Figure 1. Figure 2. Cells access the information stored in DNA by creating RNA to direct the synthesis of proteins through the process of translation. Proteins within a cell have many functions, including building cellular structures and serving as enzyme catalysts for cellular chemical reactions that give cells their specific characteristics.
If DNA serves as the complete library of cellular information, mRNA serves as a photocopy of specific information needed at a particular point in time that serves as the instructions to make a protein.
The mRNA then interacts with ribosomes and other cellular machinery Figure 3 to direct the synthesis of the protein it encodes during the process of translation see Protein Synthesis. Figure 3. In eukaryotes, synthesis, cutting, and assembly of rRNA into ribosomes takes place in the nucleolus region of the nucleus, but these activities occur in the cytoplasm of prokaryotes. Neither of these types of RNA carries instructions to direct the synthesis of a polypeptide, but they play other important roles in protein synthesis.
Ribosomes are composed of rRNA and protein. The rRNA ensures the proper alignment of the mRNA, tRNA, and the ribosomes; the rRNA of the ribosome also has an enzymatic activity peptidyl transferase and catalyzes the formation of the peptide bonds between two aligned amino acids during protein synthesis. Although rRNA had long been thought to serve primarily a structural role, its catalytic role within the ribosome was proven in Because of the importance of this work, Steitz shared the Nobel Prize in Chemistry with other scientists who made significant contributions to the understanding of ribosome structure.
It carries the correct amino acid to the site of protein synthesis in the ribosome. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to be inserted in the polypeptide chain being synthesized Figure 4.
Any mutations in the tRNA or rRNA can result in global problems for the cell because both are necessary for proper protein synthesis Table 1. Figure 4. A tRNA molecule is a single-stranded molecule that exhibits significant intracellular base pairing, giving it its characteristic three-dimensional shape. Thus, RNA clearly does have the additional capacity to serve as genetic information.
Gopal et al. What about the spatial extent of these mRNAs in comparison with the proteins they code for? By way of contrast, mRNA is more likely to have a linear structure punctuated by secondary structures in the form of hairpin stem-loops and pseudoknots, but is generally much more diffuse and extended.
This is an overestimate since these structures are riddled with branches and internal loops which will shorten the overall linear dimension.
Recent advances have made it possible to visualize large RNA molecules in solution using small angle X-ray scattering and cryo EM as shown in Figure 2.
One of the useful statistical measures of the spatial extent of such structures is the so-called radius of gyration which can be thought of as the radius of a sphere of an equal effective size. Hence, contrary to the expectation of our uncoached intuition, we note that like the mass ratio, the spatial extent of the characteristic mRNA is about 10 fold larger than the characteristic globular protein. Which is bigger, mRNA or the protein it codes for? The genes in DNA encode protein molecules, which are the "workhorses" of the cell , carrying out all the functions necessary for life.
For example, enzymes, including those that metabolize nutrients and synthesize new cellular constituents, as well as DNA polymerases and other enzymes that make copies of DNA during cell division , are all proteins.
In the simplest sense, expressing a gene means manufacturing its corresponding protein, and this multilayered process has two major steps.
The resulting mRNA is a single-stranded copy of the gene, which next must be translated into a protein molecule. Figure 1: A gene is expressed through the processes of transcription and translation. The pre-mRNA is processed to form a mature mRNA molecule that can be translated to build the protein molecule polypeptide encoded by the original gene. Figure Detail During translation , which is the second major step in gene expression , the mRNA is "read" according to the genetic code , which relates the DNA sequence to the amino acid sequence in proteins Figure 2.
Each group of three bases in mRNA constitutes a codon , and each codon specifies a particular amino acid hence, it is a triplet code. The mRNA sequence is thus used as a template to assemble—in order—the chain of amino acids that form a protein.
Figure 2: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. The codons are written 5' to 3', as they appear in the mRNA. Figure Detail But where does translation take place within a cell? What individual substeps are a part of this process? And does translation differ between prokaryotes and eukaryotes? The answers to questions such as these reveal a great deal about the essential similarities between all species. Within all cells, the translation machinery resides within a specialized organelle called the ribosome.
In eukaryotes, mature mRNA molecules must leave the nucleus and travel to the cytoplasm , where the ribosomes are located. On the other hand, in prokaryotic organisms, ribosomes can attach to mRNA while it is still being transcribed.
In all types of cells, the ribosome is composed of two subunits: the large 50S subunit and the small 30S subunit S, for svedberg unit, is a measure of sedimentation velocity and, therefore, mass. Each subunit exists separately in the cytoplasm, but the two join together on the mRNA molecule. The tRNA molecules are adaptor molecules—they have one end that can read the triplet code in the mRNA through complementary base-pairing, and another end that attaches to a specific amino acid Chapeville et al.
The idea that tRNA was an adaptor molecule was first proposed by Francis Crick, co-discoverer of DNA structure, who did much of the key work in deciphering the genetic code Crick, 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.
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