Question

All of the following describe an operon except A. it is a control mechanism for eukaryotic genes. B. it includes structural genes. C. it is expected to code for polycistronic mRNA. D. it contains control sequences such as an operator. E. it can have multiple promoters.

Short answer

Answer: A. it is a control mechanism for eukaryotic genes.

Step-by-step solution

Statement A

The first statement says that an operon is a control mechanism for eukaryotic genes. However, operons are known to be found in prokaryotes, not eukaryotes. Eukaryotes have a different mechanism of gene regulation. So, this statement does not describe an operon.

Statement B

The second statement indicates that an operon includes structural genes. This is true, as these structural genes are coding sequences for proteins and are transcribed together under the control of a common promoter.

Statement C

The statement claims that an operon is expected to code for polycistronic mRNA. This is true because the co-transcription of multiple genes in an operon results in the production of a single polycistronic mRNA, which contains multiple coding sequences for different polypeptides.

Statement D

The fourth statement mentions that an operon contains control sequences such as an operator. This is correct, as the operator is a DNA sequence within an operon, which regulatory proteins can bind to, affecting the transcription of the structural genes in the operon.

Statement E

The fifth statement says that an operon can have multiple promoters. This is true, as some operons can have multiple promoters, and the use of these alternative promoters could modulate the transcription of the structural genes in different contexts. Based on the analysis, the correct answer is: A. it is a control mechanism for eukaryotic genes.

Key concepts

Prokaryotic Gene Regulation

Understanding how genes are regulated in prokaryotes is essential for comprehending how these organisms adapt to different environments. Unlike eukaryotic cells, prokaryotic cells — those without a distinct nucleus, like bacteria — manage gene regulation primarily through structures called operons. An operon is a cluster of genes that function together and are transcribed as a single mRNA molecule. A primary feature of operon-mediated gene regulation is the coordinated expression of functionally related genes.

Control of these gene clusters is typically under the jurisdiction of regulatory elements such as promoters, operators, and sometimes activator binding sites. The promoter is a DNA sequence that signals the RNA polymerase where to start transcription. The operator is another sequence that can either enable or block transcription, acting as a switch controlled by proteins called repressors or activators. When conditions are favorable for the expression of operon's genes, activators may bind or repressors may be released, paving the way for RNA polymerase to transcribe the genes. Conversely, under unfavorable conditions, repressors can bind to the operator, preventing the transcription of the genes in the operon.

This model of gene regulation is efficient, as it allows the bacteria to quickly respond to environmental changes by adjusting the production of proteins necessary for survival, with minimal energy expenditure.

Polycistronic mRNA

Polycistronic mRNA is a single molecule of mRNA that encodes multiple different protein products. This term is derived from 'cistron', another word for a gene that encodes a single protein. In prokaryotes, polycistronic mRNA is a common result of the operon system, which entails the transcription of a group of genes into one single mRNA transcript. This means that when this mRNA is translated by the cell's machinery, it will produce several proteins from the multiple coding sequences contained within it.

Such a system is a significant departure from the eukaryotic approach where one mRNA usually corresponds to a single gene product, a concept known as monocistronic mRNA. Polycistronic mRNA is advantageous in prokaryotic organisms because it maximizes the efficient use of genetic material and permits the coordinated regulation of genes with related functions. For instance, if all genes needed for a metabolic pathway are on the same operon and hence the same mRNA, it ensures their synchronous regulation and the production of each protein at balanced levels, which is critical for the pathway's proper functionality.

Gene Control Sequences

Gene control sequences are essential DNA elements that regulate the expression of genes. They are often located near the genes they control and consist of several different types of sequences, each with a particular role in gene regulation. One of the key components is the promoter region, which acts as a docking site for RNA polymerase, the enzyme responsible for transcribing DNA into RNA.

Another crucial control sequence is the operator, which serves as a regulatory gate. When a repressor protein binds to the operator, it blocks the progression of RNA polymerase along the DNA, thereby halting transcription. The regulator gene, which is often located just upstream or within an operon, codes for the repressor or activator proteins that interact with the operator. Additional control elements, such as enhancers and silencers, can further up-regulate or down-regulate gene expression by binding transcription factors, but these are typically more prominent in eukaryotic gene expression.

Understanding how these control sequences interact with transcriptional machinery and regulatory proteins is vital to grasping the intricate dance of gene expression. The operon model exemplifies the use of control sequences in prokaryotic gene regulation, providing a framework for how genes adapt to their cellular environment by being switched on or off according to cellular needs.