Cell Bio Exam 1 answers

You wish to explore how mutations in specific genes affecting sugar metabolism might alter tooth development. Which organism is likely to provide the best model system for your studies, and why? (a) horses (b) mice (c) E. coli (d) Arabidopsis

(b) Mice are likely to provide the best model system. Mice have teeth and have long been used as a model organism. 1. mice are small and reproduce quickly 2. a lot of research on them has already been done and people know how to manipulate their genetics 3. horses are not ideal model organisms 4. E. Coli and Arabidopsis don't have teeth.

You fertilize egg cells from a healthy plant with pollen (which contains the male germ cells) that has been treated with DNA-damaging agents. You find that some of the offspring have defective chloroplasts, and that this characteristic can be passed on to future generations. This surprises you at first because you happen to know that the male germ cell in the pollen grain contributes no chloroplasts to the fertilized egg cell and thus to the offspring. What can you deduce from these results?

1. results indicate that some of the DNA that encodes for the making of a chloroplast must be found in the nucleus 2. the female gamete has all of the chloroplasts and they were not mutated 3. the pollen grain contributes half of the nuclear DNA in fertilization 4. since this DNA was damaged it has to be the source of the defect 5. some info for making chloroplasts is found in nuclear DNA to conclude

Nucleosomes are formed when DNA wraps _____ times around the histone octamer in a ______ coil.

1.7, left-handed

What term is used to describe the changes in gene sequences that have occurred? How can we use what we know about this process to construct a time line showing when various sequence changes occurred and when they led to the modern sequences that we know today?

1.Genes change through mutation and homologous genes can diverge. 2. the more recent they diverge the less of a difference (vice versa) 3. can estimate time in between versions by knowing average mutation rate 4. If there are 3 family members, A, B, and C, and A and B have a common sequence at a site but C is different, then C likely has a more recent mutation particular to itself

Which of the following is NOT true?

A CELL CAN TEMPORARILY DECONDENSE ITS CHROMATIN TO SILENCE GENES DURING DIFFERENTIATION. A cell can temporarily decondense its chromatin to allow access to specific DNA sequences for replication, repair, or gene expression. Histone tail modifications establish and maintain the different chromatin structures found in heterochromatin and euchromatin. Histone proteins have a high proportion of positively charged amino acids, which attract the negatively charged phosphate groups on DNA.

The chemical properties of amino acid side chains include charged, uncharged polar, and nonpolar. A) True B) False

A) True

The relative distribution of polar and nonpolar amino acids in a folded protein is determined largely by hydrophobic interactions, which favor the clustering of nonpolar side chains in the interior. A) True B) False

A) True

A. Primitive plant, animal, and fungal cells probably acquired mitochondria after they diverged from a common ancestor. B. Protozoans are single-celled eukaryotes with cell morphologies and behaviors that can be as complex as those of some multicellular organisms. C. The first eukaryotic cells on Earth must have been aerobic; otherwise, they would not have been able to survive when the planet's atmosphere became oxygen-rich.

A. False. The mitochondria in modern plant, animal, and fungal cells are very similar, implying that these lines diverged after the mitochondrion was acquired by the ancestral eukaryote. B. True. C. False. The first eukaryotic cells likely contained a nucleus but no mitochondria. These ancestral eukaryotes subsequently adapted to survive in a world filled with oxygen by engulfing primitive aerobic prokaryotic cells.

A. The Paramecium is a multicellular microorganism covered with hairlike cilia. B. Cells of different types can have different chemical requirements. C. The branchlike extensions that sprout from a single nerve cell in a mammalian brain can extend over several hundred micrometers.

A. The Paramecium is a single-celled organism. B. True C. True

A. Oxygen is toxic to certain prokaryotic organisms. B. Mitochondria are thought to have evolved from anaerobic bacteria. C. Photosynthetic bacteria contain chloroplasts.

A. True. B. False. Mitochondria use oxygen to generate energy and are thought to have evolved from aerobic bacteria. C. False. Photosynthetic bacteria have enzyme systems similar to those found in chloroplasts, which allow them to harvest light energy to fix carbon dioxide.

A. The human genome is roughly 30 times larger than the Arabidopsis genome, but contains approximately the same number of protein-coding genes. B. The variation in genome size among protozoans is larger than that observed across all species of mammals, birds, and reptiles. C. The vast majority of our genome encodes functional RNA molecules or proteins and most of the intervening DNA is nonfunctional.

A. True. B. True. C. False. It is a relatively small proportion of our DNA that encodes RNA and protein molecules. The majority of nonencoding sequences is probably involved in critical regulatory processes.

A. Membrane components in the cell are made in the endoplasmic reticulum. B. The Golgi apparatus is made up of a series of membrane-enclosed compartments through which materials destined for secretion must pass. C. Lysosomes are small organelles where fatty acid synthesis occurs.

A. True. B. True. C. False. Lysosomes house enzymes that break down nutrients for use by the cell and help recycle materials that cannot be used, which will later be excreted from the cell.

Genes that have homologs in a variety of species have been discovered through the analysis of genome sequences. In fact, it is not uncommon to find a family of homologous genes encoding proteins that are unmistakably similar in amino acid sequence in organisms as diverse as budding yeast, archaea, plants, and humans. Even more remarkably, many of these proteins can substitute functionally for their homologs in other organisms. Explain what it is about the origins of cells that makes it possible for proteins expressed by homologous genes to be functionally interchangeable in different organisms.

All living being likely came from a common ancestor. Essential challenges that face a cell (synthesis of proteins, lipids, and DNA) have been solved for the ancestral cell, and it could carry out these functions. Since living cells face the same issues, ancestral solutions can work for modern cells. Although there is speciation, organisms retain basic biochemical characteristics of the ancestral protein, so homologous proteins can interact with specific protein targets across different organisms. In sum, homologous proteins can substitute across different organisms.

The protozoan Didinium feeds on other organisms by engulfing them. Why are bacteria, in general, unable to feed on other cells in this way?

Didinium engulfs prey by changing its shape, and for this it uses its cytoskeleton. Bacteria have no cytoskeleton and cannot easily change their shape because they are generally surrounded by a tough cell wall.

"Why are we spending millions of dollars studying fruit flies? How can that possibly help us find a cure for cancer?" Use your knowledge of model organisms to explain why studies in D. melanogaster (the fruit fly) are actually an excellent use of research funding.

Funding research on D. melanogaster is a worthwhile investment for several reasons: (1) working with insect animal models is relatively inexpensive; (2) fruit flies have historically proven useful in helping understand eukaryotic chromosome behavior; and (3) many of the genes in Drosophila are highly similar in sequence to the homologous human genes, and thus can be used to study human diseases.

What is the result of heterochromatin formation?

Gene Silencing

The complete set of information found in a given organism's DNA is called its ______________.

Genome

What is the most highly condensed form of chromatin?

Heterochromatin

What type of bond connects base pairs?

Hydrogen Bond

What structure in an interphase cell contains ribosomal RNA and proteins for the formation of ribosomes?

Nucleolus

Histone proteins pack DNA into a repeating array of DNA-protein particles called:

Nucleosomes

What type of macromolecule helps package DNA in eukaryotic chromosomes?

Proteins

The information stored in the DNA sequences is used directly as a template to make ________________.

RNA

How does cellular specialization serve multicellular organisms and how might a high degree of specialization be detrimental?

The specialization of cells creates a division of labor and each type of cell relies on the activities of other cell types for survival. This cooperation between specialized cells is essential for the organism as a whole. If a specialized cell were on its own, it couldn't generate offspring and would probably not even live very long.

In a DNA double helix, _____________________.

The two DNA strands run antiparallel.

Knowing that there are 20 different possible amino acids that can be used at each position in a polypeptide, calculate the number of different polypeptides that could theoretically be produced for a protein that is 180 amino acids in length. Do you expect to find all of these possible protein sequences produced in living systems? Explain your answer.

There are 20180 possible sequences for a 180 amino acid polypeptide (20 different possible amino acids for each position). No, we would not expect all of the theoretically possible proteins to be made. A much smaller subset can be expected in living systems because it is not likely that all sequences would lead to a stably folded protein. Natural selection favors the retention of genes that encode proteins with stable conformations.

Your friend has just returned from a deep-sea mission and claims to have found a new single-celled life-form. He believes this new life-form may not have descended from the common ancestor that all types of life on Earth share. You are convinced that he must be wrong, and you manage to extract DNA from the cells he has discovered. He says that the mere presence of DNA is not enough to prove the point: his cells might have adopted DNA as a useful molecule quite independently of all other known life-forms. What could you do to provide additional evidence to support your argument?

You could use modern technology to discover the sequence of the DNA. If you are right, you would expect to find parts of this sequence that are unmistakably similar to corresponding sequences in other, familiar, living organisms; it would be highly improbable that such similar sequences would have evolved independently. You could, of course, also analyze other features of the chemistry of his cells; for example, do they contain proteins made of the same set of 20 amino acids? This could all be supporting evidence that this newly discovered species arose from the same common ancestral cells as all other life on Earth.

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. composition covalent denatured highest irreversible lowest noncovalent renatured reversible sequence stable unstable A newly synthesized protein generally folds up into a __________ conformation. All the information required to determine a protein's conformation is contained in its amino acid __________. On being heated, a protein molecule will become __________ as a result of breakage of __________ bonds. On removal of urea, an unfolded protein can become __________. The final folded conformation adopted by a protein is that of __________ energy.

stable sequence denatured noncovalent renatured lowest

A. Plants do not require a cytoskeleton because they have a cell wall that lends structure and support to the cell. B. The cytoskeleton is used as a transportation grid for the efficient, directional movement of cytosolic components. C. Thermal energy promotes random movement of proteins, vesicles, and small molecules in the cytosol.

A. False. Although plant cells do have a cell wall that lends structure and support, they still need a cytoskeleton, which also helps with connections between cells and the transport of vesicles inside the cell. B. True. C. True.

A. The terms "prokaryote" and "bacterium" are synonyms. B. Prokaryotes can adopt several different basic shapes, including spherical, rod-shaped, and spiral. C. Some prokaryotes have cell walls surrounding the plasma membrane.

A. False. Archaea make up a class of prokaryotic organisms that are significantly different from bacteria. B. True. C. True.

A. With respect to cellular respiration, the only organelles used by animal cells are mitochondria, while plant cells use both mitochondria and chloroplasts. B. The number of mitochondria inside a cell remains constant over the life of the cell.

A. False. In plants, only mitochondria perform cellular respiration (using oxygen to break down organic molecules to produce carbon dioxide) just as in animal cells. Chloroplasts perform photosynthesis in which water molecules are split to generate oxygen and fix carbon dioxide molecules. B. False. Mitochondria have their own division cycle and their numbers change based on the rate of division.

A. The nucleus of an animal cell is round, small, and difficult to distinguish using light microscopy. B. The presence of the plasma membrane can be inferred by the well-defined boundary of the cell. C. The cytosol is fairly empty, containing a limited number of organelles, which allows room for rapid movement via diffusion.

A. False. The nucleus is one of the largest organelles and is the easiest organelle to discern within a typical cell. B. True. C. False. The cytosol is actually brimming with individual proteins, protein fibers, extended membrane systems, transport vesicles, and small molecules. And although cellular components do move by diffusion, the rate of movement is limited by the space available and the size of the component in question.

The antibiotic streptomycin inhibits protein synthesis in bacteria. If this antibiotic is added to a culture of animal cells, protein synthesis in the cytosol continues normally. However, over time, the population of mitochondria in the cell becomes depleted. Specifically, it is observed that the protein-synthesis machinery inside the mitochondria is inhibited. A. Explain this observation based on what you know about the origins of the modern eukaryote. B. What do you expect to observe if, in a new experiment, animal cells are treated with diphtheria toxin, a compound that is known to block cytosolic protein synthesis but does not have any impact on bacterial growth?

A. If the mitochondria originated from an ancient aerobic bacterium that was engulfed by an ancient eukaryote, as postulated, it is possible that an antibiotic that inhibits protein synthesis in bacteria could also block that process in mitochondria. B. We would expect that although cytosolic protein synthesis would stop, mitochondrial protein synthesis should still occur normally (at least for a little while). This result would lend further support to the idea that mitochondria are derived from a noneukaryotic organism. If this were not the case, these compounds would be expected to affect protein synthesis at both locations.

It is possible to express an ancestral gene sequence in modern organisms and subsequently compare the function of its product with that of the modern protein. Why might this approach give misleading conclusions?

Although an inferred ancestral sequence can be reconstructed and the protein expressed, you would be placing an inferred, ancient protein in the context of a modern cell. If there are important interacting partners for the modern protein, there is a chance they may not recognize the ancestral protein, and therefore any information about its function may be inaccurate.

Proteins bind selectively to small-molecule targets called ligands. The selection of one ligand out of a mixture of possible ligands depends on the number of weak, noncovalent interactions in the protein's ligand-binding site. Where is the binding site typically located in the protein structure? A) on the surface of the protein B) inside a cavity on the protein surface C) buried in the interior of the protein D) forms on the surface of the protein in the presence of ligand

B) inside a cavity on the protein surface

The variations in the physical characteristics between different proteins are influenced by the overall amino acid compositions, but even more important is the unique amino acid A) number. B) sequence. C) bond. D) orientation.

B) sequence.

Protein structures have several different levels of organization. The primary structure of a protein is its amino acid sequence. The secondary and tertiary structures are more complicated. Consider the definitions below and select the one that best fits the term "protein domain." A) a small cluster of α helices and β sheets B) the tertiary structure of a substrate-binding pocket C) a complex of more than one polypeptide chain D) a protein segment that folds independently

D) a protein segment that folds independently

β sheets can participate in the formation of amyloid fibers, which are insoluble protein aggregates. What drives the formation of amyloid fibers? A) denaturation of proteins containing β sheets B) extension of β sheets into much longer β strands C) formation of biofilms by infectious bacteria D) β-sheet stabilization of abnormally folded proteins

D) β-sheet stabilization of abnormally folded proteins

The "polypeptide backbone" refers to all atoms in a polypeptide chain, except for those that form the peptide bonds. A) True B) False

False Peptide bonds are planar amide bonds that are central to the polypeptide backbone formation. The atoms in the amino acid side chains are not considered to be part of the backbone.

Generally, the total number of nonpolar amino acids has a greater effect on protein structure than the exact order of amino acids in a polypeptide chain. A) True B) False

False The order in which amino acids are linked is unique for each protein and is the most important factor in determining overall protein structure.

The classic "beads-on-a-string" structure is the most decondensed chromatin structure possible and is produced experimentally. Which chromatin components are not retained when this structure is generated?

Linker H1 Histones

Cellular processes are often regulated by unknown mechanisms. In many cases, biologists work backward in an attempt to understand a process in which they are interested. This was the case when Nurse and Hartwell were trying to understand how cell division is controlled in yeast. Describe the process by which they "broke" the system and then supplied the "missing parts" to get the cell cycle running again. What further evidence did they collect to show that human cells and yeast cells regulate the cell cycle using a similar mechanism?

Nurse and Hartwell first treated yeast cells with a chemical mutagen. The mutated population of cells was then grown and observed. Cells that demonstrated defects in cell-cycle regulation (characterized by cell-cycle arrest, larger-than-normal cells, and smaller-than-normal cells) were then isolated. The use of a library of plasmids that each express a normal gene from yeast cells allowed the scientists to identify exactly which gene could be used to "rescue" the mutant, because when the normal gene is expressed again, the cells return to a normal cell cycle. After this big result, the scientists went on to show that the homologous gene from other organisms could also rescue the mutant phenotype. The most exciting result was obtained with the human version of the cdc2 gene, which demonstrated that there are common principles underlying cell-cycle regulation across a large range of eukaryotic organisms.

Interphase cells contain chromosomes that are less densely packed and __________________________.

Occupy discrete territories in the nucleus.

Which of the following chemical groups is not used to construct a DNA molecule?

Six-carbon sugar

The structures that cap the ends of eukaryotic chromosomes are called:

Telomeres

Employ the principles of evolution discussed in this chapter to explain how the specific features and predatory behaviors of some primitive eukaryotes may have given them a selective advantage over others 1.5 billion years ago.

The Earth's atmosphere became oxygen-rich roughly 1.5 billion years ago. If some primitive predator eukaryotic cells were similar to modern-day protozoans, they may have been mobile and able to engulf other cells. These characteristics would have been advantageous in the face of a changing atmosphere, and the establishment of a symbiotic relationship with an engulfed aerobe would have been selected for in the eukaryotic cell populations.