Questions and Answers of Essential Cell Biology

FADH2 is produced in the citric acid cycle by a membrane-embedded enzyme complex, called succinate dehydrogenase, that contains bound FAD and carries out the reactions succinate + FAD → fumarate +
Which of the following substances would you expect to spread from one cell to the next through (a) Gap junctions (b) Plasmodesmata: glutamic acid, mRNA, cyclic AMP, Ca2+, proteins, and
The sketch in Figure Q15–22 is a schematic drawing of the electron micrograph shown in the third panel of Figure 15–20A. Name the structures that are labeled in the sketch.Figure Q15–22Figure
One important feature of any intracellular signaling pathway is its ability to be turned off. Consider the pathway shown in Figure 16−28. Where would off switches be required? Which ones do you
Gel-filtration chromatography separates molecules according to their size (Panel 4−4). Smaller molecules diffuse faster in solution than larger ones, yet smaller molecules migrate more slowly
The total length of DNA in one copy of the human genome is about 1 m, and the diameter of the double helix is about 2 nm. Nucleotides in a DNA double helix are stacked (see Figure 5–4B) at an
Some protein sequences evolve more rapidly than others. But how can this be demonstrated? One approach is to compare several genes from the same two species, as shown for rat and human in the table.
One of the first organisms that was genetically modified using recombinant DNA technology was a bacterium that normally lives on the surface of strawberry plants. This bacterium makes a protein,
Water molecules are said “to reorganize into a cagelike structure” around hydrophobic compounds (e.g., see Figure 11–9). This seems paradoxical because water molecules do not interact with the
Five students in your class always sit together in the front row. This could be because (A) they really like each other or (B) nobody else in your class wants to sit next to them. Which explanation
It seems paradoxical that a lipid bilayer can be fluid yet asymmetrical. Explain.
For the two detergents shown in Figure 11–26, explain why the blue portions of the molecules are hydrophilic and the red portions hydrophobic. Draw a short stretch of a polypeptide chain made up of
Explain why the polypeptide chain of most transmembrane proteins crosses the lipid bilayer as an α helix or a β barrel.
Look carefully at the transmembrane proteins shown in Figure 11−29B. What can you say about their mobility in the membrane?Figure 11−29B (A) 5 μm spectrin dimer actin (B) transmembrane
Describe the different methods that cells use to restrict proteins to specific regions of the membrane. Can a membrane with many of its proteins restricted still be fluid?
Which of the following statements are correct? Explain your answers.A. Lipids in a lipid bilayer spin rapidly around their long axis.B. Lipids in a lipid bilayer rapidly exchange positions with one
What is meant by the term “two-dimensional fluid”?
The structure of a lipid bilayer is determined by the particular properties of its lipid molecules. What would happen if:A. Phospholipids had only one hydrocarbon tail instead of two?B. The
What are the differences between a phospholipid molecule and a detergent molecule? How would the structure of a phospholipid molecule need to change to make it a detergent?
A. Membrane lipid molecules exchange places with their lipid neighbors every 10–7 second. A lipid molecule diffuses from one end of a 2-μm-long bacterial cell to the other in about 0.2 seconds.
Why does a red blood cell plasma membrane need transmembrane proteins?
Consider a transmembrane protein that forms a hydrophilic pore across the plasma membrane of a eukaryotic cell. When this protein is activated by binding a specific ligand on its extracellular side
Draw a schematic diagram that shows a close-up view of two plasma membranes as they come together during cell fusion, as shown in Figure 11–30. Show membrane proteins in both cells that were
In the membrane of a human red blood cell, the ratio of the mass of protein (average molecular weight 50,000) to phospholipid (molecular weight 800) to cholesterol (molecular weight 386) is about
Compare the hydrophobic forces that hold a membrane protein in the lipid bilayer with those that help proteins fold into a unique three-dimensional structure.
Predict which one of the following organisms will have the highest percentage of unsaturated phospholipids in its membranes. Explain your answer.A. Antarctic fishB. Desert snakeC. Human beingD. Polar
Which of the three 20-amino-acid sequences listed below in the single-letter amino acid code is the most likely candidate to form a transmembrane region (α helix) of a transmembrane protein? Explain
Figure Q11–20 shows the structure of triacylglycerol. Would you expect this molecule to be incorporated into the lipid bilayer? If so, which part of the molecule would face the interior of the
A simple enzyme reaction can be described by the equation E + S ↔ ES ↔ E + P where E is the enzyme, S the substrate, P the product, and ES the enzyme substrate complex.A. Write a corresponding
The figure shows a recording from a patch-clamp experiment in which the electrical current passing across a patch of membrane is measured as a function of time. The membrane patch was plucked from
A rise in the intracellular Ca2+ concentration causes muscle cells to contract. In addition to an ATPdriven Ca2+ pump, muscle cells that contract quickly and regularly, such as those of the
Using the Nernst equation and the ion concentrations given in Table 12–1, calculate the equilibrium membrane potential of K+ and Na+ that is, the membrane potential where there would be no net
Electron micrographs show that mitochondria in heart muscle have a much higher density of cristae than mitochondria in skin cells. Suggest an explanation for this observation.
When the drug dinitrophenol (DNP) is added to mitochondria, the inner membrane becomes permeable to protons (H+). In contrast, when the drug nigericin is added to mitochondria, the inner membrane
The electron micrographs shown in Figure Q17–20A were obtained from a population of microtubules that were growing rapidly. Figure Q17–20B was obtained from microtubules undergoing
Consider the following statement: “All present-day cells have arisen by an uninterrupted series of cell divisions extending back in time to the first cell division.” Is this strictly true?
When two individuals from different isolated, inbred subpopulations of a species come together and mate, their offspring often show “hybrid vigor”: that is, they appear more robust, healthy, and
A small amount of cytoplasm isolated from a mitotic cell is injected into an unfertilized frog oocyte, causing the oocyte to enter M phase (see Figure 18−7A). A sample of the injected oocyte’s
It is easy to see how deleterious mutations in bacteria, which have a single copy of each gene, are eliminated by natural selection: the affected bacteria die and the mutation is thereby lost from
Which of the following statements are correct? Explain your answers.A. The egg and sperm cells of animals contain haploid genomes.B. During meiosis, chromosomes are allocated so that each germ cell
What might cause chromosome nondisjunction, where two copies of the same chromosome end up in the same daughter cell? What could be the consequences of this event occurring (a) In
In the pedigree shown in Figure Q19–14, the first born in each of three generations is the only person affected by a dominant genetically inherited disease, D. Your friend concludes that the first
Why do sister chromatids have to remain paired in division I of meiosis? Does the answer suggest a strategy for washing your socks?
Early genetic studies in Drosophila laid the foundation for our current understanding of genes. Drosophila geneticists were able to generate mutant flies with a variety of easily observable
Suppose one person in 100 is a carrier of a fatal recessive mutation, such that babies homozygous for the mutation die soon after birth. In a population where there are 1,000,000 births per year, how
Certain mutations are called dominant-negative mutations. What do you think this means and how do you suppose these mutations act? Explain the difference between a dominant-negative mutation and a
Mutations in the genes encoding collagens often have detrimental consequences, resulting in severely crippling diseases. Particularly devastating are mutations that change glycines, which are
What are single-nucleotide polymorphisms (SNPs), and how can they be used to locate a mutant gene by linkage analysis?
Cells in the stem of a seedling that is grown in the dark orient their microtubules horizontally. How would you expect this to affect the growth of the plant?
Proteoglycans are characterized by the abundance of negative charges on their sugar chains. How would the properties of these molecules differ if the negative charges were not as abundant?
Analogs of hemidesmosomes are the focal contacts described, which are also sites where the cell attaches to the extracellular matrix. These junctions are prevalent in fibroblasts but largely absent
Gap junctions are dynamic structures that, like conventional ion channels, are gated: they can close by a reversible conformational change in response to changes in the cell. The permeability of gap
Why does ionizing radiation stop cell division?
Why do you suppose epithelial cells lining the gut are lost and replaced (renewed) frequently, whereas most neurons last for the lifetime of the organism?
About 1016 cell divisions take place in a human body during a lifetime, yet an adult human body consists of only about 1013 cells. How can you reconcile these apparently conflicting two numbers?
Ignoring the effects of chromosome crossovers, an individual human can in principle produce 223 = 8.4 × 106 genetically different gametes. How many of these possibilities can be “sampled” in the
Why do you suppose cells have evolved a special G0 phase to exit from the cell cycle, rather than just stopping in G1 and not moving on to S phase?
Discuss the following statement: “If plant cells contained intermediate filaments to provide the cells with tensile strength, their cell walls would be dispensable.”
Heavy smokers or industrial workers exposed for a limited time to a chemical carcinogen that induces mutations in DNA do not usually begin to develop cancers characteristic of their habit or
Carefully consider the graph in Figure 20−43, which shows the number of cases of colon cancer diagnosed per 100,000 women per year as a function of age. Why is this graph so steep and curved, if
Leukemias that is, cancers arising through mutations that cause excessive production of white blood cells have an earlier average age of onset than other cancers. Propose an explanation for why this
“The structure of an organism is determined by the genome that the fertilized egg contains.” What is the evidence on which this statement is based? Indeed, a friend challenges you and suggests
Gelatin is primarily composed of collagen, which is responsible for the remarkable tensile strength of connective tissue. It is the basic ingredient of jello; yet, as you probably experienced many
Through the exchange of small metabolites and ions, gap junctions provide metabolic and electrical coupling between cells. Why, then, do you suppose that neurons communicate primarily through
Which of the following statements are correct? Explain your answers.A. Gap junctions connect the cytoskeleton of one cell to that of a neighboring cell or to the extracellular matrix.B. A wilted
Is cancer hereditary?
High levels of the female sex hormone estrogen increase the risk of some forms of cancer. Thus, some early types of contraceptive pills containing high concentrations of estrogen were eventually
Susan’s grandfather was deaf, and passed down a hereditary form of deafness within Susan’s family as shown in Figure Q19–12.A. Is this mutation most likely to be dominant or recessive?B. Is it
You have been given three wrinkled peas, which we shall call A, B, and C, each of which you plant to produce a mature pea plant. Each of these three plants, once self-pollinated, produces only
Distinguish between the following genetic terms:A. Gene and allele.B. Homozygous and heterozygous.C. Genotype and phenotype.D. Dominant and recessive
Given that the mutation causing deafness in the family shown in Figure 19−26 is very rare, what is the most probable genotype of each of the four children in generation II?Figure 19−26 generation
A. What is the sequence of the DNA that was used in the sequencing reaction shown in Figure Q10–9? The four lanes show the products of sequencing reactions that contained ddG (lane 1), ddA (lane
Hemoglobin-like proteins were discovered in legumes, where they function in root nodules to lower the oxygen concentration, allowing the resident bacteria to fix nitrogen. These plant
A. If the PCR shown in Figure 10–12 is carried through an additional two rounds of amplification, how many of the DNA fragments (gray, green, red, or outlined in yellow) will be produced? If many
A molecule of double-stranded DNA was cleaved with restriction enzymes, and the resulting products were separated by gel electrophoresis (Figure Q10–11). You do not know if the molecule is linear
There has been a colossal snafu in the maternity ward of your local hospital. Four sets of male twins, born within an hour of each other, were inadvertently shuffled in the excitement occasioned by
After decades of work, Dr. Ricky M. isolated a small amount of attractase an enzyme that produces a powerful human pheromone from hair samples of Hollywood celebrities. To take advantage of
A. How many different DNA fragments would you expect to obtain if you cleaved human genomic DNA with HaeIII? How many fragments would you expect with EcoRI?B. Human genomic libraries used for DNA
Almost all the cells in an individual animal contain identical genomes. In an experiment, a tissue composed of several different cell types is fixed and subjected to in situ hybridization with a DNA
What are the consequences for a dideoxy DNA sequencing reaction if the ratio of dideoxyribonucleoside triphosphates to deoxyribonucleoside triphosphates is increased? What happens if this ratio is
Which of the following statements are correct? Explain your answers.A. Restriction enzymes cut DNA at specific sites that are always located between genes.B. DNA migrates toward the positive
Assume you want to make and study fragments of a protein. Would you expect that any fragment of the polypeptide chain would fold the same way as it would in the intact protein? Consider the protein
Protein A binds to protein B to form a complex, AB. At equilibrium in a cell the concentrations of A, B, and AB are all at 1 μM.A. Referring to Figure 3−19, calculate the equilibrium constant for
A motor protein moves along protein filaments in the cell. Why are the elements shown in the illustration not sufficient to mediate directed movement (Figure Q4–19)? With reference to Figure
In a clever experiment performed in 1962, a cysteine already attached to its tRNA was chemically converted to an alanine. These “hybrid” tRNA molecules were then added to a cellfree translation
DNA sequencing of your own two β-globin genes (one from each of your two Chromosome 11s) reveals a mutation in one of the genes. Given this information alone, should you worry about being a carrier
Discuss the following statement: “From the nucleotide sequence of a cDNA clone, the complete amino acid sequence of a protein can be deduced by applying the genetic code. Thus, protein biochemistry
Use the genetic code shown in Figure 7–27 to identify which of the following nucleotide sequences would code for the polypeptide sequence arginine glycine-aspartate:1. 5ʹ-AGA-GGA-GAU-3ʹ2.
You are interested in finding out the function of a particular gene in the mouse genome. You have determined the nucleotide sequence of the gene, defined the portion that codes for its protein
Why do you expect to encounter a stop codon about every 20 codons or so in a random sequence of DNA?
Which of the processes listed below contribute significantly to the evolution of new protein-coding genes?A. Duplication of genes to create extra copies that can acquire new functions.B. Formation of
The accuracy of DNA replication in the human germ cell line is such that on average only about 0.6 out of the 6 billion nucleotides is altered at each cell division. Because most of our DNA is not
Reverse transcriptases do not proofread as they synthesize DNA using an RNA template. What do you think the consequences of this are for the treatment of AIDS?
Which products result when the double-stranded DNA molecule below is digested with (A) EcoRI, (B) HaeIII, (C) HindIII, or (D) all three of these enzymes together? (See Figure 10−2 for the target
Which of the following statements are correct? Explain your answers.A. The plasma membrane is highly impermeable to all charged molecules.B. Channels have specific binding pockets for the solute
The diagram in Figure 12–9 shows a transporter that mediates the passive transfer of a solute down its concentration gradient across the membrane. How would you need to change the diagram to the
Explain as precisely as you can, but in no more than 100 words, the ionic basis of an action potential and how it is passed along an axon.
Describe the different methods that cells use to restrict proteins to specific regions of the membrane. Can a membrane with many of proteins restricted still be fluid?
Amino acids are taken up by animal cells using a symport in the plasma membrane. What is the most likely ion whose electrochemical gradient drives the import? Is ATP consumed in the process? If so,
One thousand Ca2+ channels open in the plasma membrane of a cell that is 1000 μm3 in size and has a cytosolic Ca2+ concentration of 100 nM. For how long would the channels need to stay open in order

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