It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site. Sections of the mRNA that do not code for amino acids, called introns, are removed. A poly-A tail, consisting of several adenine bases, is added to one end of the mRNA, while a guanosine triphosphate cap is added to the other end.
Ime Ica Each amino acid is specified by one or more nucleotide triplets codons in the DNA. Translation takes place on ribosomes, complexes of protein and rRNA that serve as the molec- ular machines coordinating the interactions between mRNA, tRNA, the enzymes, and the pro- tein factors required for protein synthesis.
Many proteins undergo posttranslational modifica- tions as they prepare to assume their ultimate roles in the cell. Important features of the genetic code include:. Each codon consists of three bases triplet. There are 64 codons. They are all written in the 5' to 3' direction. So this is a strand of tRNA, you get a sense of, okay, it's a sequence of RNA right over here, this it's, I guess you could say, you could think of it, it's two dimensional structure.
But then it wraps around itself to form this fairly complex molecule. And the anticodon, which is right here, it's kind of in the middle of the sequence, it forms the basis for this end of the molecule, that's the part that's gonna pair with the codon on the mRNA, and then at the other end of the molecule, at the other end of the molecule is where you actually bind to the appropriate amino acid.
So I know what you're thinking, alright, I see that the ribosome, it knows where to start, it starts at the start codon. I see how the appropriate tRNA can bring the appropriate amino acid, but how does the chain actually form? And you can view this in three steps, and associated with those three steps are three sites on the ribosome.
And the three sites, we call this the A-site, you're not gonna be able to see it if I write it in black. A, or yellow, alright, let me write it in blue. So that is the A-site. This is the P-site, and this is the E-site. And I'll talk in a second why we call them A, P and E.
And so you can see, we're starting the translation process, the next thing that's going to happen is another tRNA, the one that is, that matches, that has an anticodon that matches the UAU, that's going to bond over here on the A-site, and it's bringing the appropriate amino acid with it, it's bringing the tyrosine with it.
So why is that called the A-site? Well A stands for aminoacyl. An easy way to remember it it's the tRNA, it's the place where the tRNA that's bound to the amino acid, just one amino acid is going to bind on the ribosome. And so once that happens, once this character comes here, let me draw that. Once this character comes right over here, it's gonna be AUA, and it's bound to the tyrosine. Well then you could have a peptide bond form between the two amino acids, and the ribosome, and the ribosome itself can move to the right.
So this, this tRNA will then be in the E-site. So what this, what do the P and E sites stand for? Well you can see a little bit more clearly right over here. So the P-site is where you have the polypeptide chain actually forming, and, so the P-site is often, well, one way to remember it is is that's where you have the polypeptide chain, and now you have a new, you have a new A-site where you can bring in a new amino acid.
Nature of the genetic code. The use of synthetic oligo- and polynucleotides in deciphering the code. Structure of the code: relatedness of synonym codons.
The wobble hypothesis. Chain initiation and N-formyl-methionine. Chain termination and nonsense codons. Mistakes in translation: ambiguity in vitro. Suppressor mutations resulting in ambiguity. Limitations in the universality of the code.
Attempts to determine the particular codons used by a species. Mechanisms of suppression, caused by a abnormal aminoacyl-tRNA, b ribosomal malfunction. Effect of streptomycin. The problem of "reading" a nucleic acid template. Different ribosomal mutants and DNA polymerase mutants might cause different mistakes.
The possibility of involvement of allosteric proteins in template reading. Selected References These references are in PubMed. This may not be the complete list of references from this article. J Exp Med. Genetic regulatory mechanisms in the synthesis of proteins. J Mol Biol. Side-chain interactions governing the pairing of half-cystine residues in ribonuclease. J Biol Chem.
And the anticodon, which is right here, it's kind of in the middle of the sequence, it forms the basis for this end of the molecule, that's the part that's gonna pair with the codon on the mRNA, and then at the other end of the molecule, at the other end of the molecule is where you actually bind to the appropriate amino acid.
Really just give you the conceptual idea of what happens. So this might be part of a gene Actually whoops, let me make sure I'm using the right tool. And you can see a little bit visually here, this is all review, we covered a lot of this in the video on transcription and the overview video on transcription and translation, is if you look at a eukaryotic cell and the bacteria in a prokaryotic cell, it's analogous, you just don't have the nuclear membrane, and you're not gonna do the processing step that I'm gonna talk about in a little bit and we went in detail on the video on transcription. And then I copy and then I paste, and it's just like that. Transcription Genetics Translation DNA Video transcript - [Voiceover] We've already talked about how DNA's structure as this double helix, this twisted ladder, makes it suitable for being the molecular basis of heredity.
Ribonucleic acid, let me write that down. Gene action. And you can see a little bit visually here, this is all review, we covered a lot of this in the video on transcription and the overview video on transcription and translation, is if you look at a eukaryotic cell and the bacteria in a prokaryotic cell, it's analogous, you just don't have the nuclear membrane, and you're not gonna do the processing step that I'm gonna talk about in a little bit and we went in detail on the video on transcription.
And so once that happens, once this character comes here, let me draw that. Nature of the genetic code. And they also, you might have more than one codon coding for the same amino acid. Different ribosomal mutants and DNA polymerase mutants might cause different mistakes.
And so it turns out that 61 of the codons, let me write this down. The DNA molecule can be tens of millions of base pairs long. Polyadenylation of pre-mRNA occurs in the nucleoplasm. Note: The 5 values are determined by behavior of the particles in an ultracentrifuge.
And when you get to one of the stop codons, then the appropriate polypeptide is going to be released, and we will have created this thing that could either be a protein, or part of a protein, so this is very exciting, because this is happening in your cells as we speak. Attempts to determine the particular codons used by a species. So how does DNA replicate? So the questions are well how does this thing happen? The R, let me make it clear where the RNA come from, the R is right over there, then you have the nucleic, that's the n, and then it's a, acid.
N-formylmethionyl-sRNA as the initiator of protein synthesis. It's a new tool I'm using, so let me make sure I'm doing it right. So maybe I'll do the new sugar phosphate backbone in yellow. Well let's think about it, how many, how many possible three letter sequences are there? Genetic regulatory mechanisms in the synthesis of proteins.
It is now ready to exit, and that's why it's called the E-site. N-formylmethionyl-sRNA as the initiator of protein synthesis.