Zool. 1114 Exam 2 prep
Zool. 1114 Exam 2 prep BIOL 1114, 001
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Chapter 3: Cells 3.4 1. How do organelles help eukaryotic cells compensate for a small ratio of surface area to volume? One way to compensate for a small ratio of surface area to volume is to use resources and energy efficiently. Thanks to specialized organelles, a eukaryotic cell produces high concentrations of biochemicals only in certain areas rather than producing them throughout the cell. 2. Which parts of the cell interact to produce and secrete a complex substance such as milk? The nucleus, ribosomes, smooth ER, Golgi apparatus, vesicles, and cell membrane interact in the production and secretion of milk. Mitochondria provide the energy necessary to make the process happen. 3. What is the function of the nucleus and its contents? The nucleus contains DNA (the molecule that contains the instructions for making proteins) and the nucleolus (where ribosomes are assembled). mRNA molecules also are produced in the nucleus, although they exit through nuclear pores before participating in protein production. 4. Which organelles are the cell’s “recycling centers”? Lysosomes, vacuoles, and peroxisomes are the cell’s recycling centers. 5. How are the functions of plastids essential to the life of a plant cell? Chloroplasts are plastids that carry out photosynthesis. Other plastids store pigments or food, or they help a plant to detect gravity. 6. Which organelle houses the reactions that extract chemical energy from nutrient molecules? Mitochondria house the reactions that extract chemical energy from nutrient molecules. 7. Which three organelles contain DNA? The nucleus, chloroplasts, and mitochondria all contain DNA. 3.5 1. What are some functions of the cytoskeleton? The cytoskeleton provides a structural framework for the cell, is a transportation system within the cell, allows the cell to move, and connects cells to one another. 2. What are the main components of the cytoskeleton? The major components of the cytoskeleton are proteins that assemble into microfilaments, intermediate filaments, and microtubules. 3. Why are flagella and cilia important? Unicellular organisms may use either flagella or cilia to move toward food or other resources. In humans, sperm cells have flagella, and cilia sweep substances along the respiratory tract and the female reproductive tract. Chapter 7: DNA 7.1 1. How did Griffith’s research, coupled with the work of Avery and his colleagues, demonstrate that DNA, not protein, is the genetic material? Griffith’s research established that a lethal strain of bacteria (type S) could transfer a thenunknown molecule to nonlethal bacteria (type R) and confer the ability to kill mice. Avery and his colleagues added enzymes that destroyed either proteins or DNA to the mixtures that Griffith used in his experiments. These experiments showed that DNA, not protein, changed type R bacteria from nonlethal to lethal. 2. How did the Hershey–Chase “blender experiments” confirm Griffith’s results? The HersheyChase “blender experiments” used radioactive sulfur to label the protein coats of one batch of bacteriophages and used radioactive phosphorus to label the DNA of another batch of bacteriophages. Both batches of viruses were allowed to infect bacteria. Then the solutions were separately blended at high speeds to separate viral protein coats from bacterial cells. Radioactively labeled bacteria were found only in the batches that had been infected by phages with radioactively labeled DNA. The protein labeled phages did not transmit radioactivity to the bacteria they had infected. These experiments confirmed Griffith’s conclusion that DNA, not protein, is the genetic material. 7.2 1. What are the components of DNA and its threedimensional structure? A DNA molecule is composed of subunits called nucleotides. Each nucleotide is composed of a deoxyribose sugar bonded to a phosphate group and a nucleotide base (adenine, thymine, cytosine, or guanine). The threedimensional structure of DNA is a double helix, which resembles a twisted ladder. 2. What evidence enabled Watson and Crick to decipher the structure of DNA? The evidence included Rosalind Franklin’s Xray diffraction photo of a crystal of DNA, plus Erwin Chargaff’s work that showed that DNA contains equal amounts of adenine and thymine and equal amounts of cytosine and guanine. 3. Identify the 3′ and 5′ ends of a DNA strand. The 3’ and 5’ designators refer to opposite ends of a single DNA strand. The 5’ end has a phosphate group attached to the 5’ carbon atom, whereas the 3’ end has the sugar’s OH (hydroxyl group) attached to the 3’ carbon atom. 7.3 1. What is the relationship between a gene and a protein? A gene is a strand of DNA that encodes a protein. 2. What are the two main stages in protein synthesis? Transcription and translation are the two main stages in protein synthesis. 3. What are the three types of RNA, and how does each contribute to protein synthesis? Messenger RNA (mRNA) carries the instructions for building the protein; transfer RNA (tRNA) carries the appropriate amino acid to the ribosome; and ribosomal RNA (rRNA) is the major component of a ribosome, which is the structure where amino acids are assembled into polypeptides. 7.4 1. What happens during each stage of transcription? The steps of transcription are initiation, elongation, and termination. During initiation, enzymes unzip the DNA, and RNA polymerase binds to the promoter. During elongation, RNA polymerase uses the DNA template strand to add complementary nucleotides to the 3’ end of the growing RNA strand. During termination, synthesis of the RNA molecule ends and the DNA molecule is “zipped” back into its double helix form. 2. Where in the cell does transcription occur? Transcription occurs in the nucleus of a eukaryotic cell. 3. What is the role of RNA polymerase in transcription? RNA polymerase is an enzyme that uses the DNA template to bind additional nucleotides to the 3’ end of the growing chain of RNA. 4. What are the roles of the promoter and terminator sequences in transcription? The promoter signals the start of a gene, and the terminator signals the end of a gene. RNA polymerase recognizes the promoter and terminator, so it starts and stops transcription at the correct positions along the DNA template strand. 5. How is mRNA modified before it leaves the nucleus of a eukaryotic cell? Before it leaves the nucleus of a eukaryotic cell, mRNA is altered in the following ways: a cap is added to the 5’ end of the mRNA molecule; a poly A tail is added to the 3’ end; introns are removed and exons are spliced together. 7.5 1. How did researchers determine that the genetic code is a triplet and learn which codons specify which amino acids? Researchers knew that life uses four nucleotides and 20 amino acids. They reasoned that the genetic code could not reflect 1base or 2base “words,” because neither could encode enough amino acids. A triplet code (3base “words”) could potentially encode 64 amino acids, which is more than enough for the 20 amino acids found in biological proteins. They deciphered the genetic code by adding synthetic mRNA molecules to test tubes containing all the ingredients needed for translation. They analyzed the sequences of the resulting polypeptides to determine which codons correspond to which amino acids. 2. What happens in each stage of translation? In initiation, ribosomal subunits bind to mRNA, and a tRNA carrying the first amino acid (methionine) attaches to the first codon. In elongation, the ribosome moves along the mRNA, adding new amino acids to the growing polypeptide. In termination, the ribosome reaches a stop codon and releases the last tRNA and the polypeptide. The ribosomal subunits then dissociate from the mRNA. 3. Where in the cell does translation occur? Translation occurs at ribosomes, which are either free in the cytoplasm or attached to the rough ER. 4. How are polypeptides modified after translation? Polypeptides must be folded to become functional proteins. In addition, sometimes amino acids are cut out of the chain, and sometimes multiple polypeptides join together. 7.6 1. What are some reasons that cells regulate gene expression? Protein production costs a lot of energy; the regulation of gene expression avoids the production of unnecessary proteins and therefore saves energy. 2. How do proteins determine whether a bacterial operon is expressed? A repressor protein binds to an operator and prevents the genes in the operon from being transcribed. 3. How do enhancers and transcription factors interact to regulate gene expression? Transcription factors bind to certain DNA sequences to regulate transcription, for example by preparing a promoter site to bind RNA polymerase. Transcription won’t occur without these factors. Enhancers are sequences of DNA outside of the promoter. Transcription factors can bind to the enhancers to help regulate gene expression. 4. What are some other ways that a cell controls which genes are expressed? Cells can keep DNA coiled or attach methyl groups that inactivate genes. After transcription, different combinations of introns can be removed. mRNA can be confined to the nucleus or rapidly degraded. Proteins can also be degraded or modified in processing. 7.7 1. What is a mutation? A mutation is a change in a DNA sequence. 2. What are the types of mutations, and how does each alter the encoded protein? In a substitution mutation, one DNA base is replaced with another. The mutation may be have no effect on the resulting protein (silent mutation), change one amino acid (missense mutations), or create a stop codon in the middle of the mRNA (nonsense mutation). Insertions and deletions add or remove nucleotides; they often shift the “reading frame” of a gene. Such a frameshift mutation may alter many amino acids in the protein, drastically changing its shape and function. An insertion of three nucleotides adds one amino acid to the encoded protein, and a deletion of three nucleotides removes one amino acid. Expanding repeat mutations increase the number of copies of threeor four nucleotide sequences over several generations. This causes extra amino acids to be inserted into a protein, deforming it. Largescale mutations delete, duplicate, or invert large portions of a chromosome. The effects depend on whether genes are disrupted. 3. What causes mutations? Mutations are often caused by DNA replication errors, exposure to chemicals or radiation, and transposons. Largescale mutations may result from errors in meiosis. 4. What is the difference between a germline mutation and a somatic mutation? A germline mutation is one that occurs in a cell that will give rise to a sperm or an egg cell. A somatic mutation occurs within a nongermline body cell. 5. How are mutations important? Some mutations cause diseases. Mutations also produce genetic variability, which is the raw material of evolution. Scientists induce mutations to learn how genes normally function and to develop new varieties of crop plants. 7.8 1. What question about the FOXP2 gene were the researchers trying to answer? Researchers wanted to know how the human version of the FOXP2 gene differs from that of other primates. They also wanted to know if humanspecific mutations could be linked to the acquisition of language. 2. What insights could scientists gain by intentionally mutating the FOXP2 gene in a developing human? Would such an experiment be ethical? Many answers are possible, but one idea would be to mutate the FOXP2 gene so that it is nonfunctional at different stages of development to learn whether it is active through development or just in a critical window. Such an experiment would not be ethical. Write It Out 1. Explain how Griffith’s experiment and Avery, MacLeod, and McCarty’s experiment determined that DNA in bacteria transmits a trait that kills mice. Some strains of Streptococcus pneumoniae bacteria (type S) cause pneumonia, whereas others (type R) do not. Griffith’s experiment determined that heatkilled type S bacteria can transform type R bacteria into pneumoniacausing killers. Avery, MacLeod, and McCarty’s followup experiment determined that DNA, not proteins, from the dead type S bacteria altered the type R bacteria. When heatkilled type S bacteria were treated with a proteindestroying enzyme, the type R bacteria still became killers. But when type S bacteria were treated with DNAdestroying enzymes, the type R bacteria remained harmless. 2. Describe the threedimensional structure of DNA. DNA is a double helix that resembles a twisted ladder. In this molecule, the “twin rails” of the ladder are alternating units of deoxyribose and phosphate, and the ladder’s rungs are AT and GC base pairs joined by hydrogen bonds. 3. Explain Chargaff’s observation that a DNA molecule contains equal amounts of A and T and equal amounts of G and C. DNA has two complementary strands. Each adenine (A) on one strand pairs with a thymine (T) on the opposite strand. Likewise, each guanine (G) on one strand pairs with a cytosine (C) on the other strand. Therefore, DNA has one T for every A and has one C for every G. 4. Write the complementary DNA sequence of each of the following base sequences: a. A G G C A T A C C T G A G T C b. G T T T A A T G C C C T A C A c. A A C A C T A C C G A T T C A The complementary sequences are: a) TCCGTATGGACTCAG b) CAAATTACGGGATGT c) TTGTGATGGCTAAGT 5. Put the following in order from smallest to largest: nucleotide, genome, nitrogenous base, gene, nucleus, cell, codon, chromosome. From smallest to largest, the order is nitrogenous base, nucleotide, codon, gene, chromosome, nucleus, and cell. 6. What is the function of DNA? The function of much of the DNA in a cell is not known, but some of it encodes the cell’s RNA and proteins. 7. Use figure 7.9 to describe the structural and functional differences between RNA and DNA. RNA nucleotides contain a sugar called ribose; DNA nucleotides contain a similar sugar called deoxyribose. RNA has the nitrogenous base uracil, which behaves similarly to the thymine in DNA that is, both uracil and thymine form complementary base pairs with adenine. RNA can be singlestranded; DNA is doublestranded. RNA can catalyze chemical reactions, a role not known for DNA. 8. Explain how information in DNA is transcribed and translated into amino acids. Transcription copies the information encoded in a DNA base sequence into the complementary language of mRNA. Once transcription is complete and mRNA is processed, the cell is ready to translate the mRNA message into a sequence of amino acids that builds a protein. Transcription occurs in the nucleus, and translation occurs at ribosomes in the cytoplasm. 9. Some people compare DNA to a blueprint stored in the office of a construction company. Explain how this analogy would extend to transcription and translation. Transcription would be the process of scanning or copying the blueprints so that the contractor would have a set at the construction site. Translation would be the process of the contractor directing the assembly of all the raw materials at the site into the finished building. 10. List the three major types of RNA and their functions. Messenger RNA (mRNA) carries the information that specifies a protein. Ribosomal RNA (rRNA) combines with proteins to form a ribosome, the physical location of protein synthesis. Transfer RNA (tRNA) molecules are “connectors” that bind mRNA codons at one end and specific amino acids at the other. Their role is to carry each amino acid to the ribosome at the correct spot along the mRNA molecule. 11. List the sequences of the mRNA molecules transcribed from the following template DNA sequences: a. T G A A C T A C G G T A C C A T A C b. G C A C T A A A G A T C The complementary sequences are: a) ACUUGAUGCCAUGGUAUG b) CGUGAUUUCUAG 12. How many codons are in each of the mRNA molecules that you wrote for question 11? a. 6 codons b. 4 codons 13. Refer to the figure to answer these questions: a. Add labels for mRNA (including the 5’ and 3’ ends) and tRNA. In addition, draw the RNA polymerase enzyme and the ribosomes, including arrows indicating the direction of movement for each. b. What are the next three amino acids to be added to polypeptide b? c. Fill in the nucleotides in the mRNA complementary to the template DNA strand. d. What is the sequence of the DNA complementary to the template strand (as much as can be determined from the figure)? e. Does this figure show the entire polypeptide that this gene encodes? How can you tell? f. What might happen to polypeptide b after its release from the ribosome? g. Does this figure depict a prokaryotic or a eukaryotic cell? How can you tell? a. Refer to figures 7.10 (Transcription Creates mRNA) and 7.15 (Translation Creates the Protein). b. LysGlySer c. The remaining mRNA nucleotides are (from left to right): CUUAGGACACC d. The complementary DNA sequence is (from left to right): CTTAGGACACC e. No, because the last codon would be a stop codon (UAA, UAG, or UGA) f. The peptide would fold into its proper shape and then either begin performing its function in the cell or be exported to the cell’s exterior. g. The figure depicts a prokaryotic cell. In eukaryotes, the mRNA is fully synthesized in the nucleus, undergoes processing, and then is transcribed in the cytoplasm. The figure shows translation occurring simultaneously with transcription, which only occurs in prokaryotes. 14. Is changing the first nucleotide in a codon more likely or less likely to change the encoded amino acid than changing the third nucleotide in a codon? Consult the dictionary of the genetic code. Changing the first nucleotide in a codon typically changes the encoded amino acid. In contrast, changing the third nucleotide of a codon often does not change the encoded amino acid (e.g., look at the codons for serine, proline, and alanine). 15. Titin is a muscle protein whose gene has the largest known coding sequence—80,781 DNA bases. How many amino acids long is titin? The titan protein is 26,927 amino acids (80,781 nucleotides divided by 3 nucleotides per amino acid). 16. If a protein is 1259 amino acids long, what is the minimum size of the gene that encodes the protein? Why might the gene be longer than the minimum? 1259 x 3 = 3,777 bases plus three bases for stop codon = 3,780 bases. The gene would have bases for the leader sequence on the mRNA and might include any number of introns. 17. How did researchers reason that a combination of at least three RNA bases must specify each amino acid? Since RNA has four types of bases and proteins have 20 types of amino acids, one RNA base could not specify each amino acid. If a combination of two RNA bases specified one amino acid, then only 16 amino acids could be encoded (four possibilities for position 1 of the codon multiplied by four possibilities for position 2 equals 16 combinations of RNA bases). Therefore, at least three RNA bases must specify each amino acid (4x4x4=64). Later studies confirmed that each codon contains three RNA bases. 18. The roundworm C. elegans has 556 cells when it hatches. Each cell contains the entire genome but expresses only a subset of the genes. Therefore, the cells “specialize” in particular functions. List all of the ways that a roundworm cell might silence the unneeded genes. An individual roundworm cell can keep some of its DNA coiled or attach methyl groups to inactivate genes. Transcription factors and enhancers needed for transcription might not be available. After transcription, different combinations of introns can be removed. mRNA can be confined to the nucleus or rapidly degraded. The proteins can also be degraded. 19. The genome of the human immunodeficiency virus (HIV) includes nine genes. Two of the genes encode four different proteins each. How is this possible? The genes each contain several introns. To make each protein, a different combination of introns is removed with the remaining mRNA spliced together. 20. The shape of a finch’s beak reflects the expression of a gene that encodes a protein called calmodulin. A cactus finch has a long, pointy beak; its cells express the gene more than a ground finch, which has a short, deep beak. When researchers boosted gene expression in a ground finch embryo, the bird’s upper beak was longer than normal. Develop a hypothesis that explains this finding. One possibility is that the calmodulin gene influences the length of the upper beak. Boosting calmodulin expression in the ground finch would therefore promote additional growth in the bird’s upper beak. Perhaps the ground finch’s lower beak was unaffected because other genes influence its size. 21. If a gene is like a cake recipe, then a mutation is like a cake recipe containing an error. List the major types of mutations, and describe an analogous error in a cake recipe. Missense: instead of baking powder, the recipe lists baking soda. Nonsense: the recipe cuts off after a partial list of ingredients. Insertion (3 nucleotides): the recipe lists one extra ingredient. Deletion (three nucleotides): the recipe leaves out one ingredient. Frameshift: the word spacing is altered, e.g., flour, wate, regg, ssuga, rsalt etc. Expanding repeat: the recipe lists an ingredient repeatedly. 22. A proteinencoding region of a gene has the following DNA sequence: T T T C A T C A G G A T G C A A C T Determine how each of the following mutations alters the amino acid sequence: a. substitution of an A for the T in the first position b. substitution of a G for the C in the 17th position c. insertion of a T between the fourth and fifth DNA bases d. insertion of a GTA between the 12th and 13th DNA bases e. deletion of the first DNA nucleotide a. Nonsense mutation; instead of encoding the amino acid lysine, the codon would recruit a release factor protein. b. Missense mutation; instead of incorporating the amino acid cysteine, the protein would incorporate serine. c. Frameshift mutation; valine is replaced by aspartic acid, and the remainder of the protein is disrupted. d. Insertion mutation; the amino acid histidine is added within the protein. e. Frameshift mutation; the entire protein is disrupted. 23. Explain how a mutation in a proteinencoding gene, an enhancer, or a gene encoding a transcription factor can all have the same effect on an organism. A mutation in the gene can lead to a polypeptide that is too short or has the wrong amino acids; in either case it will not fold properly, and therefore will not function properly. This means that the organism will not express the effects of that protein. A mutation to either an enhancer or a gene encoding a transcription factor can leave the transcription factor unable to bind to the gene, blocking transcription. 24. How can a mutation alter the sequence of DNA bases in a gene but not produce a noticeable change in the gene’s polypeptide product? How can a mutation alter the amino acid sequence of a polypeptide yet not alter the organism? A mutation may alter the sequence of a gene but not produce a noticeable change in the gene’s polypeptide sequence because several different codons encode most amino acids. A mutation may alter the amino acid sequence but not alter the phenotype because the protein’s shape may not change, other proteins may take over the altered protein’s function, or the protein may not be essential. 25. Describe the mutation shown in figure 7.26 and explain how the mutation affects the amino acid sequence encoded by the gene. Figure 7.26 shows a deletion mutation. Since exactly three nucleotides are deleted, the reading frame of the gene remains the same. One amino acid is deleted from the protein. 26. Parkinson disease causes rigidity, tremors, and other motor symptoms. Only 2% of cases are inherited, and these tend to have an early onset of symptoms. Some inherited cases result from mutations in a gene that encodes the protein parkin, which has 12 exons. Indicate whether each of the following mutations in the parkin gene would result in a smaller protein, a larger protein, or no change in the size of the protein. a. deletion of exon 3 b. deletion of six consecutive nucleotides in exon 1 c. duplication of exon 5 d. disruption of the splice site between exon 8 and intron 8 e. deletion of intron 2 a) Smaller protein. b) Smaller protein. c) Larger protein. d) No change. e) No change. 27. Consult the genetic code to write codon changes that could account for the following changes in amino acid sequence. a. tryptophan to arginine b. glycine to valine c. tyrosine to histidine Multiple answers are possible; these are examples. a) UGG to CGG. b) GGU to GUU. c) UAC to CAC. 28. Researchers use computer algorithms that search DNA sequences for indications of specialized functions. Explain the significance of detecting the following sequences: a. a promoter b. a sequence of 75 to 80 nucleotides that folds into a backwards letter L c. RNAs with poly A tails a) A promoter signals the start of a gene. b) These nucleotides compose a tRNA molecule. c) The poly A tails signal an mRNA. 29. In a disorder called gyrate atrophy, cells in the retina begin to degenerate in late adolescence, causing night blindness that progresses to blindness. The cause is a mutation in the gene that encodes an enzyme, ornithine aminotransferase (OAT). Researchers sequenced the OAT gene for five patients with the following results: • Patient A: A change in codon 209 of UAU to UAA • Patient B: A change in codon 299 of UAC to UAG • Patient C: A change in codon 426 of CGA to UGA • Patient D: A twonucleotide deletion at codons 64 and 65 that results in a UGA codon at position 79 • Patient E: Exon 6, including 1071 nucleotides, is entirely deleted. a. Which patient(s) have a frameshift mutation? b. How many amino acids is patient E missing? c. Which patient(s) will produce a shortened protein? a) Patient D. b) 357 amino acids. c) All will produce a shortened protein. Pull It Together 1. Why is protein production essential to cell function? Cell structure and function depend on proteins. Enzymes are proteins and are required for almost all chemical reactions to occur within a cell. Without enzymes, the cell could not synthesize ATP, which the cell uses for energy. In addition, proteins embedded within cell membranes have several important functions such as adhesion, cell recognition, and transport of watersoluble molecules; without protein production, new cell membrane proteins could not be produced when the cell divides. 2. Where do promoters, terminators, stop codons, transcription factors, RNA polymerase, and enhancers fit into this concept map? Both “Transcription factors” and “RNA polymerase” can connect with the phrase “bind to” to “Promoters.” Both “Promoters” and “Terminators” can connect with the phrase “are noncoding sequences of” to “DNA.” “Promoters” can also connect with the phrase “signals the starting point for ” to “Transcription.” “Terminators” can also connect with the phrase “signals the end point for” to “Transcription.” “Stop codons” can connect with the phrase “ends the process of” to “Translation.” “Transcription factors” can connect with the phrase “bind to” to “Enhancers.” 3. Use the concept map to explain how DNA nucleotides are related to amino acids. DNA nucleotides are transcribed to RNA nucleotides. An mRNA molecule is divided into threenucleotide codons, each of which corresponds to one amino acid. 4. Use the concept map to explain why a mutation in DNA sometimes causes protein function to change. A mutation is a change in a DNA sequence. If the mutation leads to a change in the encoded amino acid sequence, the protein’s shape could be altered or destroyed. Therefore, mutations could lead to changes in protein function. (A gene that undergoes a neutral mutation, however, encodes the same amino acid sequence. The protein’s function therefore does not change.) Chapter 12: Evolutionary Change 12.1 1. What are two ways to define evolution? One way to define evolution is “descent with modification.” On a population genetics scale, evolution is a change in allele frequencies in a population over time. 2. Why can’t evolution act on individuals? Evolution is a change in allele frequencies in a population over time. Allele frequencies are a property of a population, not an individual; an individual’s alleles do not change within a lifetime. The only way evolution can occur is if allele frequencies in a population change from generation to generation. 12.2 1. How does the history of evolutionary thought illustrate the process of science? Scientists (and others) wondered about the origin of life’s diversity. They proposed many possible explanations and tested them by using multiple lines of evidence (including fossils, geology, and observations of existing species). Over time, the evidence falsified some of these ideas while supporting the theory of evolution by natural selection. 2. What did Darwin observe that led him to develop his ideas about the origin of species? As Darwin journeyed in the HMS Beagle, he observed the uniformity of geological processes such as volcanism, earthquakes, and erosion. He collected fossils and living specimens, and he observed that each location had unique species that were different from those on other continents. On the Galapagos, he observed and collected finches from different islands and noted differences in giant tortoises on different Galapagos Islands. These observations led Darwin to the idea of “descent with modification.” 3. How might artificial selection and natural selection produce the same result? Which process would be faster? Why? In artificial selection, a human chooses which individuals get to breed, in an effort to select for one or more desirable traits. In natural selection, the environment might select for the same traits, provided that those traits promote reproductive success. Artificial selection would probably be faster because the breeder can select only those individuals that come closest to the “ideal” and prevent all other individuals from reproducing in each generation. It is therefore likely to be more efficient than natural selection. 4. What is the modern evolutionary synthesis? The modern evolutionary synthesis combines Darwin’s idea of natural selection with a modern understanding of genes, chromosomes, and the origins of variation. 12.3 1. What is an adaptation, and how do adaptations become more common within a population? An adaptation is a characteristic that helps an individual survive and reproduce in its environment. Adaptations become more common within a population when they are heritable and when they increase the odds of survival and reproduction. 2. What is the role of genetic variation in natural selection? Genetic variation is the raw material that natural selection acts on. Genetic variation creates variations in phenotypes, some of which are more likely than others to lead to reproductive success in the current environment. 3. How can natural selection favor different phenotypes at different times? Natural selection can favor different phenotypes at different times because environments and selective forces are always shifting. 4. Why doesn’t natural selection produce perfectly adapted organisms? Natural selection does not produce perfectly adapted organisms because each population’s evolution is constrained by its existing gene pool, which may not contain every allele necessary to confront every challenge. Chance events may also wipe out adaptive allele combinations. Genetic illnesses that produce symptoms only after reproductive age also can maintain harmful alleles in a population. 5. What is evolutionary fitness? Evolutionary fitness is measured by reproductive success. Chapter 16: Viruses 16.1 1. What features do all viruses share? All viruses contain genetic information (either DNA or RNA) and are surrounded by a protein coat. 2. What determines a virus’s host range? A virus’s host range is determined by the presence of a specific receptor on the host cell. 3. How do viruses evolve? Viruses evolve by natural selection. Mutations in viral DNA or RNA create genetic variation; some variants are more successful than others at infecting cells and leaving descendants. 16.2 1. Describe the five steps in viral replication. (1) Attachment: virus adheres to host cell receptor. (2) Penetration: virus enters the cell. (3) Synthesis: multiple copies of the viral genome and proteins are produced by the host cell. (4) Assembly: the viral genetic information is packaged in a protein coat. (5) Release: new viruses leave host cell. 2. What is the source of energy and raw materials for the synthesis of viruses in a host cell? The source of energy and raw materials is the host cell’s ATP and its stores of nucleotides and amino acids. 16.4 1. How can a person acquire and transmit a viral infection? Viral infections can be acquired by inhaling respiratory droplets or ingesting contaminated food and water. Some viruses are also acquired through the bloodstream and transmitted by blood transfusion, sexual contact, or contaminated needles. 2. How do symptoms of a viral infection develop? A viral infection can cause symptoms by killing host cells in the respiratory tract, skin, immune system, or other body parts. In addition, the viral infection triggers an immune response that causes wholebody symptoms like fever and inflammation. 3. What is a latent animal virus? In a latent infection, a virus has infected a host cell, but the viral genetic information is not being expressed. The host cell is therefore not producing new viruses. 4. How are some latent viral infections linked to cancer? Some latent viruses signal host cells to divide continuously, a strategy that increases the number of infected cells but can also cause cancer. 5. Describe how HIV replicates in host cells. After attachment and penetration, the HIV enzyme reverse transcriptase copies the viral RNA to DNA. The viral DNA then inserts itself into the host cell's DNA. New HIV particles are synthesized and assembled inside the host cell and are released by budding. A new virus acquires its envelope from the host cell’s membrane as it emerges. 6. How are viral infections treated and prevented? Viral infections are difficult to treat, in part because viruses infect living host cells; it is difficult to destroy the virus without also destroying the host cell. A few antiviral drugs do prevent viral replication, but viruses are genetically variable, and new treatments quickly select for resistant varieties. The best weapon for prevention is the vaccine, which builds immunity to a virus before a person is exposed. Chapter 17: Bacteria & Archea 17.1 1. What are two domains that contain prokaryotes? Bacteria and Archaea are the two domains that include prokaryotes. 2. List several ways that prokaryotes have influenced evolution. Prokaryotes have influenced evolution by being the first cells on Earth, contributing O to2 the atmosphere, helping to create the ozone layer, and becoming incorporated into eukaryotic cells as mitochondria and chloroplasts. 3. In what habitats do bacteria and archaea live? Prokaryotes live in nearly every conceivable habitat; few areas on or near Earth’s surface are free of microbes. 4. Why are most species of prokaryotes little understood? The bestunderstood microbes are the ones that can be cultivated in laboratories. However, many types of microbes cannot grow in the laboratory. We know these organisms exist because their DNA sequences have been discovered, but we know little about their abundance, distribution, metabolic activities, or roles in ecosystems. 17.4 1. In what ways are bacteria and archaea essential to eukaryotic life? Microbes are decomposers, photosynthetic organisms, food sources, and nitrogen fixers. 2. How are the microbes that colonize your body beneficial? The microbes that colonize the human body help prevent attack by pathogenic bacteria, help train the immune system to ignore harmless substances, and produce vitamins. 3. What adaptations enable pathogenic bacteria to enter the body and cause disease? Pathogenic bacteria enter the body with insect or tick bites, during sexual activity, in food, air, or water, or by direct contact with wounded skin surfaces. Once inside the body, pili help the bacteria to attach to host cells, and bacterial enzymes attack host tissues. Bacterial toxins can disable the host’s circulatory, digestive, or nervous system. 4. What are some practical uses of bacteria and archaea? Bacteria and archaea are used in the production of food, vitamins, and other useful substances. Transgenic bacteria produce human proteins for use in pharmaceutical drugs. Bacteria and archaea also participate in water and waste treatment. Chapter 26: The Nervous System 26.2 1. Describe the parts of a typical neuron. Dendrites are branches that convey sensory input to the neuron’s cell body. The cell body, which contains the nucleus, mitochondria, and ribosomes, carries out the neuron’s metabolic functions. The axon is a long fiber extending from the cell body. An axon branches at its tip and transmits information from the cell body to other neurons, muscles, or glands. 2. Where is the myelin sheath located? The myelin sheath surrounds the axon of some neurons. 3. In what direction does a message move in a neuron? Typically a message moves from dendrites to cell body to axon. 4. What are the functions of each of the three classes of neurons? Sensory neurons bring information to the central nervous system. Motor neurons convey information from the central nervous system to muscles and glands. Interneurons are central nervous system neurons that connect one neuron to another. 26.3 1. Describe the forces that maintain the distribution of K and Na across the cell membrane in a neuron at rest. At rest, a neuron has a high concentration of K inside the cell (relative to the outside) and a high concentration of Na outside the cell (relative to the inside). The sodium + + + potassium pump moves Na out of the cell as it moves K in. In addition, K is simultaneously repelled by the Na outside and attracted to the negatively charged proteins inside the cell. 2. In what way is the term resting potential misleading? The term “resting potential” is misleading because the neuron is ready to fire and not really “at rest.” Also it is expending almost 75% of its energy to maintain the distribution of K and Na while “at rest.” 3. Differentiate among a graded potential, the threshold potential, and an action potential. A graded potential is a local flow of electricity that depends on stimulus strength and weakens with distance. A graded potential may depolarize the cell to i+s threshold potential (around 50mV), which is a signal to open the cell’s Na channels (the first step in an action potential). An action potential is a brief depolarization that propagates along a nerve fiber. 4. How does an axon generate and transmit a neural impulse? + If the “trigger zone” reaches the threshold potential, Na channels in the axon will briefly + open, depolarizing the axon. This depolarization then propagates down the axon as Na in one local area diffuses into the next and brings it to threshold. 5. What prevents action potentials from spreading in both directions along an axon? During the refractory period, a local area of the axon that has depolarized returns to resting potential and cannot generate action potentials. Action potentials therefore move only from the trigger zone toward the tip of the axon. 6. How do action potentials indicate stimulus intensity and type? The rate of action potentials indicates the strength of a stimulus. The type of neuron stimulated and the part of the brain that receives the signal indicates the type of stimulus. 7. How do myelin and the nodes of Ranvier speed neural impulse transmission along an axon? Myelincovered parts of the axon lack sodium channels; nodes of Ranvier are gaps in the myelin sheath where Na channels allow depolarization. This arrangement speeds neural impulse transmission because it allows the action potential to jump between nodes. 26.4 1. Describe the structure of a synapse. The presynaptic cell ends in a synaptic terminal. Calcium channels are embedded in the membrane of the terminal, and synaptic vesicles filled with neurotransmitters occupy the synaptic terminal. The postsynaptic cell has ion channels with receptors for the neurotransmitters. Between the two cells is a small gap called the synaptic cleft. 2. What event stimulates a presynaptic neuron to release neurotransmitters? When action potentials depolarize the synaptic terminal, calcium channels open and calcium diffuses in. This influx triggers the release of the neurotransmitters into the synaptic cleft. 3. What happens to a neurotransmitter after its release? After a neurotransmitter is released, some of it travels to receptors on the postsynaptic cell. Some diffuses away, some is enzymatically inactivated, and some is taken back into vesicles within the presynaptic cell. 4. How does synaptic integration determine whether a neuron transmits action potentials? Like in a voting system, synaptic integration involves “adding up” the number of excitatory and inhibitory signals. If excitatory signals predominate then there will be an action potential. 26.5 1. What is the difference between cranial and spinal nerves? A cranial nerve exits the central nervous system at the brain, and a spinal nerve exits at the spinal cord. 2. How do the sensory and motor pathways of the peripheral nervous system differ? The sensory pathways transmit action potentials from the peripheral nervous system to the central nervous system, and the motor pathways carry information from the central nervous system to muscles and glands. 3. Describe the relationships among the autonomic, sympathetic, and parasympathetic nervous systems. The autonomic nervous system carries signals to muscles and glands that are under involuntary control. The autonomic system is further divided into the sympathetic system, which dominates under stress, and the parasympathetic system, which restores body systems to normal during relaxed times. 4. How do the sympathetic and parasympathetic nervous systems maintain homeostasis? These systems continually work together, in opposition to each other, to adjust blood pressure, breathing rate, nutrient availability, and other body functions to meet the body’s needs. Chapter 27: The Senses 27.1 1. What role do the senses play in maintaining homeostasis? The senses monitor internal and external stimuli, including blood pH, body temperature, levels of ions and water in interstitial fluids, and a host of other physical and chemical conditions. Information about these stimuli is transmitted to the central nervous system for processing and may trigger hormonal, chemical, or behavioral adjustments that maintain homeostasis. 2. Distinguish between sensation and perception. A sensation is the raw input of a receptor as it arrives at the central nervous system. Perception is the interpretation of the sensation in the central nervous system, and it reflects an integration of all sensory input along with memory. 3. What are the major types of sensory receptors? The major types of sensory receptors are mechanoreceptors, thermoreceptors, pain receptors, proprioceptors, photoreceptors, and chemoreceptors. 4. What is a receptor potential? A receptor potential is a graded potential that occurs in a sensory receptor. If the receptor potential is large enough, it will generate an action potential in the sensory receptor. 5. What is sensory adaptation, and how is it beneficial? Sensory adaptation is a reduced response to a stimulus. It enables us to tune out sensations that are the equivalent of irrelevant “background noise.” 27.2 1. Which structures provide the senses of touch, temperature, pain, and position? Mechanoreceptors in the skin provide the sense of touch. Some free nerve endings in the skin are thermoreceptors, whereas other free nerve endings detect mechanical damage and produce the sensation of pain. Position is detected by proprioceptors. 2. What is the role of the cerebral cortex in integrating information about the general senses? A portion of the cerebral cortex receives information about the general senses. That input is mapped to specific body locations so that the sensation can be interpreted. 27.3 1. How does the brain distinguish one odor from another? A portion of the cerebral cortex distinguishes one odor from another based on the specific combination of receptor proteins that have transmitted the impulse from the olfactory epithelium. 2. How does a taste bud function? A taste bud is a cluster of taste receptor cells. When a food molecule binds to a chemoreceptor in a taste bud, sensory neurons convey the message to a portion of the cerebral cortex for processing and integration. 27.4 1. Describe three types of invertebrate eyes. Some invertebrates have cupshaped eyes made of photoreceptor cells, as in the eyespots of a flatworm. Others have compound eyes with tightly packed photoreceptors, as in insects. Finally, some invertebrates have a single lens eye that is fluid filled and houses photoreceptors at the back, as in cephalopods. 2. What are the parts of the vertebrate eye? The sclera includes the white of the eye and the cornea. The choroid is internal to the sclera; it includes the iris, pupil, and lens. The retina, the innermost layer of the eye, consists of photoreceptor cells. Most of the eye’s volume is filled with the jellylike vitreous humor; the watery aqueous humor fills the space between the cornea and lens. 3. How do rods, cones, and pigments participate in vision? Rod cells and cone cells detect light. Rod cells provide blackandwhite vision in dim light, and cone cells provide color vision in bright light. Both cell types contain light sensitive pigments that absorb photons of light and trigger receptor potentials that are passed on to other neurons that send action potentials to the brain. 4. Trace the pathway of information flow from the retina to the visual cortex of the brain. In the retina, light sensitive pigments in rods and cones absorb light energy of different wavelengths. In the presence of light, the pigment molecule changes shape and triggers a receptor potential that stimulates the retina’s bipolar neurons. These send the message to the ganglion cells. If they become depolarized, the ganglion cells send action potentials through the visual pathway to the optic nerve. The optic nerve exits the eyeball, traveling from the retina to the brain. Optic nerves first go to the thalamus. Then the visual information goes to neurons in the primary visual cortex of the brain. 27.5 1. What are the parts of the ear, and how do they transmit sound? The outer ear funnels sound waves into the auditory canal that ends in the eardrum. In response to sound waves, the eardrum and bones of the middle ear move; their movements jiggle the fluid of the inner ear’s cochlea. Vibration of the fluid in the cochlea causes cilia of hair cells to move relative to the tectorial membrane. This movement, in turn, causes the hair cells to release a neurotransmitter that triggers action potentials in the auditory nerve. 2. How does the vestibular apparatus provide the sense of equilibrium? The vestibular apparatus includes the utricle and saccule, along with the three semicircular canals. The utricle and saccule detect whether the head is accelerating horizontally or vertically. Calcium carbonate granules cause hair cells to bend, triggering action potentials that the brain senses as acceleration. The semicircular canals detect whether the head is tilting or rotating. These motions shift the fluid in the semicircular canals; the fluid, in turn, bends the cilia of hair cells that transmit action potentials to the nearby cranial nerve and from there to the brain. Chapter 30: Circulatory System 30.6 1. Where does lymph come from? Lymph is blood plasma that leaks out of capillaries. 2. List the functions of each component of the lymphatic system. Lymph capillaries absorb lymphatic fluid from tissues and send it to lymph nodes, which contain white blood cells that remove foreign cells and tissue debris from the fluid. Eventually, lymph vessels empty lymphatic fluid into the bloodstream in veins in the chest. The spleen is an organ of the lymphatic system that releases white blood cells. T cells mature in the thymus. 3. How does lymph travel within lymph capillaries? Lymph is pushed through lymph capillaries when the surrounding skeletal muscles contract. Chapter 34: Immune System 34.1 1. List the cell types that are important in the body’s defenses, along with some of their functions. Neutrophils act as phagocytes, which engulf bacteria and debris; monocytes also develop into macrophages. Eosinophils defend against worms and other multicellular parasites. Basophils and mast cells release chemicals to trigger inflammation and allergies. Two types of lymphocytes (T cells and B cells) coordinate defenses against specific pathogens. Natural killer cells are lymphocytes that attack cancerous and virusinfected cells. 2. List and describe the components of the lymphatic system. Lymph is the colorless fluid that carries white blood cells and that the lymphatic system transports, cleanses, and returns to the bloodstream. Lymphoid organs include the thymus, spleen, and lymph nodes (collections of lymphocytes embedded in loose connective tissue). Lymph vessels are the tubes that absorb and carry lymph. 3. How does the immune system interact with the circulatory system? Phagocytes, antibodies, and other immune system cells and substances originate in the immune system but travel in the bloodstream. In addition, lymph originates as blood plasma that leaks out of blood vessels. Once cleansed, lymph returns to the blood. 4. What are the two subdivisions of the immune system? The immune system is divided into innate defenses and adaptive immunity. 34.2 1. List five categories of innate defenses. Barriers, white blood cells, inflammation, the complement system, and fever are the innate defenses. 2. What are some barriers to infection in the human body? Barriers to infection in the human body include the skin, mucous membranes in the nose and throat, earwax, tears, cilia in the respiratory system, acid secretions of the stomach, and the normal bacteria in the gut and elsewhere. 3. How do white blood cells and macrophages destroy invaders? Macrophages and neutrophils destroy invaders by phagocytosis. Other white blood cells trigger inflammation or secrete chemicals that destroy pathogens. 4. How can inflammation be both helpful and harmful? Inflammation can be helpful because it recruits immune components, helps clear debris, and creates an environment hostile to microorganisms around the site of injury or infection. Inflammation can also be harmful because it can cause the site to become swollen and painful. Also, joints and other body parts can become chronically inflamed even if they are uninfected, causing great discomfort. 5. How is fever protective? Fever creates an environment that is hostile to bacteria and viruses. Not only does the warmer temperature inhibit some bacteria and viruses, it also reduces the iron level in the blood, depriving bacteria and fungi of iron and reducing their replication. Fever also increases the activity level of phagocytes. 34.3 1. What is the relationship between antigens and antibodies? Antigens are molecules that trigger the production of antibodies. 2. What are the two subdivisions of adaptive immunity, and which cell types participate in each? The two subdivisions of adaptive immunity are the cellular response and the humoral response. Helper T cells, memory T cells, and cytotoxic T cells participate in the cellular response. Memory B cells and plasma cells participate in the humoral response. 3. In your own words, write a paragraph describing the events of adaptive immunity, beginning with a pathogen entering a host’s body and ending with the production of memory cells. When a pathogen enters a host’s body, it may be present in body fluids and infect living cells. A macrophage engulfs a pathogen in body fluids and displays the pathogen’s antigens on its surface. The antigenpresenting macrophage enters a lymph node, where the combination of self protein plus antigen attracts helper T cells. Activated helper T cells divide into memory cells and effector cells, which secrete chemicals that activate B cells and cytotoxic T cells with receptors for the antigen. The activated B cells divide and differentiate into plasma cells that produce antibodies; some B cells also differentiate into memory cells. Likewise, activated cytotoxic T cells differentiate into memory cells and effector T cells that attack host cells already infected by the pathogen. 4. Describe the structure and function of an antibody. An antibody is a Yshaped protein that matches a specific antigen. Upon encountering an invader with a matching antigen, the antibody binds to the antigen. This may make the antigen more noticeable to macrophages, inactivate a microbe, or neutralize its toxins. Viruses that are coated with antibodies may not be able to attach to target cells. Antibodies also trigger the production of complement proteins that destroy microbes. 5. What happens if an immune reaction persists after a pathogen is eliminated? An immune system response that persists after a pathogen is eliminated may attack and damage the body’s own tissues. 6. Explain the difference between the primary and secondary immune response. The primary immune response is triggered the first time the body encounters a pathogen; a secondary immune response is triggered on subsequent exposure. A primary immune response may take days or weeks to respond to the infection. The secondary immune response is much quicker and stronger than the primary response. 34.4 1. What is a vaccine? A vaccine is a mixture containing antigens that stimulate the immune response without actually causing disease. 2. List the main types of vaccine formulations. Vaccines can be composed of live but weakened pathogens, inactivated pathogens or toxins, or subunits of pathogens. 3. Why haven’t scientists been able to develop vaccines against HIV and the common cold? These viruses mutate too frequently for a vaccine to be effective. 34.5 1. What is autoimmunity? In autoimmunity, the immune system attacks the body’s own molecules, potentially leading to severe tissue and organ damage. 2. Which immune system cells does HIV attack, and what is the consequence? HIV attacks helper T cells, causing the body’s immune response to fail. Once too many T cells are lost, the person becomes susceptible to opportunistic illnesses and infections. 3. Which cells and biochemicals participate in an allergic reaction?
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