Which of the following describes polycistronic mRNA?
a) It codes for only a single protein
b) It is present in eukaryotes
c) It is transcribed from a single gene
d) It is present in prokaryotes
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Understanding prokaryotic gene expression is essential for the MCAT exam. In prokaryotes, gene expression is tightly regulated. Structural proteins that have related functions are encoded together within the genome in various blocks called operons. An operon is a DNA sequence that includes a promoter, an operator, and the genes that are regulated together. The promoter is a DNA sequence that is recognized by RNA polymerase for transcription. The operator is a DNA sequence that a repressor can bind to, and repressors are proteins that can either allow or inhibit transcription. The set of genes in an operon are transcribed together by way of the promoter, creating a polycistronic transcript. In other words, multiple proteins are encoded by a single mRNA molecule.
In many ways, prokaryotic gene regulation through the use of an operon is extremely efficient. In order to carry out a particular biochemical process, instead of producing a separate mRNA for each protein or enzyme required for the process, prokaryotes can produce a single mRNA for all of the necessary proteins and enzymes in one nucleotide molecule.
Prokaryotic gene expression can be controlled in two ways: positive control and negative control. In negative control of prokaryotic gene expression, a repressor protein will bind to the operator. When this happens, RNA polymerase is blocked, so transcription cannot occur. However, the repressor protein is regulated as well. Whether or not the regulator protein will be able to bind to the operator depends on the presence or absence of an inducer molecule. An inducer molecule will bind to the repressor protein and alter its conformation. When the conformation of the repressor protein is changed, this will either allow or prevent the repressor protein from binding to the operator, so transcription can freely occur.
The lac operon is a negative inducible operon. It is ‘negative’ because it is under negative control, meaning it involves a repressor protein. ‘Inducible’ means that the lac operon is an operon whose transcription can be induced, or turned on, under the right conditions. The lac operon codes for proteins involved in the metabolism of lactose, and includes a promoter, an operator, and the genes for lactose metabolism. Initially, in the absence of lactose, the repressor protein is bound to the operator (Figure 1). When the repressor protein is in this position, it prevents RNA polymerase from transcribing the operon. In this way, no mRNA is produced.
However, if lactose is present, lactose will bind to the repressor (Figure 2). When this happens, the repressor will no longer be able to bind to the operator. In this way, RNA polymerase is free to transcribe the operon, resulting in the production of the enzymes for lactose metabolism. This process is quite logical. In the absence of lactose, the prokaryotic cell should not be making proteins and enzymes for lactose metabolism. However, in the presence of lactose, it would be beneficial for the cell to make proteins and enzymes for lactose metabolism.
The Trp Operon is a negative repressible operon. It is ‘negative’ because it is under negative control, meaning it involves a repressor protein. ‘Repressible’ means that the activity of this operon is usually on, and it can be repressed or turned off. The Trp operon encodes the proteins for tryptophan biosynthesis and has much the same structure as the lac operon, including a promoter, an operator, and all the genes for tryptophan biosynthesis. If tryptophan is at low concentrations in the cell, the repressor protein is inactive. In this way, the trp operon is transcribed, and the proteins for tryptophan biosynthesis are made (Figure 3).
However, at high concentrations of tryptophan, tryptophan itself acts as the repressor molecule and binds to the repressor protein (Figure 4). When the repressor protein binds tryptophan, the repressor protein undergoes a conformational change that allows it to bind to the operator. The binding of the repressor protein to the operator blocks RNA polymerase from transcribing the operon. In this way, the proteins and enzymes for tryptophan biosynthesis can no longer be generated.
Again, this process makes sense, given that tryptophan is an essential amino acid, and its production takes place within the cell. If the cell does not have enough tryptophan, then it should be able to make the enzymes to produce more. Likewise, if there is sufficient tryptophan or even too much tryptophan in the cell, then the tryptophan should act as a negative feedback regulator, and prevent transcription of mRNA for its production.
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