CHAPTER 8 From DNA To Proteins

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CHAPTER8From DNA toProteinsK E Y CO N C E P T S8.1 Identifying DNA as the Genetic MaterialDNA was identified as the genetic material through a seriesof experiments.8.2 Structure of DNADNA structure is the same in all organisms.8.3 DNA ReplicationDNA replication copies the genetic information of a cell.8.4 TranscriptionTranscription converts a gene into a single-stranded RNA molecule.8.5 TranslationTranslation converts an mRNA message into a polypeptide, or protein.8.6 Gene Expression and RegulationGene expression is carefully regulated in both prokaryoticand eukaryotic cells.8.7 MutationsMutations are changes in DNA that may or may not affect phenotype.BIOLOGYCL ASSZONE .COMRESOURCE CENTERBIOLOGYView animated chapterconcepts. DNA Replication Build a Protein224Unit 3: GeneticsKeep current with biology news. Featured stories News feeds Strange BiologyGet more information on DNA RNA Mutations

Why is thismouse glowing?ConnectingThis mouse’s eerie green glow comesfrom green fluorescent protein (GFP),which glows under ultraviolet light. Scientists put a gene from a glowing jellyfishinto a virus that was allowed to infect amouse egg. The jellyfish gene became partof the mouse’s genes. As a result, the mousecells produce the same protein. Researchershope to track cancer cells using GFP.CONCEPTSTranslation This computermodel of GFP shows theamino acids (purple) in thecenter of the protein thatmake the protein glow. Thegenetic code is universal,which means that a gene fromone organism can be correctlytranslated into a protein inanother organism. Althoughthe gene for GFP comes froma jellyfish, GFP has been madein bacteria, yeast, slime mold,plants, fruit flies, zebrafish,and mammals.Chapter 8: From DNA to Proteins 225

8.1Identifying DNA as theGenetic MaterialKEY CONCEPTDNA was identified as the genetic material through a series of experiments.MAIN IDEASVOCABULARY Griffith finds a “transforming principle.” Avery identifies DNA as the transforming principle. Hershey and Chase confirm that DNA is thegenetic material.bacteriophage, p. 228Reviewdeoxyribonucleic acid (DNA), gene, enzymeConnect Some people think a complicated answer is better than a simple one. Ifthey have a head cold, for instance, they may use all sorts of pills, syrups, andsprays, when they simply need rest, water, and warm chicken soup. In the early1900s, most scientists thought DNA’s structure was too repetitive for it to be thegenetic material. Proteins, which are more variable in structure, appeared to be abetter candidate. Starting in the 1920s, experiments provided data that did notsupport this idea. By the 1950s, sufficient evidence showed that DNA—the samemolecule that codes for GFP in the glowing mouse—carries genetic information.MAIN IDEAGriffith finds a “transforming principle.”TAKING NOTESMake a table to keep track ofthe experiments discussed inthis section and how they contributed to our understandingof DNA.Experiment ResultsGriffth’smiceA transferablematerial changedharmless bacteriainto disease-causingbacteria.In 1928 the British microbiologist Frederick Griffith was investigating twoforms of the bacterium that causes pneumonia. One form is surrounded by acoating made of sugar molecules. Griffith called these bacteria the S formbecause colonies of them look smooth. The second form of bacteria do nothave a smooth coating and are called the R, or rough, form. As you can see inFIGURE 8.1, when Griffith injected the two types of bacteria into mice, only the Stype killed the mice. When the S bacteria were killed with heat, the mice wereunaffected. Therefore, only live S bacteria would cause the mice to die.FIGURE 8.1 Griffith’s ExperimentsThe S form of the bacterium is deadly; the R form is not.226Unit 3: Geneticsa kZH WVXiZg Va kZG WVXiZg V]ZVi" aaZYH WVXiZg V]ZVi" aaZY H WVXiZg V a kZ G WVXiZg VYZVY bdjhZa kZ bdjhZa kZ bdjhZYZVY bdjhZ

Griffith next injected mice with a combination of heat-killed S bacteriaand live R bacteria. To his surprise, the mice died. Even more surprising, hefound live S bacteria in blood samples from the dead mice. Griffith concludedthat some material must have been transferred from the heat-killed S bacteriato the live R bacteria. Whatever that material was, it contained informationthat changed harmless R bacteria into disease-causing S bacteria. Griffith calledthis mystery material the “transforming principle.”ConnectingCONCEPTSMicrobiology Much of ourknowledge of the chemical basisof genetics has come from thestudy of bacteria. You will learnmuch more about bacteria inChapter 18.Infer What evidence suggested that there was a transforming principle?MAIN IDEAAvery identifies DNA as the transforming principle.What exactly is the transforming principle that Griffith discovered? Thatquestion puzzled Oswald Avery and his fellow biologists. They worked formore than ten years to find the answer. Avery’s team began by combiningliving R bacteria with an extract made from S bacteria. This procedureallowed them to directly observe the transformation of R bacteria intoS bacteria in a petri dish.Avery’s group next developed a process to purifyFIGURE 8.2 Avery’s Discoveriestheir extract. They then performed a series of testsCHEMICAL ANALYSIS OF TRANSFORMING PRINCIPLEto find out if the transforming principle was DNAor protein.% Nitrogen% PhosphorusRatio of N(N)(P)to P Qualitative tests Standard chemical testsshowed that no protein was present. In contrast,Sample A14.218.571.66tests revealed that DNA was present.Sample B15.939.091.75 Chemical analysis As you can see in FIGURE 8.2,the proportions of elements in the extractSample C15.369.041.69closely matched those found in DNA. ProteinsSample D13.408.451.58contain almost no phosphorus.Known Enzyme tests When the team added to thevalue for15.329.051.69extract enzymes known to break down proteins,DNAthe extract still transformed the R bacteria toSource: Avery, O. T. et al., The Journal ofthe S form. Also, transformation occurred whenExperimental Medicine 79:2.researchers added an enzyme that breaks downAnalyze How do the data support theRNA (another nucleic acid). Transformationhypothesisthat DNA, not protein, isfailed to occur only when an enzyme was addedthe transforming principle?to destroy DNA.In 1944 Avery and his group presented this and other evidence to supporttheir conclusion that DNA must be the transforming principle, or geneticmaterial. The results created great interest. However, some scientists questionedwhether the genetic material in bacteria was the same as that in other organisms. Despite Avery’s evidence, some scientists insisted that his extract musthave contained protein.Oswald AverySummarize List the key steps in the process that Avery’s team used to identify thetransforming principle.Chapter 8: From DNA to Proteins 227

MAIN IDEAHershey and Chase confirm that DNA is thegenetic material.FIGURE 8.3 This micrographshows the protein coat of abacteriophage (orange) afterit has injected its DNA into anE. coli bacterium (blue). (coloredTEM; magnification 115,000 )Conclusive evidence for DNA as the genetic material came in 1952 from twoAmerican biologists, Alfred Hershey and Martha Chase. Hershey andChase were studying viruses that infect bacteria. This type of virus, called abacteriophage (bak-TEER-ee-uh-FAYJ), or “phage” for short, takes over abacterium’s genetic machinery and directs it to make more viruses.Phages like the ones Hershey and Chase studied are relatively simple—littlemore than a DNA molecule surrounded by a protein coat. This two-partstructure of phages offered a perfect opportunity to answer the question, Isthe genetic material made of DNA or protein? By discovering which part of aphage (DNA or protein) actually entered a bacterium, as shown in FIGURE 8.3,they could answer this question once and for all.Hershey and Chase thought up a clever procedure that made use of thechemical elements found in protein and DNA. Protein contains sulfur but verylittle phosphorus, while DNA contains phosphorus but no sulfur. The researchers grew phages in cultures that contained radioactive isotopes of sulfuror phosphorus. Hershey and Chase then used these radioactively taggedphages in two experiments. Experiment 1 In the first experiment, bacteria were infected with phagesthat had radioactive sulfur atoms in their protein molecules. Hershey andChase then used an ordinary kitchen blender to separate the bacteria fromthe parts of the phages that remained outside the bacteria. When theyexamined the bacteria, they found no significant radioactivity. Experiment 2 Next, Hershey and Chase repeated the procedure withphages that had DNA tagged with radioactive phosphorus. This time,radioactivity was clearly present inside the bacteria.From their results, Hershey and Chase concluded that the phages’ DNAhad entered the bacteria, but the protein had not. Their findings finallyconvinced scientists that the genetic material is DNA and not protein.Apply How did Hershey and Chase build upon Avery’s chemical analysis results?8.1ONLINE QUIZASSESSMENTREVIEWINGMAIN IDEAS1. What was “transformed” in Griffith’sexperiment?2. How did Avery and his group identifythe transforming principle?3. Summarize how Hershey and Chaseconfirmed that DNA is the geneticmaterial.228 Unit 3: GeneticsClassZone.comCRITICAL THINKING4. Summarize Why was the bacteriophage an excellent choice forresearch to determine whethergenes are made of DNA or proteins?5. Analyze Choose one experimentfrom this section and explain howthe results support the conclusion.ConnectingCONCEPTS6. Mendelian Genetics Describehow Mendel’s studies relate tothe experiments discussed inthis section.

CHAPTER 8I N V E S T I G AT I O NMATERIALSExtracting DNA balance10 g raw wheat germlaboratory spatulatest tubetest tube rack10 mL warm distilledwater2 eyedroppers4 10-mL graduatedcylinders20 mL detergentsolution3 g meat tenderizer20 mL salt solution10 mL cold isopropylalcoholglass stirring rodPROCESS SKILLS Observing AnalyzingOswald Avery wrote in a scientific article, “At a critical concentration . . . of alcoholthe active material separates out in the form of fibrous strands that windthemselves around the stirring rod.” In this lab, you can observe the same thingAvery observed as you extract DNA from wheat germ. This procedure is asimplified version of the one scientists commonly use to extract DNA today.P R O B L E M How do you extract the DNA from plant cells?PROCEDURE1. Place a small amount of wheat germ in a test tube. Thewheat germ should be about 1 cm high in the test tube.2. Add enough distilled water to wet and cover all of thewheat germ.3. Add 25–30 drops of detergent solution to the test tube.For 3 minutes, gently swirl the test-tube contents. Avoidmaking bubbles.4. Add 3 g of meat tenderizer.5. Add 25–30 drops of salt solution to the test tube.Swirl for 1 minute.6. Tilt the test tube at an angle as shown. Slowly add alcohol sothat it runs down the inside of the test tube to form aseparate layer on top of the mixture in the tube. Add enoughalcohol to double the total volume in the tube. Let the testtube stand for 2 minutes.7. Watch for stringy, cloudy material to rise from the bottom layerinto the alcohol layer. This is the DNA.8. Use the glass stirring rod to remove some DNA. Be carefulto probe only the alcohol layer.9. Draw in your lab report what the mixture and DNA looked likein steps 2–7. Be sure to include color, texture, and whathappened after a new solution was added.step 6step 8ANALYZE AND CONCLUDE1. Connect Consider what you know about cell structure and the location of DNA.Suggest a reason for adding detergent solution to the test tube.2. Predict What do you think might happen if the alcohol were added quickly andthe two layers mixed?3. Infer Meat tenderizer contains enzymes that break down proteins. What do youthink is the purpose of adding meat tenderizer in this procedure?4. Connect In what type of real-life situation would the extraction of DNAbe useful?EXTEND YOUR INVESTIGATIONDetermine a method to calculate what percentage of the wheat germconsists of DNA.Chapter 8: From DNA to Proteins 229

8.2Structure of DNAKEY CONCEPTDNA structure is the same in all organisms.MAIN IDEAS DNA is composed of four types of nucleotides. Watson and Crick developed an accuratemodel of DNA’s three-dimensional structure. Nucleotides always pair in the same way.VOCABULARYnucleotide, p. 230double helix, p. 232base pairing rules, p. 232Reviewcovalent bond, hydrogen bondConnect The experiments of Hershey and Chase confirmed that DNA carriesthe genetic information, but they left other big questions unanswered: Whatexactly is this genetic information? How does DNA store this information?Scientists in the early 1950s still had a limited knowledge of the structure ofDNA, but that was about to change dramatically.MAIN IDEADNA is composed of four types of nucleotides.ConnectingCONCEPTSBiochemistry The nucleotides ina strand of DNA all line up in thesame direction. As a result, DNAhas chemical polarity, whichmeans that the two ends of theDNA strand are different. The 5 carbon is located at one end ofthe DNA strand, and the 3 carbon is located at the other end.When the two strands of DNApair together, the 5 end of onestrand aligns with the 3 end ofthe other strand. @ @6I 8I @230Unit 3: Genetics6 @Since the 1920s, scientists have known that the DNA molecule is a very longpolymer, or chain of repeating units. The small units, or monomers, that makeup DNA are called nucleotides (NOO-klee-oh-TYDZ). Each nucleotide hasthree parts.VISUAL VOCAB A phosphate group (onephosphorus with four oxygens)The small units, or monomers, thatmake up a strand of DNA are called A ring-shaped sugar callednucleotides. Nucleotides havedeoxyribosethree parts. A nitrogen-containing base (aphosphate group nitrogen-containingsingle or double ring built aroundbasenitrogen and carbon atoms)One molecule of human DNAcontains billions of nucleo

8.2 Structure of DNA DNA structure is the same in all organisms. 8.3 DNA Replication DNA replication copies the genetic information of a cell. 8.4 Transcription Transcription converts a gene into a single-stranded RNA molecule. 8.5 Translation Translation converts an mRNA message into a polypeptide, or protein. 8.6 Gene Expression and Regulation

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