Test: Gene Control - 2 - MCAT MCQ

An operon is a regulatory unit in bacterial and some eukaryotic genomes that consists of a cluster of genes with related functions, along with the regulatory elements that control their expression. It typically includes a promoter, operator, and one or more structural genes. The promoter is the DNA sequence where RNA polymerase binds to initiate transcription. The operator is a regulatory region where a repressor protein can bind and control the transcription of the genes within the operon.

The operon concept was first described by François Jacob and Jacques Monod in their studies of the lac operon in E. coli. The operon allows for the coordinated expression of multiple genes involved in a specific metabolic pathway or cellular function. By controlling the expression of these genes together, the cell can efficiently respond to environmental changes and regulate the production of proteins needed for specific functions.

In a bacterium possessing the lac operon, when glucose is low and lactose is abundant, the lac operon is induced to allow the transport and metabolism of lactose. In this condition, the lac repressor protein is not bound to the operator region of the operon, and as a result, the lacZ and lacY genes are transcribed.

The lacY gene encodes for lactose permease, a transporter protein that facilitates the uptake of lactose into the bacterial cell. When lactose is abundant, the presence of lactose molecules induces the expression of the lacY gene and increases the production of lactose permease. This enhances the transport of lactose into the cell, enabling the bacterium to utilize lactose as an energy source in the absence of glucose.

Therefore, option D is the correct answer as it describes the enhanced transport of lactose into the bacterial cell in the presence of low glucose and abundant lactose.

Allolactose, an isomer of lactose, acts as an inducer of the lac operon in bacteria. When allolactose is present, it binds to the lac repressor protein, causing a conformational change in the repressor. This conformational change prevents the repressor from binding to the operator region of the lac operon.

As a result, the lac operon is derepressed, allowing for the transcription and expression of the genes involved in lactose metabolism.

Therefore, option C is the correct answer as it describes the effect of allolactose on inducing the lac operon by inducing a conformational change in the repressor protein.

Deacetylation of histones involves the removal of acetyl groups from histone proteins, leading to the tightening or coiling of the chromatin structure. This compacted state of chromatin makes it less accessible to the transcriptional machinery, such as transcription factors and RNA polymerase, thereby inhibiting gene expression.

Methylation of CpG islands, which are regions of DNA rich in CpG dinucleotides, plays a crucial role in the stable silencing of DNA during cellular differentiation. CpG islands are often found in the promoter regions of genes, and methylation of these CpG sites can lead to gene silencing by inhibiting the binding of transcription factors and other proteins necessary for gene expression. This process is particularly important during cellular differentiation when specific sets of genes need to be turned off or silenced in order to establish and maintain specialized cell types with distinct functions.

The role of an activator is to enhance the interaction between RNA polymerase and the promoter region of a gene. Activator proteins are transcription factors that bind to specific DNA sequences known as enhancer regions, which are usually located upstream or downstream of the promoter. By binding to the enhancer, the activator protein can interact with other components of the transcription machinery, including RNA polymerase, and facilitate their recruitment to the promoter.

The activator protein helps to stabilize the formation of the transcription initiation complex and promotes efficient transcriptional initiation. It can also interact with other coactivator proteins to enhance the transcriptional activity of RNA polymerase.

The spliceosome is a complex of RNA and proteins that plays a crucial role in the process of RNA splicing. RNA splicing is a post-transcriptional modification in which introns, non-coding regions within a pre-mRNA molecule, are removed and the remaining exons are joined together to produce a mature mRNA molecule that can be translated into a protein.

The spliceosome recognizes specific sequences at the boundaries of introns, known as splice sites, and catalyzes the cleavage of the RNA at these sites. It then brings together the adjacent exons and ligates them, resulting in the removal of the intron and the formation of a continuous coding sequence.

In summary, the function of the spliceosome is to cleave introns from RNA molecules and ligate the cut ends, allowing for the production of mature mRNA molecules that can be translated into proteins.

The 5’ terminus of mRNA is involved in various functions, but promoting polyadenylation of the 3’ terminus is not one of them. Polyadenylation is the process of adding a poly(A) tail to the 3’ end of mRNA and is important for stability and translation efficiency.

MicroRNAs (miRNAs) primarily function in gene regulation by silencing gene expression at the post-transcriptional level. They do so through translational repression or target degradation.

When miRNAs bind to target messenger RNA (mRNA) molecules, they can inhibit translation, preventing the synthesis of the corresponding protein. This translational repression occurs through base pairing between the miRNA and the complementary sequences on the target mRNA, leading to the formation of an RNA-induced silencing complex (RISC). The RISC complex inhibits translation initiation or promotes mRNA degradation, ultimately reducing the protein levels.

A proto-oncogene is a normal gene that has the potential to become an oncogene, which is a gene that promotes cancer development. Various genetic alterations can convert a proto-oncogene into an oncogene. These alterations can lead to uncontrolled cell growth and division, contributing to tumor formation.

Increased repressor binding: This option does not represent a mechanism for the conversion of a proto-oncogene into an oncogene. Increased binding of repressor proteins to a proto-oncogene would generally inhibit its expression and reduce its potential to promote cancer development.