Objectives• Describe the interaction of the four

nucleotides that make up DNA.• Describe the three-dimensional

structure of DNA.

Section ResourcesUnit Resource Book

Study Guide pp. 65–66Power Notes p. 67Reinforcement p. 68Pre-AP Activity pp. 91–92

Interactive Reader Chapter 8Spanish Study Guide pp. 75–76

Biology Toolkit pp. C3, C9, C31

TechnologyPower Presentation 8.2Media Gallery DVDOnline Quiz 8.2

Activate Prior Knowledge Tell students that scientists found it hard to accept DNA as the genetic material because of its structural simplicity. Ask

• 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 DNA? alphabet and computer code because both are informational units

Have students compare the structure of the nucleotide monomer in VISUAL VOCAB to the DNA polymer shown in Connecting Concepts. Ask

• What part of the monomer serves to connect the two DNA strands? connection between bases

• What is the pattern of that connection? Single ring bonds to double ring.

Plan and PreparePlan and Prepare

TeachTeach

TEACH FROM VISUALSTEACH FROM VISUALS

U

The small units, or monomers, that make up a strand of DNA are called nucleotides. nucleotides. Nucleotides have three parts.

VISUAL VOCAB

nitrogen-containing base

phosphate group

deoxyribose (sugar)

Biochemistry The nucleotides in a strand of DNA all line up in the same direction. As a result, DNA has chemical polarity, which means that the two ends of the DNA strand are different. The 5�carbon is located at one end of the DNA strand, and the 3� car-bon is located at the other end. When the two strands of DNA pair together, the 5� end of one strand aligns with the 3� end of the other strand.

Connecting CONCEPTS

8.2 Structure of DNAKEY CONCEPT DNA structure is the same in all organisms.

MAIN IDEAS• DNA is composed of four types of nucleotides.

• Watson and Crick developed an accurate model of DNA’s three-dimensional structure.

• Nucleotides always pair in the same way.

VOCABULARYnucleotide,nucleotide, p. 230

double helix,double helix, p. 232

base pairing rules,base pairing rules, p. 232

Reviewcovalent bond, hydrogen bond

Connect The experiments of Hershey and Chase confirmed that DNA carries the genetic information, but they left other big questions unanswered: What exactly is this genetic information? How does DNA store this information? Scientists in the early 1950s still had a limited knowledge of the structure of DNA, but that was about to change dramatically.

MAIN IDEA

DNA is composed of four types of nucleotides.Since the 1920s, scientists have known that the DNA molecule is a very long polymer, or chain of repeating units. The small units, or monomers, that make up DNA are called nucleotidesnucleotides (NOO-klee-oh-TYDZ). Each nucleotide has three parts.

• A phosphate group (one phosphorus with four oxygens)

• A ring-shaped sugar called deoxyribose

• A nitrogen-containing base (a single or double ring built around nitrogen and carbon atoms)

One molecule of human DNA contains billions of nucleotides, but there are only four types of nucleotides in DNA. These nucleotides differ only in their nitrogen-containing bases.

The four bases in DNA are shown in FIGURE 8.4. Notice that the bases cytosine (C) and thymine (T) have a single-ring structure. Adenine (A) 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 DNA was made up of equal amounts of the four nucleotides, and so the DNA in all organisms was exactly the same. That hypothesis was a key reason that it was so hard to convince scientists that DNA was the genetic material. They reasoned that identical molecules could not carry different instructions across all organisms.

230 Unit 3: Genetics

5.a Anaylze and explain the molecular basis of heredity and the inheritance of traits to succes-sive generations by using the Cen-tral Dogma of Molecular Biology.

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Differentiated Instruction

SECTION 8.2

BELOW LEVELPrepare a series of short-answer questions or true/false statements to check students knowledge of DNA 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.

Biology Toolkit, Anticipation Guide, p. C3

PRE-APHave 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

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ONLINE BIOLOGY Have students evaluate the genomes of four different species in terms of Chargaff’s rules. Go to Data Analysis Online in Options for Inquiry on page 257.

The Inside StoryIn 1951, Rosalind Franklin was invited to King’s College London to be part of a team working on DNA analysis. Franklin used x-rays to study DNA’s crystalline structure. Meanwhile, at Cambridge University, James Watson and Francis Crick were feverishly working to be the first to accurately describe DNA’s structure. They knew DNA’s compo-nents, 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 all-important clue—a DNA molecule was a two-stranded helix of a constant width. Within weeks, Watson and Crick had figured out the structure of DNA.

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.

AnswersA Compare cytosine, because it has a

single ring

B Summarize Their nitrogen-containing bases differ from each other.

nucleotides.

nucleotide,double helix,base pairing rules,

nucleotides

AM

FIGURE 8.4 The Four Nitrogen-Containing Bases of DNAPYRIMIDINES = SINGLE RING PURINES = DOUBLE RING

Name of Base Structural Formula Model Name of Base Structural Formula Model

Compare Which base is most similar in structure to thymine?

thymine

cytosine

adenine

guanine

Rosalind Franklin

FIGURE 8.5 Rosalind Franklin (above) produced x-ray photo-graphs of DNA that indicated it was a helix. Her coworker, Mau-rice Wilkins, showed the data without Franklin’s consent to Watson and Crick, which helped them discover DNA’s structure.

VOCABULARYAn amine is a molecule that contains nitrogen. Notice that the four DNA bases end in -ine and all contain nitrogen.

By 1950 Erwin Chargaff changed the thinking about DNA by analyzing theDNA of several different organisms. Chargaff found that the same four basesare found in the DNA of all organisms, but the proportion of the four basesdiffers somewhat from organism to organism. In the DNA of each organism,the amount of adenine approximately equals the amount of thymine. Simi-larly, the amount of cytosine roughly equals the amount of guanine. TheseA = T and C = G relationships became known as Chargaff ’s rules.

Summarize What is the only difference among the four DNA nucleotides?

MAIN IDEA

Watson and Crick developed an accurate model of DNA’s three-dimensional structure.

The breakthrough in understanding the structure of DNA came in the early1950s through the teamwork of American geneticist James Watson andBritish physicist Francis Crick. Watson and Crick were supposed to be study-ing the structure of proteins. Both men, however, were more fascinated by thechallenge of figuring out DNA’s structure. Their interest was sparked not onlyby the findings of Hershey, Chase, and Chargaff but also by the work of thebiochemist Linus Pauling. Pauling had found that the structure of someproteins was a helix, or spiral. Watson and Crick hypothesized that DNA mightalso be a helix.

X-Ray EvidenceAt the same time, Rosalind Franklin, shown in FIGURE 8.5, and MauriceWilkins were studying DNA using a technique called x-ray crystallography.When DNA is bombarded with x-rays, the atoms in DNA diffract the x-raysin a pattern that can be captured on film. Franklin’s x-ray photographs ofDNA showed an X surrounded by a circle. Franklin’s data gave Watson andCrick the clues they needed. The patterns and angle of the X suggested that

Chapter 8: From DNA to Proteins 231

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A

B

INCLUSIONFor students who have difficulty sorting out the relationship between different aspects of the text, use DNA’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; A-T: Acorns grow on trees. Or point out that the letters C and G are both round, whereas the letters A and T are made of straight lines.

Chapter 8: From DNA to Protein 231

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B

A

B

ONLINE BIOLOGY Go to the chapter Resource Center at ClassZone.com for additional resources and information on DNA.

VocabularyGreek 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.” A single molecule of DNA molecule has two helixes, or strands, making it a double helix.

Science TriviaInterpretation of Rosalind Franklin’s x-ray image showed the following:

• DNA 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 DNA.

Chemical Bonds Helping students understand the relative strengths of covalent bonds and hydrogen bonds lays the groundwork for understanding DNA replication in Section 8.3. Because hydrogen bonds between the bases are easily broken, the two strands of DNA can be readily separated, while the strong covalent bonds between nucleo-tides keep the individual strands intact.

AnswersA Apply Because A pairs only with T,

and C pairs only with G, DNA will always have approximately the same proportion of A and T and the same proportion of C and G.

Teach continuedTeach continued

Connecting CONCEPTSConnecting CONCEPTS

l C d 2 03240 Fil N bh 030802 i dd U d d L M difi d 7/5/06 3 36 PM

FIGURE 8.6 James Watson (left) and Francis Crick (right) used a model to fig-ure out DNA’s structure. Their model was influ-enced by data from other researchers, including an x-ray image (far right) taken by Rosalind Franklin. When x-rays bounce off vertically suspended DNA, they form this characteristic x-shaped pattern.

James Watson and Francis Crick

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.

Connecting CONCEPTS

The Double HelixBack in their own laboratory, Watson and Crick made models of metal andwood to figure out the structure of DNA. Their models placed the sugar-phosphate backbones on the outside and the bases on the inside. At first,Watson reasoned that A might pair with A, T with T, and so on. But the basesA and G are about twice as wide as C and T, so this produced a helix thatvaried in width. Finally, Watson and Crick found that if they paired double-ringed nucleotides with single-ringed nucleotides, the bases fit like a puzzle.

In April 1953 Watson and Crick published their DNA model in a paper inthe journal Nature. FIGURE 8.6 shows their double helixdouble helix (HEE-lihks) model, inwhich two strands of DNA wind around each other like a twisted ladder. Thestrands are complementary —they fit together and are the opposite of eachother. That is, if one strand is ACACAC, the other strand is TGTGTG. Thepairing of bases in their model finally explained Chargaff ’s rules.

Apply How did the Watson and Crick model explain Chargaff’s rules?

MAIN IDEA

Nucleotides always pair in the same way.The DNA nucleotides of a single strand are joined together by covalent bondsthat connect the sugar of one nucleotide to the phosphate of the next nucleo-tide. The alternating sugars and phosphates form the sides of a double helix,sort of like a twisted ladder. The DNA double helix is held together by hydro-gen bonds between the bases in the middle. Individually, each hydrogen bondis weak, but together, they maintain DNA structure.

As shown in FIGURE 8.7, the bases of the two DNA strands always pair up inthe same way. This is summarized in the base pairing rules: base pairing rules: thymine (T)always pairs with adenine (A), and cytosine (C) always pairs with guanine (G).These pairings occur because of the sizes of the bases and the ability of the

232 Unit 3: Genetics

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base pairing rules nucleotides

nucleotides

base pairing rules

double helix

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A

A

B

Differentiated Instruction

ENGLISH LEARNERSUse the Assessment 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.

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.

Biology Toolkit, QAR, p. C9

232 Unit 3: Genetics

00.2 ASSESSMENT

B

A

B

FIGURE 8.7 Help students decode the figure. Ask

• How many rings are there in each base pair? three

• Why is DNA’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.

AnswersA Synthesize C and G, because they

are connected by three hydrogen bonds, whereas A and T are held by only two

B Apply ACTGAT

Assess Use the Online Quiz or Section Quiz (Assessment Book, p. 148).Reteach Have students make models of DNA by cutting out and arranging cardboard shapes representing deoxyri-bose, phosphate groups, and the four bases. Encourage students to make their model at least four nucleotides long.

TEACH FROM VISUALSTEACH FROM VISUALS

Assess and ReteachAssess and Reteach

8.2 ASSESSMENT

Connecting CONCEPTS

ONLINE QUIZClassZone.com

FIGURE 8.7 Base Pairing Rules

The base pairing rules base pairing rules describe how nucleotidesnucleotides form pairs in DNA. T always pairs with A, and G always pairs with C.

This ribbonlike part represents the phosphate groups and deoxyribose sugar molecules that make up DNA’s “backbone.”

Synthesize Which base pairs do you think are held moretightly together? Why?

hydrogen bond

G C

AT

GC

T A

The nitrogen-containing bases bond in the middle to form the rungs of the DNA ladder.

covalent bond

bases to form hydrogen bonds with each other. Due to the arrangement of their molecules, A can form unique hydrogen bonds with T, and C with G. Notice that A 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 A and T pair together by default. If a sequence of bases on one strand of DNA is CTGCTA, you know the other DNA strand will be GACGAT.

Apply What sequence of bases would pair with the sequence TGACTA?

REVIEWING MAIN IDEAS

1. How many types of nucleotides nucleotides are in DNA, and how do they differ?

2. How are the base pairing rules base pairing rules related to Chargaff’s research on DNA?

3. Explain how the doubledouble helixhelix model of DNA built on the research of Rosalind Franklin.

CRITICAL THINKING

4. Infer Which part of a DNA mol-ecule carries the genetic instruc-tions that are unique for each individual: the sugar-phosphate backbone or the nitrogen-contain-ing bases? Explain.

5. Predict In a sample of yeast DNA, 31.5% of the bases are adenine (A). Predict the approximate percent-ages of C, G, and T. Explain.

6. Evolution The DNA 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?

Chapter 8: From DNA to Proteins 233

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A

B

8.2 ASSESSMENT 1. Four; their nitrogen-containing bases differ. 2. Because A pairs only with T, and C pairs

only with G, DNA will always have approxi-mately the same proportion of A and T and the same proportion of C and G.

3. Franklin’s data suggested that DNA 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 DNA. The nitrogen-containing bases provide the unique instructions.

5. Matching A, T is approximately 31.5 percent. Thus, C and G together make up 37 percent of the bases, so each makes up approxi-mately 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 DNA to Protein 233

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IntroduceHistograms differ from bar graphs in that a histogram shows data intervals that are continuous. Ask

• How are the data intervals continu-ous 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 50–59. Very young and very old winners are less common.

Unit Resource Book, Data Analysis, p. 89

Science Trivia• The youngest person to receive a

Nobel Prize was Sir William Law-rence 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 DNA.

• Raymond Davis, Jr., was the oldest Nobel Prize laureate. He was almost 88 years old when he won the Nobel Prize for Physics in 2002.

DiscussIt is estimated that humans have between 20,000 and 25,000 genes. Ask, In which bar in Graph 2 would humans be included? the sixth bar, on the far right

Unit Resource Book, Data Analysis, p. 89

U

DATA ANALYSISClassZone.com

GRAPH 1. NOBEL PRIZE WINNERS BY AGE

D ATA A N A LY S I S I N T E R P R E T I N G H I S T O G R A M S

Frequency DistributionsA 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.

EXAMPLEThe 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 1962.

According 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.

ANALYZE A HISTOGRAMThe histogram below categorizes data collected based on the number of genes in 11 species.

1. Identify How many species had between 10,001 and 15,000 genes?

2. Analyze Are the data in graph 2 sufficient to reveal a trend in the number of genes per species? Explain your reasoning.

GRAPH 2. NUMBER OF GENES IN SELECT SPECIES

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replication,DNA polymerase,

replication.

b10h

D A T A A N A LY S I S

Answers 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.

234 Unit 3: Genetics

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