Chapter 18

If a particular operon encodes enzymes for making an essential amino acid and is regulated like the \(trp\) operon, then \begin{equation} \begin{array}{l}{\text { (A) the amino acid inactivates the repressor. }} \\\ {\text { (B) the repressor is active in the absence of the }} \\ {\text { amino acid. }} \\ {\text { (C) the amino acid acts as a corepressor. }} \\\ {\text { (D) the amino acid turns on transcription of the operon. }}\end{array} \end{equation}

Muscle cells differ from nerve cells mainly because they \begin{equation} \begin{array}{l}{\text { (A) express different genes. }} \\ {\text { (B) contain different genes. }} \\ {\text { (C) use different genetic codes. }} \\\ {\text { (D) have unique ribosomes. }}\end{array} \end{equation}

The functioning of enhancers is an example of $$\begin{array}{l}{(\text { A) a eukaryotic equivalent of prokaryotic promoter functioning. }} \\ {\text { (B) transcriptional control of gene expression. }} \\ {\text { (C) the stimulation of translation by initiation factors. }} \\\ {\text { (D) post-translational control that activates certain proteins. }}\end{array}$$

Cell differentiation always involves \begin{equation} \begin{array}{l}{\text { (A) transcription of the myoD gene. }} \\ {\text { (B) the movement of cells. }} \\ {\text { (C) the production of tissue- specific proteins. }} \\ {\text { (D) the selective loss of certain genes from the genome. }}\end{array} \end{equation}

Which of the following is an example of post-transcriptional control of gene expression? \begin{equation} \begin{array}{l}{\text { (A) the addition of methyl groups to cytosine bases of DNA }} \\ {\text { (B) the binding of transcription factors to a promoter }} \\ {\text { (C) the removal of introns and alternative splicing of exons }} \\ {\text { (D) gene amplification contributing to cancer }}\end{array} \end{equation}

What would occur if the repressor of an inducible operon were mutated so it could not bind the operator? \begin{equation} \begin{array}{l}{\text { (A) irreversible binding of the repressor to the promoter }} \\ {\text { (B) reduced transcription of the operon's genes }} \\\ {\text { (C) buildup of a substrate for the pathway controlled by the }} \\\ {\text { operon }} \\ {\text { (D) continuous transcription of the operon's genes }}\end{array} \end{equation}

Absence of bicoid mRNA from a Drosophila egg leads to the absence of anterior larval body parts and mirror-image duplication of posterior parts. This is evidence that the product of the bicoid gene \begin{equation} \begin{array}{l}{\text { (A) normally leads to formation of head structures. }} \\ {\text { (B) normally leads to formation of tail structures. }} \\\ {\text { (C) is transcribed in the early embryo. }} \\ {\text { (D) is a protein present in all head structures. }}\end{array} \end{equation}

Which of the following statements about the DNA in one of your brain cells is true? \begin{equation} \begin{array}{l}{\text { (A) Most of the DNA codes for protein. }} \\ {\text { (B) The majority of genes are likely to be transcribed. }} \\ {\text { (C) It is the same as the DNA in one of your liver cells. }} \\ {\text { (D) Each gene lies immediately adjacent to an enhancer. }}\end{array} \end{equation}

Within a cell, the amount of protein made using a given mRNA molecule depends partly on \begin{equation} \begin{array}{l}{\text { (A) the degree of DNA methylation. }} \\ {\text { (B) the rate at which the mRNA is degraded. }} \\ {\text { (C) the number of introns present in the mRNA. }} \\ {\text { (D) the types of ribosomes present in the cytoplasm. }}\end{array} \end{equation}

Proto-oncogenes can change into oncogenes that cause cancer. Which of the following best explains the presence of these potential time bombs in eukaryotic cells? \begin{equation} \begin{array}{l}{\text { (A) Proto-oncogenes first arose from viral infections. }} \\ {\text { (B) Proto-oncogenes are mutant versions of normal genes. }} \\ {\text { (C) Proto-oncogenes are genetic "junk." }} \\ {\text { (D) Proto-oncogenes normally help regulate cell division. }}\end{array} \end{equation}

EVOLUTION CONNECTION DNA sequences can act as "tape measures of evolution" (see Concept 5.6 ). Scientists analyzing the human genome sequence were surprised to find that some of the regions of the human genome that are most highly conserved (similar to comparable regions in other species) don't code for proteins. Propose a possible explanation for this observation.

SCIENCE, TECHNOLOGY, AND SOCIETY Trace amounts of dioxin were present in Agent Orange, a defoliant sprayed on vegetation during the Vietnam War. Animal tests suggest that dioxin can cause birth defects, cancer, liver and thymus damage,and immune system suppression, sometimes leading to death. But the animal tests are equivocal; a hamster is not affected by a dose that can kill a guinea pig. Dioxin acts like a steroid hormone, entering a cell and binding to a cytoplasmic receptor that then binds the cell's DNA. \begin{equation} \begin{array}{l}{\text { (a) Discuss how this mechanism might help explain the vari- }} \\ {\text { ety of dioxin's effects on different body systems and in }} \\ {\text { different animals. }}\end{array} \end{equation} \begin{equation} \begin{array}{l}{\text { (b) Discuss how you might determine whether a type of illness }} \\ {\text { is related to dioxin exposure. Next, discuss how you might }} \\ {\text { determine whether a particular individual became ill as a }} \\ {\text { result of exposure to dioxin. Which would be more difficult }} \\ {\text { to demonstrate? Why? }}\end{array} \end{equation}

SYNTHESIZE YOUR KNOWLEDGE The flashlight fish has an organ under its eye that emits light, which serves to startle predators and attract prey, and allows the fish to communicate with other fish. Some species can rotate the organ inside and then out, so the light appears to flash on and off. The light is actually emitted by bacteria (of the genus Vibrio) that live in the organ in a mutualistic relationship with the fish. (The bacteria receive nutrients from the fish.) The bacteria must multiply until they reach a certain density in the organ (a "quorum"; see Concept \(11.1 ),\) at which point they all begin emitting light at the same time. There is a group of six or so genes, called lux genes, whose gene products are necessary for light formation. Given that these bacterial genes are regulated together, propose a hypothesis for how the genes are organized and regulated.