three parts. deoxyribose Differentiated Instruction

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SECTION 8.2 Plan and Prepare Objectives Describe the interaction of the four nucleotides that make up DN. Describe the three-dimensional structure of DN.

1 SECTION 8.2 Plan and Prepare Objectives Describe the interaction of the four nucleotides that make up DN. Describe the three-dimensional structure of DN. Section Resources Unit Resource Book Study Guide pp Power Notes p. 67 Reinforcement p. 68 Pre-P ctivity pp Interactive Reader Chapter 8 Spanish Study Guide pp Biology Toolkit pp. C3, C9, C31 Technology Power Presentation 8.2 Media Gallery DVD Online Quiz 8.2 ctivate Prior Knowledge Tell students that scientists found it hard to accept DN as the genetic material because of its structural simplicity. sk What are some examples of simple units that can be used to produce great complexity? letters of an alphabet, 0s and 1s of computer code, building blocks Which of these examples offer the best analogy to DN? alphabet and computer code because both are informational units Teach Biochemistry The nucleotides in a strand of DN all line up in the same direction. s a result, DN has chemical polarity, which means that the two ends of the DN strand are different. The 5 carbon is located at one end of the DN strand, and the 3 carbon is located at the other end. When the two strands of DN pair together, the 5 end of one strand aligns with the 3 end of the other strand a naylze and explain the molecular basis of heredity and the inheritance of traits to successive generations by using the Central Dogma of Molecular Biology. Structure of DN KEY CONCEPT DN structure is the same in all organisms. MIN IDES DN is composed of four types of nucleotides. Watson and Crick developed an accurate model of DN s three-dimensional structure. Nucleotides always pair in the same way. VOCBULRY nucleotide, p. 230 double helix, p. 232 base pairing rules, p. 232 Review covalent bond, hydrogen bond Connect The experiments of Hershey and Chase confirmed that DN carries the genetic information, but they left other big questions unanswered: What exactly is this genetic information? How does DN store this information? Scientists in the early 1950s still had a limited knowledge of the structure of DN, but that was about to change dramatically. MIN IDE DN is composed of four types of nucleotides. Since the 1920s, scientists have known that the DN molecule is a very long polymer, or chain of repeating units. The small units, or monomers, that make up DN are called nucleotides (NOO-klee-oh-TYDZ). Each nucleotide has three parts. phosphate group (one VISUL VOCB phosphorus with four oxygens) The small units, or monomers, that ring-shaped sugar called make up a strand of DN are called deoxyribose nucleotides. Nucleotides have three parts. nitrogen-containing base (a phosphate group single or double ring built around nitrogen-containing base nitrogen and carbon atoms) One molecule of human DN contains billions of nucleotides, but there are only four types of nucleotides deoxyribose (sugar) in DN. These nucleotides differ only in their nitrogen-containing bases. The four bases in DN are shown in FIGURE 8.4. Notice that the bases cytosine (C) and thymine (T) have a single-ring structure. denine () and guanine (G) have a larger, double-ring structure. The letter abbreviations refer both to the bases and to the nucleotides that contain the bases. For a long time, scientists hypothesized that DN was made up of equal amounts of the four nucleotides, and so the DN in all organisms was exactly the same. That hypothesis was a key reason that it was so hard to convince scientists that DN was the genetic material. They reasoned that identical molecules could not carry different instructions across all organisms. TECH FROM VISULS Have students compare the structure of the nucleotide monomer in VISUL VOCB to the DN polymer shown in Connecting Concepts. sk What part of the monomer serves to connect the two DN strands? connection between bases What is the pattern of that connection? Single ring bonds to double ring. 230 Unit 3: Genetics Differentiated Instruction BELOW LEVEL Prepare a series of short-answer questions or true/false statements to check students knowledge of DN structure. This will help students focus on the critical information in the section. Students should answer the questions before reading and then check and correct their responses as they read. 0hspe indd 230 Biology Toolkit, nticipation Guide, p. C3 U PRE-P Have students prepare a timeline of the discoveries detailed in Sections 8.2 and 8.3. Have them identify what piece of information about the genetic material of a cell was revealed at each point in time and who contributed to the discovery. Biology Toolkit, Timeline, p. C31 9/9/09 4:43:33 PM 230 Unit 3: Genetics

2 B FIGURE 8.4 The Four Nitrogen-Containing Bases of DN PYRIMIDINES = SINGLE RING PURINES = DOUBLE RING Name of Base Structural Formula Model Name of Base Structural Formula Model thymine cytosine adenine guanine Compare Which base is most similar in structure to thymine? By 1950 Erwin Chargaff changed the thinking about DN by analyzing the DN of several different organisms. Chargaff found that the same four bases are found in the DN of all organisms, but the proportion of the four bases differs somewhat from organism to organism. In the DN of each organism, the amount of adenine approximately equals the amount of thymine. Similarly, the amount of cytosine roughly equals the amount of guanine. These = T and C = G relationships became known as Chargaff s rules. Summarize What is the only difference among the four DN nucleotides? MIN IDE Watson and Crick developed an accurate model of DN s three-dimensional structure. The breakthrough in understanding the structure of DN came in the early 1950s through the teamwork of merican geneticist James Watson and British physicist Francis Crick. Watson and Crick were supposed to be studying the structure of proteins. Both men, however, were more fascinated by the challenge of figuring out DN s structure. Their interest was sparked not only by the findings of Hershey, Chase, and Chargaff but also by the work of the biochemist Linus Pauling. Pauling had found that the structure of some proteins was a helix, or spiral. Watson and Crick hypothesized that DN might also be a helix. X-Ray Evidence t the same time, Rosalind Franklin, shown in FIGURE 8.5, and Maurice Wilkins were studying DN using a technique called x-ray crystallography. When DN is bombarded with x-rays, the atoms in DN diffract the x-rays in a pattern that can be captured on film. Franklin s x-ray photographs of DN showed an X surrounded by a circle. Franklin s data gave Watson and Crick the clues they needed. The patterns and angle of the X suggested that INCLUSION For students who have difficulty sorting out the relationship between different aspects of the text, use DN s base pairs as a point of reference. Show students where the base pairs are found in various diagrams and photos in the section. Suggest a mnemonic to help students remember the pairs: C-G: Cars need gas; -T: corns grow on trees. Or point out that the letters C and G are both round, whereas the letters and T are made of straight lines. VOCBULRY n amine is a molecule that contains nitrogen. Notice that the four DN bases end in -ine and all contain nitrogen. Rosalind Franklin FIGURE 8.5 Rosalind Franklin (above) produced x-ray photographs of DN that indicated it was a helix. Her coworker, Maurice Wilkins, showed the data without Franklin s consent to Watson and Crick, which helped them discover DN s structure. Chapter 8: From DN to Proteins 231 bhspe indd 231 1/5/07 11:42:27 ONLINE BIOLOGY Have students evaluate the genomes of four different species in terms of Chargaff s rules. Go to Data nalysis Online in Options for Inquiry on page 257. The Inside Story In 1951, Rosalind Franklin was invited to King s College London to be part of a team working on DN analysis. Franklin used x-rays to study DN s crystalline structure. Meanwhile, at Cambridge University, James Watson and Francis Crick were feverishly working to be the first to accurately describe DN s structure. They knew DN s components, as did many others, but no one could figure out how the pieces fit together. In 1953, Maurice Wilkins, Franklin s coworker, showed Watson one of Franklin s x-rays, which provided an allimportant clue a DN molecule was a two-stranded helix of a constant width. Within weeks, Watson and Crick had figured out the structure of DN. In 1962, Watson, Crick, and Wilkins shared the Nobel Prize for Physiology or Medicine for this work. Franklin had died of cancer four years earlier, at the age of 37. Some attribute her cancer to exposure to x-rays. While the Nobel Prize is never awarded posthumously, no mention of Franklin s contribution was made at that time. Her work and its oversight have now ensured her a place in history. nswers Compare cytosine, because it has a single ring B Summarize Their nitrogen-containing bases differ from each other. Chapter 8: From DN to Protein 231

single molecule of DN molecule has two helixes, or strands, making it a double helix.

3 Teach continued ONLINE BIOLOGY Go to the chapter Resource Center at ClassZone.com for additional resources and information on DN. Vocabulary Greek and Latin Word Origins The words spiral and helix are synonymous. The word spiral comes from a Latin root meaning coil. The word helix comes from a Greek root meaning to wrap around. single molecule of DN molecule has two helixes, or strands, making it a double helix. Science Trivia Interpretation of Rosalind Franklin s x-ray image showed the following: DN has a width of about 2 nanometers (10 9 m). One complete turn of the helix occurs every 3.4 nanometers. There are ten base pairs in each turn of the helix, so the base pairs are stacked 0.34 nanometers apart. There are approximately 3 billion base pairs in human DN. Chemical Bonds Helping students understand the relative strengths of covalent bonds and hydrogen bonds lays the groundwork for understanding DN replication in Section 8.3. Because hydrogen bonds between the bases are easily broken, the two strands of DN can be readily separated, while the strong covalent bonds between nucleotides keep the individual strands intact. nswers pply Because pairs only with T, and C pairs only with G, DN will always have approximately the same proportion of and T and the same proportion of C and G. 232 Unit 3: Genetics FIGURE 8.6 James Watson (left) and Francis Crick (right) used a model to figure out DN s structure. Their model was influenced by data from other researchers, including an x-ray image (far right) taken by Rosalind Franklin. When x-rays bounce off vertically suspended DN, they form this characteristic x-shaped pattern. Chemical Bonds Recall from Chapter 2 that a covalent bond is a strong bond in which two atoms share one or more pairs of electrons. Hydrogen bonds are much weaker than covalent bonds and can easily be broken. Differentiated Instruction 232 Unit 3: Genetics hspe indd Sec2:232 James Watson and Francis Crick ENGLISH LERNERS Use the ssessment question in this section to go over question-answer relationships. Tell students the way they look for an answer depends on the type of question asked. Tell students that there are two basic categories: in the book and in my head. Tell students that the review questions at the end of a section are in the book questions. The answers can be found either right there (in the text), as with question 1, or the question may require students to think and search, as with question 2. The Double Helix Back in their own laboratory, Watson and Crick made models of metal and wood to figure out the structure of DN. Their models placed the sugarphosphate backbones on the outside and the bases on the inside. t first, Watson reasoned that might pair with, T with T, and so on. But the bases and G are about twice as wide as C and T, so this produced a helix that varied in width. Finally, Watson and Crick found that if they paired doubleringed nucleotides with single-ringed nucleotides, the bases fit like a puzzle. In pril 1953 Watson and Crick published their DN model in a paper in the journal Nature. FIGURE 8.6 shows their double helix (HEE-lihks) model, in which two strands of DN wind around each other like a twisted ladder. The strands are complementary they fit together and are the opposite of each other. That is, if one strand is CCC, the other strand is TGTGTG. The pairing of bases in their model finally explained Chargaff s rules. pply How did the Watson and Crick model explain Chargaff s rules? MIN IDE Nucleotides always pair in the same way. The DN nucleotides of a single strand are joined together by covalent bonds that connect the sugar of one nucleotide to the phosphate of the next nucleotide. The alternating sugars and phosphates form the sides of a double helix, sort of like a twisted ladder. The DN double helix is held together by hydrogen bonds between the bases in the middle. Individually, each hydrogen bond is weak, but together, they maintain DN structure. s shown in FIGURE 8.7, the bases of the two DN strands always pair up in the same way. This is summarized in the base pairing rules: thymine (T) always pairs with adenine (), and cytosine (C) always pairs with guanine (G). These pairings occur because of the sizes of the bases and the ability of the For in my head questions, the answers are not directly stated in the text. With critical thinking questions, students use information from the text to work out the answer. Such questions are categorized as author and you. Other questions, such as those in Connecting Concepts, require students to answer on my own. The answer may rely on something the student learned earlier or gained from personal experience Fil N bh i dd U d d L M difi d 7/5/ PM Biology Toolkit, QR, p. C9 7/6/06 9:36:44 M

FIGURE 8.7 Base Pairing Rules TECH FROM VISULS The base pairing rules describe how nucleotides form pairs in DN. T always pairs with, and G always pairs with C.

The nitrogen-containing bases bond in the middle to form the rungs of the DN ladder. T G C C G T FIGURE 8.7 Help students decode the figure. sk How many rings are there in each base pair?

The left diagram can be superimposed onto the right diagram, with the sugar-phosphate backbones on the spiral strands and the bases on the rungs connecting the strands.

4 FIGURE 8.7 Base Pairing Rules TECH FROM VISULS The base pairing rules describe how nucleotides form pairs in DN. T always pairs with, and G always pairs with C. hydrogen bond covalent bond This ribbonlike part represents the phosphate groups and deoxyribose sugar molecules that make up DN s backbone. The nitrogen-containing bases bond in the middle to form the rungs of the DN ladder. T G C C G T FIGURE 8.7 Help students decode the figure. sk How many rings are there in each base pair? three Why is DN s shape called a double helix? It is made of two spirals that are twisted around each other. How are the two diagrams related to each other? The left diagram can be superimposed onto the right diagram, with the sugar-phosphate backbones on the spiral strands and the bases on the rungs connecting the strands. B B Synthesize Which base pairs do you think are held more tightly together? Why? bases to form hydrogen bonds with each other. Due to the arrangement of their molecules, can form unique hydrogen bonds with T, and C with G. Notice that and T form two hydrogen bonds, whereas C and G form three. You can remember the rules of base pairing by noticing that the letters C and G have a similar shape. Once you know that C and G pair together, you know that and T pair together by default. If a sequence of bases on one strand of DN is CTGCT, you know the other DN strand will be GCGT. pply What sequence of bases would pair with the sequence TGCT? nswers Synthesize C and G, because they are connected by three hydrogen bonds, whereas and T are held by only two B pply CTGT ssess and Reteach 8.2 SSESSMENT REVIEWING MIN IDES 1. How many types of nucleotides are in DN, and how do they differ? 2. How are the base pairing rules related to Chargaff s research on DN? 3. Explain how the double helix model of DN built on the research of Rosalind Franklin. CRITICL THINKING 4. Infer Which part of a DN molecule carries the genetic instructions that are unique for each individual: the sugar-phosphate backbone or the nitrogen-containing bases? Explain. 5. Predict In a sample of yeast DN, 31.5% of the bases are adenine (). Predict the approximate percentages of C, G, and T. Explain. ONLINE QUIZ ClassZone.com 6. Evolution The DN of all organisms contains the same four bases (adenine, thymine, cytosine, and guanine). What might this similarity indicate about the origins of life on Earth? ssess Use the Online Quiz or Section Quiz (ssessment Book, p. 148). Reteach Have students make models of DN by cutting out and arranging cardboard shapes representing deoxyribose, phosphate groups, and the four bases. Encourage students to make their model at least four nucleotides long. 8.2 SSESSMENT 00.2 SSESSMENT 1. Four; their nitrogen-containing bases differ. 2. Because pairs only with T, and C pairs only with G, DN will always have approximately the same proportion of and T and the same proportion of C and G. Chapter 8: From DN to Proteins Franklin s data suggested that DN was a helix made of two strands an even width apart. From this, Watson and Crick realized that a base with one ring would bond with a base with two rings. 4. The backbone is the same in all DN. The nitrogen-containing bases provide the unique instructions. b10hspe indd 233 9/2/08 12:32:59 PM 5. Matching, T is approximately 31.5 percent. Thus, C and G together make up 37 percent of the bases, so each makes up approximately 18.5 percent of the bases. 6. It suggests that the wide diversity of life that we see might have stemmed from a common ancestor. Chapter 8: From DN to Protein 233

DT NLYSIS Introduce Histograms differ from bar graphs in that a histogram shows data intervals that are continuous. sk How are the data intervals continuous in Graph 1?

$He won the prize in physics jointly with his father in 1915 for work in x-ray diffraction, which later paved the way for Rosalind Franklin s work with DN. Raymond Davis, Jr.$

5 DT NLYSIS Introduce Histograms differ from bar graphs in that a histogram shows data intervals that are continuous. sk How are the data intervals continuous in Graph 1? The ages are divided into continuous 10-year intervals from age 30 to age 89. What trend is shown in Graph 1? The most common age of the Nobel laureates is Very young and very old winners are less common. Unit Resource Book, Data nalysis, p. 89 Science Trivia The youngest person to receive a Nobel Prize was Sir William Lawrence Bragg, at age 25. He won the prize in physics jointly with his father in 1915 for work in x-ray diffraction, which later paved the way for Rosalind Franklin s work with DN. Raymond Davis, Jr., was the oldest Nobel Prize laureate. He was almost 88 years old when he won the Nobel Prize for Physics in Discuss It is estimated that humans have between 20,000 and 25,000 genes. sk, In which bar in Graph 2 would humans be included? the sixth bar, on the far right Unit Resource Book, Data nalysis, p. 89 DT NLYSIS Frequency Distributions INTERPRETING HISTOGRMS histogram is a graph that shows the frequency distribution of a data set. First, a scientist collects data. Then, she groups the data values into equal intervals. The number of data values in each interval is the frequency of the interval. The intervals are shown along the x-axis of the histogram, and the frequencies are shown on the y-axis. EXMPLE The histogram at right shows the frequency distribution of the ages of winners of the Nobel Prize in Medicine at the time of winning. Francis Crick was 46 and James Watson was 34 when they were jointly awarded a Nobel Prize in Medicine in ccording to the histogram, the most winners have been between 50 and 59 years old at the time of winning. Only five scientists have been between the ages of 80 and 89 at the time of winning a Nobel Prize in Medicine. GRPH 1. NOBEL PRIZE WINNERS BY GE NLYZE HISTOGRM The histogram below categorizes data collected based on the number of genes in 11 species. GRPH 2. NUMBER OF GENES IN SELECT SPECIES 1. Identify How many species had between 10,001 and 15,000 genes? 2. nalyze re the data in graph 2 sufficient to reveal a trend in the number of genes per species? Explain your reasoning. DT NLYSIS ClassZone.com 234 Unit 3: Genetics nswers 1. three 2. No, there is no obvious correlation. The highest number of species (3) and the lowest number of species (1) differ by only two. This difference is not significant. Data from 11 species are inadequate to reveal a trend for species in general. 10hspe da.indd 234 U 9/2/08 12:33:07 PM 234 Unit 3: Genetics

Lesson Overview The Structure of DNA

Lesson Overview The Structure of DNA 12.2 THINK ABOUT IT The DNA molecule must somehow specify how to assemble proteins, which are needed to regulate the various functions of each cell. What kind of structure could serve this purpose without