The rRNA catalyzes the attachment of each new amino acid to the growing chain. Interestingly, not all regions of an mRNA molecule correspond to particular amino acids. In particular, there is an area near the 5' end of the molecule that is known as the untranslated region UTR or leader sequence. This portion of mRNA is located between the first nucleotide that is transcribed and the start codon AUG of the coding region, and it does not affect the sequence of amino acids in a protein Figure 3.
So, what is the purpose of the UTR? It turns out that the leader sequence is important because it contains a ribosome-binding site. A similar site in vertebrates was characterized by Marilyn Kozak and is thus known as the Kozak box. If the leader is long, it may contain regulatory sequences, including binding sites for proteins, that can affect the stability of the mRNA or the efficiency of its translation.
Figure 4: The translation initiation complex. When translation begins, the small subunit of the ribosome and an initiator tRNA molecule assemble on the mRNA transcript. The small subunit of the ribosome has three binding sites: an amino acid site A , a polypeptide site P , and an exit site E. Here, the initiator tRNA molecule is shown binding after the small ribosomal subunit has assembled on the mRNA; the order in which this occurs is unique to prokaryotic cells.
In eukaryotes, the free initiator tRNA first binds the small ribosomal subunit to form a complex. Figure Detail Although methionine Met is the first amino acid incorporated into any new protein, it is not always the first amino acid in mature proteins—in many proteins, methionine is removed after translation. In fact, if a large number of proteins are sequenced and compared with their known gene sequences, methionine or formylmethionine occurs at the N-terminus of all of them.
However, not all amino acids are equally likely to occur second in the chain, and the second amino acid influences whether the initial methionine is enzymatically removed. For example, many proteins begin with methionine followed by alanine.
In both prokaryotes and eukaryotes, these proteins have the methionine removed, so that alanine becomes the N-terminal amino acid Table 1. However, if the second amino acid is lysine, which is also frequently the case, methionine is not removed at least in the sample proteins that have been studied thus far. These proteins therefore begin with methionine followed by lysine Flinta et al. Table 1 shows the N-terminal sequences of proteins in prokaryotes and eukaryotes, based on a sample of prokaryotic and eukaryotic proteins Flinta et al.
In the table, M represents methionine, A represents alanine, K represents lysine, S represents serine, and T represents threonine. Once the initiation complex is formed on the mRNA, the large ribosomal subunit binds to this complex, which causes the release of IFs initiation factors. The large subunit of the ribosome has three sites at which tRNA molecules can bind. The A amino acid site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain.
The P polypeptide site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. Finally, the E exit site is the location at which the "empty" tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process.
The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond Figure 5. Figure 5: The large ribosomal subunit binds to the small ribosomal subunit to complete the initiation complex.
The initiator tRNA molecule, carrying the methionine amino acid that will serve as the first amino acid of the polypeptide chain, is bound to the P site on the ribosome. The A site is aligned with the next codon, which will be bound by the anticodon of the next incoming tRNA. Next, peptide bonds between the now-adjacent first and second amino acids are formed through a peptidyl transferase activity. For many years, it was thought that an enzyme catalyzed this step, but recent evidence indicates that the transferase activity is a catalytic function of rRNA Pierce, After the peptide bond is formed, the ribosome shifts, or translocates, again, thus causing the tRNA to occupy the E site.
The tRNA is then released to the cytoplasm to pick up another amino acid. In addition, the A site is now empty and ready to receive the tRNA for the next codon.
This process is repeated until all the codons in the mRNA have been read by tRNA molecules, and the amino acids attached to the tRNAs have been linked together in the growing polypeptide chain in the appropriate order. At this point, translation must be terminated, and the nascent protein must be released from the mRNA and ribosome.
No tRNAs recognize these codons. Thus, in the place of these tRNAs, one of several proteins, called release factors, binds and facilitates release of the mRNA from the ribosome and subsequent dissociation of the ribosome.
The translation process is very similar in prokaryotes and eukaryotes. Though the molecule folds into a 3-leaf clover structure, notice the anticodon arm in the lower segment of the molecule, with the amino acid attached at the opposite end of the molecule acceptor stem. After a polypeptide chain is synthesized, it may undergo additional processes. For example, it may assume a folded shape due to interactions among its amino acids.
It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to the Golgi apparatus to be modified for the specific job they will do.
Translation occurs at a ribosome. During translation, a protein is synthesized using the codons in mRNA as a guide. All three types of RNA play a role in translation. Making Connections. Since there are no tRNA molecules that can recognize these codons, the ribosome recognizes that translation is complete. The new protein is then released, and the translation complex comes apart. Further Exploration Concept Links for further exploration gene expression frameshift mutation nonsense mutation RNA intron exon codon amino acid chromosome mutation protein genetic code gene tRNA proteome ribosome peptide cytoplasm splicing transcription.
Related Concepts You have authorized LearnCasting of your reading list in Scitable. That translation is done not in individual letters. It's very much like the human language or any other language that, in this case, all the words are the same length. They're all three words long, and the reader in this case is something called a ribosome, which is this big, multi-subunit molecular machine that travels along the mRNA, and it reads much like a person reading Braille does.
It reads along, detects what are these letters underneath it, and when it detects what those three letters are, it decides what the amino acid is supposed to be that it adds to the growing amino acid chain, polypeptide chain, to become a protein.
0コメント