Chapter 8 DNA STRUCTURE AND CHROMOSOMAL ORGANIZATION Chapter Summary Even though DNA has been known as a biochemical compound for over 100 years, it was not implicated as the carrier of hereditary information until much more recently. A number of experiments showed in a variety of ways that DNA, and not protein (as was originally thought), was capable of transmitting information from one generation to the next. Once the scientific community was convinced of the role of DNA, more work was needed to show how a chemical molecule can carry hereditary information. DNA, and its close relative RNA, are composed of repeating subunits known as nucleotides. As there are only four kinds of DNA nucleotides and four kinds of RNA nucleotides, the answer to the mystery was found to lie in the sequence in which these building blocks occurred. The names most closely associated with the details of DNA structure and function are Watson and Crick, who received the Nobel Prize for this contribution. A DNA molecule looks like a ladder with side rails formed from alternating sugar and phosphate molecules, and the rungs formed from paired bases of two single DNA strands. The entire ladder is twisted into a spiral. Since each side rail can be visualized as making a spiral or helix, the DNA molecule is often called a double helix. In bacteria and in the organelles called mitochondria, the DNA forms a circular molecule. In the nuclei of higher organisms, the DNA is wrapped around protein molecules that stabilize the DNA. When chromosomes replicate, the two sides of the DNA separate, a process similar to cutting a ladder from top to bottom down the middle of the rungs. Each single strand of DNA can then act as a pattern for the synthesis of the other half of the molecule. The result is two molecules of DNA, each composed of one old strand and one new strand.
60 Chapter 8 Learning Objectives At the end of this chapter, you should have a thorough understanding of: a. the chemical composition and structure of the components of DNA. b. the historical background and experiments leading to the recognition of DNA as the molecule that carries the hereditary information. c. the Watson-Crick model of DNA. d. chromosome structure. e. the mechanism by which DNA is able to replicate faithful copies. Key Terms transformation transforming factor covalent bonds hydrogen bond deoxyribonucleic acid (DNA) ribonucleic acid (RNA) nucleotide nitrogen-containing base purine pyrimidine pentose sugar adenine and guanine thymine, uracil and cytosine ribose and deoxyribose template chromatin histones nucleosome semiconservative replication DNA polymerase Multiple choice Circle the letter of the best answer. 1. Which of the following best characterizes the structure of DNA? a. a double helix in which purines are bonded to purines and pyrimidines to pyrimidines b. a double helix in which the strands are held together by deoxyribose c. two strands of nucleotides held together by hydrogen bonds and formed into a double helix d. two strands of amino acids held together by peptide bonds and twisted into a spiral secondary structure
2. The amount of adenine in a sample of DNA a. equals the amount of thymine b. equals the amount of guanine c. equals the amount of cytosine d. equals the amount of uracil DNA Structure and Chromosomal Organization 61 3. In that same sample (question #2), the amount of adenine a. can be found in a 1:1:1:1 ratio b. might be 20 percent of the total bases present in the DNA molecules c. could be as high as 50 percent of the total bases present d. could reach 100 percent of the total bases present 4. A cycling cell has a certain amount of DNA (x amount) in its nucleus at G 1. When this cell enters prophase of mitosis, the amount of DNA could be represented as a. 4x b. 2x c. x d. x/2 5. The same type of cell enters Prophase I of meiosis. Then the amount of DNA in the cell would be represented as a. 4x b. 2x c. x d. x/2 6. At the end of Prophase II, the DNA in a secondary oocyte will be a. 2x b. x c. x/2 d. x/4 7. A sperm would be found to contain DNA equal to a. 2x b. x c. x/2 d. x/4 8. Compared with a normal diploid cell, the amount of DNA in a triploid cell will be a. 1.5x the amount in a normal diploid cell b. 3x the amount in a normal diploid cell c. the amount in the diploid cell plus the amount in one chromosome d. the same amount that the diploid cell has
62 Chapter 8 9. A cell from an individual with Down syndrome (Trisomy 21) will contain a. 1.5x the amount of DNA in a diploid cell from a normal individual b. 3x the amount in a diploid cell from a normal individual c. the amount in a diploid cell plus the amount in one chromosome d. the same amount that the diploid cell from a normal individual has 10. The difference between a covalent bond and a hydrogen bond is a. hydrogen bonds are stronger than covalent bonds b. hydrogen bonds are found in DNA and nowhere else c. hydrogen bonds connect two atoms d. hydrogen bonds are more easily broken than are covalent bonds 11. The bonding pattern of the two DNA strands has a. guanine bonding to adenine b. adenine bonding to cytosine c. each strand covalently bonded together d. purines bonding to pyrimidines 12. The replication of DNA occurs during a. translation b. interphase c. transcription d. prophase e. none of these 13. Guanine is the complementary base to a. cytosine b. uracil c. thymine d. adenine e. all of the above 14. What is meant by DNA replication being a semiconservative process? a. The replicated DNA consists of one old strand and one new. b. The replication process is one in which A bonds to T and C bonds to G. c. The replicated DNA is retained by the cell producing it. d. The replication process produces two new strands that remain together.
DNA Structure and Chromosomal Organization 63 Fill-ins Supply the word or words to complete the statement. 1. Adenine and guanine are two-ring structures called, whereas cytosine and thymine are single ring structures called. 2. The enzyme that is responsible for linking DNA nucleotides together is called. 3. Griffith was the first researcher to propose that. 4. Avery, McLeod, and McCarty built on Griffith s work and identified the transforming factor as. 5. The only nucleic acid to contain the base uracil is. 6. A human chromosome could be described as a single long molecule of wrapped around. 7. If one strand of a DNA molecule contains the base sequence AATGCCGAT, the other strand will have the base sequence. 8. The two strands of DNA in question #7 are not identical to each other, but are called. 9. A nucleotide is composed of a five-carbon, a phosphate group, and a nitrogen-containing. 10. A covalent bond is formed when two atoms a pair of electrons; this electron sharing does not occur in bonds. Editing If the statement is true, label it T. If the statement is false, change the underlined word or phrase to make it true. Merely adding no or not is not an acceptable change. 1. Following replication a chromosome is composed of two sister chromatids. Each chromatid contains a DNA molecule. The DNA molecule in one chromatid is the original DNA molecule that was in the pre-replication chromosome.
64 Chapter 8 2. A Nobel Prize was awarded to Franklin and Miescher for their contributions to a description of the structure of DNA. 3. The two strands of a DNA molecule are held together with covalent bonds. 4. When two nucleotides are bonded together, the pentose sugar of one molecule is joined with the pentose sugar of the other nucleotide. 5. The double helix shape of DNA was first seen in x-rays. 6. The relative amounts of the four nitrogen-containing bases led Chargaff to the model that A pairs with A or G and T pairs with T or C. 7. A strand of RNA and a strand of DNA each contain the same three types of nitrogencontaining bases; in addition, DNA has the base thymine, which is not found in RNA. 8. If a woman has a disorder caused by a defect in her mitochondrial DNA, she will pass that defect to one-half of her sons. 9. Griffith s experiments demonstrated that hereditary material from dead virulent bacteria caused cancer. 10. Referring to the two ends of a DNA molecule as the 5 end and the 3 end is a shorthand way of indicating that each end begins with the same kind of molecule. 11. The human genome consists of about three billion base pairs and 100,000 genes. Short Discussion Questions/Problems 1. A single strand of a DNA molecule is represented by ATGCCTAAGT. a. Using this strand as a template, write the base sequence of the complementary strand. b. List the steps that have to happen to replicate this piece of DNA. c. What molecules must be present in order for this DNA to be replicated? d. Draw a diagram showing the two molecules of DNA as they appear following replication. e. In your diagram, use arrows to indicate which parts of the two molecules represent the original DNA strands and which parts are newly synthesized.
DNA Structure and Chromosomal Organization 65 2. You are trying to determine the hereditary material of a primitive organism from Mars. What characteristics does a good candidate molecule have? 3. In your studies of the primitive organism from Mars (question #2) suppose that two compounds have been suggested as being candidates for the hereditary material. Each one contains an element not found in the other. How could you use radioactive isotopes to determine which of the two compounds is replicated as the cells of the organism divide? 4. When prominent scientists disagree over a basic issue, such as DNA or protein as the hereditary material, how does a non-scientist decide whom to believe? 5. When prominent scientists disagree over a basic issue, how do members of the scientific community decide whom to believe? 6. Why is it important that a scientist not be too fond of his own theory? What is meant by objectivity? 7. One of the pitfalls in doing good science is assuming that because they have always said something is the case, it really is so. Often it is necessary to design experiments to test whether long-standing beliefs are true. Can you think of another instance of this happening? 8. If mitochondrial genes are maternally inherited, does that mean that they are X-linked? Explain.