SECTION 8.2 Plan and Prepare 8.2 Structure of DNA

SECTION 8.2

Plan and Prepare

Objectives

? Describe the interaction of the four nucleotides that make up DNA.

? Describe the three-dimensional structure of DNA.

Section Resources

Unit Resource Book Study Guide pp. 65?66 Power Notes p. 67 Reinforcement p. 68 Pre-AP Activity pp. 91?92

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

Biology Toolkit pp. C3, C9, C31

Technology Power Presentation 8.2 Media Gallery DVD Online 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

Teach

8.2

Structure of DNA

KEY 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.

VOCABULARY 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 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.

Connecting CONCEPTS

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 carbon 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.

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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 nucleotides (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)

VISUAL VOCAB

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

phosphate group nitrogen-containing base

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.

deoxyribose (sugar)

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.

TEACH FROM VISUALS

230 Unit 3: Genetics

Differentiated Instruction

Have students compare the structure of the nucleotide monomer in VISUAL

B E LOW L E V E L b10hspe-030802.indd 230

VOCAB to the DNA polymer shown in

Prepare a series of short-answer questions or

Connecting Concepts. Ask

true/false statements to check students

knowledge of DNA structure. This will help

? What part of the monomer serves

students focus on the critical information in

to connect the two DNA strands?

the section. Students should answer the

connection between bases

questions before reading and then check and

? What is the pattern of that

correct their responses as they read.

connection? Single ring bonds to

double ring.

Biology Toolkit, Anticipation Guide, p. C3

U PRE-AP

9/9/09 4:43:33 PM

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

230 Unit 3: Genetics

FIGURE 8.4 The Four Nitrogen-Containing Bases of DNA

PYRIMIDINES = SINGLE RING

PURINES = DOUBLE RING

Name of Base thymine

Structural Formula

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8 8=( 8

C= 8 8D

=8 C=

cytosine

C='

8C

=8

8D

=8 C=

Model

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8

Name of Base adenine guanine

Structural Formula

=8

C 8

=C 8

C=' 8

C

C 8=

=8 =C

C 8

8

D 8

C=

C8

C='

A Compare Which base is most similar in structure to thymine?

By 1950 Erwin Chargaff changed the thinking about DNA by analyzing the DNA of several different organisms. Chargaff found that the same four bases are found in the DNA of all organisms, but the proportion of the four bases differs somewhat from organism to organism. In the DNA 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 A = T and C = G relationships became known as Chargaff 's rules.

B 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 early 1950s through the teamwork of American 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 DNA'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 DNA might also be a helix.

X-Ray Evidence

At the same time, Rosalind Franklin, shown in FIGURE 8.5, and Maurice Wilkins were studying DNA using a technique called x-ray crystallography. When DNA is bombarded with x-rays, the atoms in DNA diffract the x-rays in a pattern that can be captured on film. Franklin's x-ray photographs of DNA 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

I N C L U S I O N bhspe-030802.indd 231

For 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.

Model

6

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.

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

Rosalind Franklin

FIGURE 8.5 Rosalind Franklin

(above) produced x-ray photographs of DNA 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 DNA's structure.

Chapter 8: From DNA to Proteins 231

1/5/07 11:42:27

The Inside Story

In 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 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 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.

Answers

A Compare cytosine, because it has a single ring

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

Chapter 8: From DNA to Protein 231

Teach continued

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

Vocabulary

FIGURE 8.6 James Watson

(left) and Francis Crick (right) used a model to figure out DNA'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 DNA, they form this characteristic x-shaped pattern.

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

Science Trivia

Interpretation of Rosalind Franklin's x-ray image showed the following:

? DNA has a width of about

2 nanometers (109 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.

James Watson and Francis Crick

The Double Helix

Back in their own laboratory, Watson and Crick made models of metal and wood to figure out the structure of DNA. Their models placed the sugarphosphate 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 bases A 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 April 1953 Watson and Crick published their DNA model in a paper in the journal Nature. FIGURE 8.6 shows their double helix (HEE-lihks) model, in which two strands of DNA 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 ACACAC, the other strand is TGTGTG. The pairing of bases in their model finally explained Chargaff 's rules.

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

MAIN IDEA

Nucleotides always pair in the same way.

Connecting CONCEPTS

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 nucleotides keep the individual strands intact.

Answers

A 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.

232 Unit 3: Genetics

Connecting CONCEPTS

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.

The DNA 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 DNA double helix is held together by hydrogen bonds between the bases in the middle. Individually, each hydrogen bond is weak, but together, they maintain DNA structure.

As shown in FIGURE 8.7, the bases of the two DNA strands always pair up in the same way. This is summarized in the 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 DUniitf3f: Geenreeticns tiated Instruction

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Use 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."

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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

Tell students that the review questions at the something the student learned earlier or

end of a section are "in the book" questions. gained from personal experience.

The answers can be found either "right there"

(in the text), as with question 1, or the

Biology Toolkit, QAR, p. C9

question may require students to "think and

search," as with question 2.

FIGURE 8.7 Base Pairing Rules

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

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This ribbonlike part represents the phosphate groups and deoxyribose sugar molecules that make up DNA's "backbone."

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

hydrogen bond

covalent bond

A A Synthesize Which base pairs do you think are held more

tightly together? Why?

TA GC AT CG

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

BB

strand of DNA is CTGCTA, you know the other DNA strand will be GACGAT.

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

8.2 ASSESSMENT

REVIEWING MAIN IDEAS

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

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

3. Explain how the double helix model of DNA built on the research of Rosalind Franklin.

CRITICAL THINKING

4. Infer Which part of a DNA 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 DNA, 31.5% of the bases are adenine (A). Predict the approximate percentages of C, G, and T. Explain.

ONLINE QUIZ

Connecting CONCEPTS

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?

TEACH FROM VISUALS

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.

Answers

A 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 and Reteach

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 deoxyribose, phosphate groups, and the four bases. Encourage students to make their model at least four nucleotides long.

8.2 ASSESSMENT 010. F.o2urA; tSheSirEnSitSroMgenE-NcoTntaining bases differ.

2. 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.

Chapter 8: From DNA to Proteins 233

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 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 DNA to Protein 233

DATA ANALYSIS

Introduce

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

? 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 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 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 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.

Discuss

It 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

DATA ANALYSIS INTE RPRETING HISTOGR AMS

Frequency Distributions

DATA ANALYSIS

A 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.

EXAMPLE 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 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.

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GRAPH 1. NOBEL PRIZE WINNERS BY AGE +% *% )% (% '% &% % (%?(. )%?). *%?*. +%?+. ,%?,. -%?-. 6\ZVii^bZd[l^cc^c\

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

GRAPH 2. NUMBER OF GENES IN SELECT SPECIES

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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.

234 Unit 3: Genetics

Answers

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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.

9/2/08 12:33:07 PM

234 Unit 3: Genetics

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