypeptide chain from the P-site tRNA to the A-site tRNA; the tRNA in the P site is released; the ribosome moves exactly three nucleotides along the mRNA, moving the tRNA in the A site to the P site as well as bringing yet another mRNA codon into the A site to be read.
The series of steps that comprises elongation is repeated continually until one of the three stop codons (none of which specifies an amino acid) is reached in the mRNA, leading to termination: which involves the release of the completed polypeptide chain and the mRNA from the ribosome, followed by the dissociation of the two ribosomal subunits.
Translation
Translation is the synthesis of proteins based upon the informational content of mRNA. So for those products of transcription that are mRNA, the next step is translation (the other RNAs are also involved in translation rRNA is a component of ribosomes, and tRNA brings the correct amino acid to the ribosome, but these two types of RNA are not themselves translated). Since the informations language is actually being changed it begins as nucleotides and ends as amino acids the term translation is appropriate here (the term transcription is inappropriate as it does not express the change in languages).
Before continuing, both codons and the genetic code should be briefly discussed. As previously mentioned, three consecutive nucleotides on a mRNA molecule, which code for a particular amino acid, are called a codon. The genetic code is the association between mRNA codons (nucleotide triplets on mRNA) and the corresponding amino acids for which they code. To call this a code is quite appropriate since without the proper deciphering mechanism the information encoded in nucleotide sequences could not be discerned by either biologists or cells (biologists use a lookup table which shows the mappings between codons and amino acids; cells use the tRNA adaptor molecules to translate between the two code sets). As with any other code, a certain sequence of symbols in the encrypted message can be decrypted into the proper symbol(s) of the natural language once the code has been broken. In the genetic code, mRNA codons code for amino acids for example, the mRNA codons GGU, GGC, GGA, and GGG all code for the single amino acid glycine, always. Note that in many instances the third nucleotide in a codon is not that important for glycine, it can be U, C, A, or G, as long as the first two nucleotides are GG. This ability for the correct amino acid to be specified by codons with different nucleotides in the third position is termed wobble to indicate the codes looseness in the third position in such cases. There are 64 possible codons when taking three nucleotides at a time (43 = 64), and each of the 20 biological amino acids is specified by from 1 to 6 of these codons. In addition, codons also specify the two punctuation marks of the code: start and stop.
In cells, the association between codons and amino acids is made by tRNA. Each tRNA molecule has one site (acceptor stem) that will bind only a certain amino acid that is, the tRNA for methionine will bind the amino acid methionine only. In addition, each tRNA has on its opposite end a particular anticodon, which is a linear sequence of three nucleotides on a tRNA molecule that is complementary to a particular mRNA codon. Therefore, since a particular mRNAs codon specifies and particular tRNA (through complementary base pairing between codon and anticodon) , and that particular tRNA specifies a particular amino acid, tRNA acts to translate languages from nucleotide triplets to single amino acids.
A functional ribosome consists of two parts: an LSU (large subunit) and an SSU (small subunit). These two subunits are separate and do not join until initiation of translation occurs. A functional ribosome has two key sites: the P site (Peptidyl site so called because throughout most of translation, the growing polypeptide chain is attached to the tRNA bound to that site) and the A site (Aminoacyl site so called because throughout most of translation, this site is occupied by the tRNA that binds the next amino acid to be incorporated into the growing polypeptide) note that there is also an E site (Exit site), but it is not terribly important to a general understanding of translation. The ribosome also contains the ribozyme peptidyl transferase, which forms the peptide bond between the amino acids attached to the tRNAs in the P and A sites, and then transfers the growing polypeptide chain from the tRNA in the P site to the tRNA in the A site (this positioning of the growing polypeptide chain, passing it on to the tRNA in the A site, is only temporary - translocation of the tRNA in the A site to the P site will occur quickly such that the P site again contains the tRNA that possesses the growing polypeptide).
Translation consists of three stages: initiation, elongation, and termination. During initiation, mRNA and a particular tRNA join with the large and small subunits of the ribosome to form an initiation complex. The first mRNA codon is always the start codon, AUG, so its associated amino acid, methionine (or a modified form), is always the first amino acid in every polypeptide. However, this single translation-initiating methionine is often removed at some point after initiation once it has served its purpose and is no longer needed.
After the initiation stage, chain elongation begins. The second amino acid, which is coded for by the mRNA codon currently associated with the A site, is brought to the ribosome by the appropriate tRNA molecule. With one tRNA in the P site and one in the A site, both with an amino acid attached, the ribosomal enzyme peptidyl transferase creates a peptide bond between the two amino acids, detaches the amino acid from the tRNA in the P site, and transfers it to the amino acid associated with the tRNA docked in the A site. The tRNA in the P site, which is no longer charged, is released from the ribosome. The next phase of elongation is traslocation. During translocation, the ribosome moves exactly three nucleotides along the mRNA it is reading, which causes the mRNA-bound tRNA in the A site - with the dipeptide attached - to be moved to the just-vacated P site, and simultaneously positions the next mRNA codon into position to be read by the ribosome (that is, the next codon in sequence becomes associated with the A site). Now the process repeats. The tRNA specified by the mRNA now associated with the A site carries the correct amino acid to the ribosome and docks into the A site; peptidyl transferase peptide bonds the growing polypeptide (at this point, just a didpeptide) attached to the tRNA in the P site to the newly-arrived tRNAs amino acid and transfers the growing polypeptide chain from the P-site tRNA to the A-site tRNA; the tRNA in the P site is released; the ribosome moves exactly three nucleotides along the mRNA, moving the tRNA in the A site to the P site as well as bringing yet another mRNA codon into the A site to be read.
The series of steps that comprises elongation is repeated continually until one of the three stop codons (none of which specifies an amino acid) is reached in the mRNA, leading to termination: which involves the release of the completed polypeptide chain and the mRNA from the ribosome, followed by the dissociation of the two ribosomal subunits.
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