Chapter 23: Classical and Modern Genetics

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Chapter 23: Classical and Modern Genetics

  1. 1. Chapter 23: Classical and Modern Genetics Key Terms: Genetics Double helix Purebred Messenger RNA (mRNA) Hybrid Transfer RNA (tRNA) Gene Genetic code Classical genetics Mutation Dominant Virus Recessive Genome Molecular genetics Human genome project Nucleic acids DNA mapping DNA DNA sequencing chromosome RNA In-Class Activities: Instructor Notes for In-Class Activity 1 Title: Personal Decision Making and Genetics Time: 10-15 minutes Materials: NA Handouts: NA Procedures: Allow the class to discuss various issues regarding genetics. You can start the ball rolling by asking, “When you have children, will you want to know the sex of the child?” Ask students why they feel the way they do. Note that one of the biggest reasons offered is economic. If they don’t pick up the discussion from there, prompt again, “What about diseases? What kinds of diseases would you want to know about? Why?” What would you do if you found out that the fetus had a debilitating physical problem, like heart problems that will require multiple surgeries throughout childhood and may be adulthood? A tendency toward addictions? A problem that would result in eventual death? What genetic problems would make you consider not having children at all? Tendencies for breast cancer? Tay-Sachs disease? Bad teeth? Hemophilia? Student NA Instructions: Specific This discussion could get heated as people’s beliefs are close held on Suggestions: many topics regarding human genetics. Be certain to begin the discussion with the need for people to raise their hands and be recognized before talking. You might also have a stopwatch and give each person only a minute or two to speak. 324
  2. 2. Chapter 23 Specific 1. When you have children, will you want to know the sex of the Questions: child? Why or why not? 2. What about diseases? What kinds of diseases would you want to know about? Why? 3. What would you do if you found out that the fetus had a debilitating physical problem, like heart problems that will require multiple surgeries throughout childhood and may be adulthood? A tendency toward addictions? A problem that would result in eventual death? 4. What genetic problems would make you consider not having children at all? Tendencies for breast cancer? Tay-Sachs disease? Bad teeth? Hemophilia? Objectives: Apply knowledge about genetics to personal decisions. 325
  3. 3. Chapter 23 Instructor Notes for In-Class Activity 2 Title: Extract Your Own DNA Time: About 10 minutes at the beginning of class if the materials are available at the front of the room Materials: This is a more “stuff intensive” activity. You will need toothpicks or cotton swabs for the class, a container (If you are generous, you can get the little containers that close up and make a small necklace. If that’s not in the budget, small clear vials will work or students can bring something the size of a baby food jar.), parafilm to close the containers, vinegar, Handouts: NA Procedures: Student Instructions: Specific Suggestions: Objectives: Extract personal DNA. 326
  4. 4. Chapter 23 Instructor Notes for In-Class Activity 3 Title: The Human Genome Project/The Mosquito Genome Project Time: Varies by which activities you choose Materials: The Human Genome Project CD – available from www.nhgri.nih.gov/educationkit The Mosquito Genome Project CD – available from www.aaas.org Handouts: NA Procedures: The CDs provide the ability to use the data from the genome projects to explore information about humans and mosquitoes and the effect this information can have on research both basic and applied. Student NA Instructions: Specific These CDs offer a variety of activities that help students understand the Suggestions: concepts involved as well as the importance of these two projects. They are well worth the time it takes to review and include activities appropriate to your students’s levels of understanding and interests. Objectives: Describe the activities and results of current research in genetics. Assess the importance of current genetic research to people in a variety of situations, cultures, geographic locations. 327
  5. 5. Chapter 23 Lecture Lead-Ins Spin a scenario that will make genetics more personal. Ask students to imagine they have recently married, and their spouse's sister has just given birth to a baby with Down's syndrome (or any other genetic disease like cystic fibrosis or hemophilia). What implications might this have for the student’s future children? What if they recall that the student’s own great-uncle also had that disease? Other scenarios are possible: You have just had your first child and it has cystic fibrosis (or some other genetic disease). What are the chances that your next child will also inherit the same disease? Your father has just been diagnosed with Huntington’s disease. What are the implications for your health? Active Learning Ideas Virtual fly lab Students can discover rules of inheritance by breeding virtual fruit flies. Enter as a guest at www.sciencecourseware.org/vcise/drosophila/Drosophila.php or go to http://www.biologylab.awlonline.com/ for a free one-day trial. At these sites students can gain access to a Virtual Fly Lab site where they can mate flies with various phenotypes, note the number of offspring, test hypotheses, calculate Chi Square and go through the virtual process of culturing fruit flies. DNA to RNA to proteins Have students translate a bit of DNA code into RNA and then form the encoded protein strand to drive home the connections between these molecules. The instructor can provide as a handout to be used in class, a short DNA nucleotide sequence. Students use base complements to transcribe the sequence of DNA into mRNA. A table of codons can then be used to determine the order of amino acids in the protein. Mutations Once students can derive proteins from DNA sequences, the consequences of DNA mutations are easily illustrated. Provide students with a DNA base sequence as well as several mutated versions of the sequence. They should then follow the procedure used above for determining the resulting protein sequence. Allow students to discover what happens to proteins as a result of different kinds of mutations: 1) A single nucleotide pair deletion or addition which creates a change in the triplet reading frame and results in a different sequence of amino acids and/or a premature stop codon. 2) A base pair change that creates a premature stop codon in the mRNA. 3) A base pair change that results in the substitution of a different amino acid. Lecture Extenders Homeobox Genes After presenting the idea that cells control when and where genes are expressed through the activity of regulatory genes, the role of “master” regulatory genes (or homeobox genes) in directing the development from the fertilized egg to the adult stage can be introduced. One amazing thing about these genes is that many of them are nearly identical in nucleotide sequence in all organisms so far studied. The highly conserved sequences of these genes suggest that they play very fundamental roles in the process of development. (See “Homeobox Genes and the Vertebrate Body Plan” by DeRobertis, Oliver and Wright, Scientific American, July 1990 and “The Molecular Architects of Body Design” by McGinnis and Kuziora, Scientific American, February 1994.) 328
  6. 6. Chapter 23 Stop and Think! Answers Page 469: Both worked with everyday materials that were readily available, yet these men thought about them more profoundly than others did. Both collected quantitative data and proposed explanations for what they observed. Both radically changed human thought and opened up new areas of science. Page 481: Certainly the sooner we are aware of new viruses, the sooner we will be able to treat or vaccinate. Global travel is so pervasive and simple today that international efforts certainly make sense. Answers to Discussion Questions 1. Yes, because this trait is due to a recessive gene, it cannot be exhibited unless two heterozygotes produce on offspring. Even then, each offspring will only have a 25% chance of inheriting both recessive alleles from the parents, which is necessary for the recessive trait to be observed. Therefore, the phenotypic expression of the trait can skip multiple generations. 2. In genetics, a hybrid refers to the offspring of genetically dissimilar parents. Gene expression depends on how alleles for a single locus on the chromosome interact to produce a phenotype. There are three types of dominance relationships. In a heterozygote with complete dominance, the recessive gene is present but it does not determine the offspring’s physical characteristics; it is not expressed. In incomplete dominance, a heterozygous genotype creates an intermediate phenotype, such as in a red flower crosses with a white flower yields a pink flower. In co-dominance, the offspring expresses both phenotypes, such as in ABO blood typing. 3. DNA sequencing determines the order of the thousands of nucleotides of a gene which is a tedious and time-consuming process, whereas mapping is a less precise process which places the gene in a location on a chromosome relative to known markers or other known genes. 4. Enzymes are proteins that facilitate cell processes. “One gene - one protein” means that one gene codes for the amino sequence of a single protein or polypeptide. In this process, DNA is transcribed in the nucleus into a strand of mRNA that can exit the nucleus and travel to the ribosome. There, tRNA molecules bring specific amino acids to peptide bond with one another in the exact sequence coded by the DNA thus producing a single protein. 5. Arguments for genetic engineering: a) It can be used to produce human proteins (e.g., clotting factors, insulin, growth hormone) which are difficult to produce otherwise because they must all be extracted from human tissues. b) It can be used to deliver “good” copies of genes into cells where the presence of mutated copies of the gene cause human diseases (e.g., currently gene therapy trials in children with ADA deficiency, which results in a nonfunctional immune system, and people with cystic fibrosis are underway.) c) It can be used to develop strains of bacteria that can metabolize and thus clean up a variety of pollutants in the environment (e.g., oil slicks on beaches), strains of plants with such characteristics as 329
  7. 7. Chapter 23 better yields, resistance to pests and, resistance to drought, and domestic animals with characteristics such as rapid growth. Arguments against genetic engineering: a) When gene therapy involves a virus as a vector for the gene, even though the virus is inactivated and cannot cause disease, it is unclear whether it would be likely to undergo recombination with other naturally infecting viruses and become an active pathogen again. 6. Since inbreeding means that a certain group of genes is being inherited in a fairly small group of people, it is more likely that traits, especially those due to rare recessive or dominant genes, will be exhibited in greater proportion for that small population than in the population at large. 7. Even though every cell has identical DNA, not all the genes composed of that DNA actually give rise to a functional protein because gene expression is controlled by regulatory genes which “turn different genes on and off” in different cells so that these cells have different functions. 8. Viruses have features in common with living cells: they have nucleic acids as their genetic material, they can evolve, and they can reproduce. However, they lack a typical cell structure, and they lack their own metabolism. Therefore, viruses must parasitize living cells in order to reproduce. Some students may feel the three features viruses have in common with cells qualify them as living, and others may feel all features of living cells must be seen to be qualified as living. 9. Exposure of the testis, where stem cells which undergo meiosis are located, to agents which can cause mutations, could lead to a mutation in a stem cell which later results in sperm carrying that mutation. If those sperm fertilize an egg, which goes on to develop, the resulting child will inherit that mutation which could possibly cause a disease. Also, exposure of the ovaries, which contain at birth all the primary oocytes that eventually go through the process of meiosis to produce eggs, to mutation causing agents can have the same outcome - inheritance of a gene that was mutated, possibly long before the child was conceived. Cells, however, are equipped with enzymes that recognize and repair many mutations before chromosomes are replicated. Once the mutation has been replicated, it usually cannot be detected by these enzymes and it becomes a permanent feature of the chromosome. 10. There are three types of RNA in a cell. Single stranded messenger RNA (mRNA) carries the information that was contained in the central double stranded DNA molecule out into the region of the cell where chemical reactions are going on. Transfer RNA (tRNA) molecules attach to specific amino acids and then bond complementary along the mRNA stand, placing their attached amino acid into the coded sequence creating a string of amino acids. This coded order of amino acids assembles a specific protein. Ribosomal RNA (rRNA) along with proteins compose the ribosome organelle. Thus all three types of RNA are involved in the synthesis of a single protein. 11. Inbreeding is the production of offspring by the mating of closely related individuals. This incest is a genetically unproductive practice because the practice restricts the possible gene pool. It provides a loss of genetic diversity and greater chance for recessive genes to be expressed phenotypically. The majority of serious genetic disorders are recessive. Cultural taboos on incest are only loosely related to genetic risk. 12. Sexual reproduction requires meiosis, a specialized type of cell division that results in the formation of egg and sperm. This process reduces the chromosome count of these gametes by half. Then when sperm and egg unite, a full complement of chromosomes in the newly formed zygote is present. 330
  8. 8. Chapter 23 Offspring resemble their parents because they inherit half their chromosomes from Mom (egg) and the other half from Dad (sperm). As the individual grows, new cells are added. In order for a cell to divide, it must first make copies of its existing chromosomes during a process called mitosis so that each new cell will have a complete set of chromosomes. These chromosomes are composed of many genes -- the basic physical and functional units of heredity. A gene, which is a strand of DNA, has a specific arrangement of nucleotide bases, whose sequences carry the information required for constructing proteins. In protein synthesis, the code in DNA is transcribed by RNA and transported to a ribosome for amino acid assembly. Many of your features, such as hair, skin, etc. are composed of these proteins, which is why offspring resemble their parents. Answers to Problems 1. If the average word is 5 characters and 8 bits/character then we have 40 bits/word. The number of bits in a set of encyclopedias will be 40 bits/word x 1500 words/page x 1000 pages/volume x 30 volumes, which equals 1,800,000,000 bits. If one nucleotide base pair is the equivalent of 2 bits of information, as there are four states (C,T,A,G) it could be in, and the length of the human genome is 3 billion bases, then 2(3 x 109) would be 6 x 109 bits of information in human DNA. To find the number of encyclopedia sets that would contain the same amount of information as in the human's DNA, divide the number of bits of information in human DNA, 6 x 109 bits, by 1.8 x 109 bits.. This equals 3.3 sets of encyclopedias. 2. In this problem, white fur is recessive to brown fur, and clear eyes are recessive to pink eyes. Using the symbols B for the brown gene, b for the white gene, P for the pink-eyed gene and p for the clear-eyed gene, the cross of a pure breeding (i.e., homozygous) brown, pink-eyed mouse to a pure breeding white, clear-eyed mouse can be represented as follows: BBPP x bbpp. All the offspring of this cross will be BbPp in genotype, and brown and pink-eyed in phenotype. 3. To determine which gene is dominant and which is recessive it would take one generation using pure breeding flies. The offspring of this cross, the F1s, would then be crossed and the resulting F2 generation should exhibit roughly a 3:1 ratio of those with the dominant trait to those with the recessive trait. Since two generations of flies are involved and each generation takes about 10 days, it will take about 20 days to repeat Mendel's experiments which investigated the inheritance of a single characteristic such as flower color. To repeat this experiment with elephants would take a bit longer. Since elephants only have one offspring every two years, it could take up to four years to produce enough F1s to cross (could take longer depending on gender produced). Then it will take 13 – 20 for the offspring to reach sexual maturity and another 22-24 years for the second generation to be ready to mate. More Practice Questions and Problems 1. What is the connection between a gene and a chromosome? (Ans. Genes are small lengths of the DNA molecule that makes up a chromosome.) 2. How does a gene determine all the inherited traits in an animal (or in any organism)? (Ans. Because genes code for and thus determine the organism's proteins, the function of the different proteins then determines the general phenotype for all inherited traits.) 331
  9. 9. Chapter 23 3. What is the connection between the nucleotide sequence of a gene and the order of amino acids in a protein? (Ans. The DNA nucleotide sequence in triplets, determines the amino acid sequence of the protein.) 4. Pretend you have analyzed a sample of DNA from an unknown virus and found that it was composed as follows: 20% A, 28% T, 12% G, and 40% C. Is this a single-stranded or double-stranded DNA? (Ans. It is made of a single strand of nucleotides. In a double strand, the %A=%T and %C=%G due to complementary pairing.) 5. When DNA replication occurs and a single, double-stranded molecule replicates to form two “progeny” double stranded molecules, would you expect to see that one of these molecules consisted of the two “old” or original nucleotide strands and the other molecule consisted of the two newly made nucleotide strands? (Ans. No, because DNA replication is semi-conservative and each “progeny” molecule will consist of one “old” nucleotide strand hydrogen bonded to one “new” nucleotide strand.) 6. Would you expect to see that the DNA coding for the protein determining genes, the rRNA genes, the tRNA genes and the regulatory genes composes about 90% of the DNA in the human genome? (Ans. No, it accounts for only about 5% of the genome and the other 95% is DNA of unknown function - sometimes called “junk” DNA.) 7. What would happen to a protein if, due to a mutation, a stop codon appeared in a mRNA after the fifth codon of a mRNA that is 105 codons in length? (Ans. The protein would only be five amino acids in length rather than 105 amino acids in length and is highly likely to be nonfunctional.) 8. If you crossed two gray cats and they had two gray kittens and five black kittens, could you conclude that black was dominant? (Ans. No, because if black were dominant one or both of the parents would have to be black. In this case, the two gray cats are heterozygous and by chance, the two recessive alleles for black got together in five of the seven fertilizations which produced the seven kittens.) Connecting Back 1. Hydrogen bonding is responsible for complementary base pairing in DNA. Find the section in chapter 10 on hydrogen bonding. When does it occur, and in what systems is it found? Connecting Ahead 1. Look ahead to chapter 24. How do cells repair DNA mutations? 2. Mutations make evolution possible. Look ahead at chapter 25 and then describe the connections between mutations, natural selection, and evolution. 332
  10. 10. Chapter 23 Further Reading “The Business of the Human Genome,” special section in Scientific American, July 2000. Capecchi, Mario R., “Targeted Gene Replacement,” Scientific American, March 1994, pp. 52-59. Cavanee, Webster K., and Raymond L. White, “The Genetic Basis of Cancer,” Scientific American, March 1995, p. 72. Horgan, John, “Eugenics Revisited,” Scientific American, June 1993, pp. 123-131. Jegalian, Karin, and Bruce T. Lahn, “Why the Y is So Weird,” Scientific American, February 2001, pp. 56-61. Paabo, Svante, “Ancient DNA,” Scientific American, November 1993, pp. 86-92. Rennie, John, “Trends in Genetics: Grading the Gene Tests,” Scientific American, June 1994, pp. 88-97. Rhodes, Daniela and Aaron Klug, “Zinc Fingers,” Scientific American, February, 1993. (an article on the proteins that regulate gene expression) Thornton, John I., “DNA Profiling,” Chemical and Engineering News, November 20, 1989, pp. 18-30. Williamson, Robert, and Rony Duncan, “DNA Testing for All,” Nature, August 8, 2002, pp. 585-586. 333
  11. 11. Chapter 23 Lecture Outline All living things use the same genetic code to guide the chemical reactions in every cell. I. Science Through the Day: A Family Resemblance II. Classical Genetics A. The Rules of Classical Genetics 1. Physical characteristics or traits are passed from parents to offspring by some unknown mechanism (we call it a gene). 2. Each offspring has two genes for each trait, one gene from each parent. 3. Some genes are dominant and some are recessive. When present together, the trait of a dominant gene will be expressed in preference to the trait of a recessive gene. B. Qualitative versus Quantitative Genetics III. DNA and the Birth of Molecular Genetics A. Nucleotides: The Building Blocks of Nucleic Acids B. DNA Structure C. RNA Structure D. The Replication of DNA IV. The Genetic Code 334
  12. 12. Chapter 23 A. Transcription of DNA B. The Synthesis of Proteins 1. One gene codes for one protein. 2. All living things on Earth use the same genetic code. C. Mutations and DNA Repair D. Why Are Genes Expressed? E. Viruses F. Viral Epidemics V. The Human Genome A. Science in the Making: Connecting Genes and DNA B. Science by the Numbers: The Human Book of Life C. Technology: New Ways to Sequence D. Science News: Shades of Flesh Tone VI. Thinking More About Genetics: The Ethics of Genes 335
  13. 13. Chapter 23 336

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