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By the end of this lesson,youshould be able to describe thestructure and main functions ofDNA.II

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• Modeling DNA• Building a DNA Sequence• Mutations Cause Diversity

Exploration Lab• Extracting DNA

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1 Predict Check T or F to show whether you thinkeach statement is true or false.

2 Describe DMA is sometimes called the

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DNA is found in the cells of allliving things.

All DNA mutations are harmful.

The cell can make copies of itsDNA.

Active R&adiwg3 Synthesize Many English words have their

roots in other languages. Use the Latin wordsbelow to make an educated guess aboutthe meanings of the words replication andmutation.

Latin word Wleaning

mutare to change

replicare to repeat

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Vocabulary TermsDflA • mutationnucleotide • RNAreplication • ribosome

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4 Identify This list contains the key termsyou'll learn In this lesson. As you read,circle the definition of each term.

Example sentenceDNA can undergo mutation.

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Example sentenceBefore cell division, DNA replication occurs.

replication:

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5 Identify As you read, underlinethe meaning of the word code.

What is DMA?The genetic material of a cell contains information needed forthe cell's growth and other activities. It also determines theinherited characteristics of an organism. The genetic materialin cells is contained in a molecule called deoxyribonucleic(dee*OK*see*ry»boh«noo*KLAY*ik) acid, or DNA for short. Youcould compare the information in DNA to the books in your locallibrary. _You might find a book describing how to bake a cake orcomplete your favorite video game. The books, however, don'tactually do any of those things—you do. Similarly, the "books" thatmake up the DNA "library" carry the information that a cell needsto function, grow, and divide. However, DNA doesn't do any ofthose things. Proteins do most of the work of a cell and also makeup much of the structure of a cell.

Scientists describe DNA as containing a code. A code is aset of rules and symbols used to carry information. For example, Iyour computer uses a code of ones and zeroes that is translatedinto numbers, letters, and graphics on a computer screen. Tounderstand how DNA functions as a code, you first need to learnabout the structure of the DNA molecule.

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DNATimeline

Revietv this timeline to learn a\>outfomeof the important scientific contributions toour unJerytanJing of DNA.

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1869 Friedrich Miescheridentifies a substance that Ewill later be known as DNA.:^^»

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1919 Phoebus Levene publishes a paper on nucleicacids. His research helps scientists determine thatDNA is made up of sugars, phosphate groups, and fournitrogen-containing bases: adenine, thymine, guanine,and'cytosine. Bases are often referred to by their firstletter: A, T, C, or G. Each base has a different shape.

6 Analyze In this model, what do P, S, and A basesrepresent?

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How was DNA discovered?The discovery of the structure and function ofDNA did nothappen overnight. Many scientists from all over the worldcontributed to our current understanding of this importantmolecule. Some scientists discovered the chemicals that make upDNA. Others learned how these chemicals fit together. Still othersdetermined the three-dimensional structure of the DNA molecule.The timeline below shows some of the key steps in this process ofdiscovery.

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Research Use the Internet orlibrary resources to research ascientist who contributed to thediscovery of DNA. Then, create aposter about the scientist. Shareyour findings with your class.

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Maurice Wilkins make images ofDNA using x-rays. When an x-raypasses through the molecule, theray bends and creates a patternthat is captured on film.

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1953 James Watson and FrancisCrick use ChargafFs rules and thex-ray images of DNA to concludethat DNA looks like a long, twistedladder. They build a large-scalemodel of DNA using simplematerials from their laboratory.

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1950 Erwin Chargaff observesthat the amount of guanine alwaysequals the amount of cytosine, andthe amount ofadenine equals theamount ofthymine. His findings arenow known as Chargaffs rules.

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1952 Alfred Hershey and MarthaChase perform experiments withviruses to confirm that DNA,not proteins, carries geneticinformation.

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The chemical components that make up DNA are too small to beobserved directly. But experiments and imaging techniques havehelped scientists to infer the shape ofDNA and the arrangement ofits parts.

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^The Shape of DNA Is a Double HelixThe structure ofDNA is a twisted ladder shape called a doublehelix. The two sides of the ladder, often referred to as the DNA

backbone, are made of alternating sugars and phosphate groups.The rungs of the ladder are made of a pair of bases, each attachedto one of the sugars in the backbone.

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Acfwe Reading Describe Where are phosphate groups found in aDNA molecule?

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DNA Is Made Up of NucleotidesA base, a sugar, and a phosphate group make abuilding block ofDNA known as a nucleotide.These repeating chemical units join togetherto form the DNA molecule. There are four

different nucleotides in DNA, identified by theirbases: adenine (A), thymine (T), cytosine (C),and guanine (G). Because of differences in sizeand shape, adenine always pairs with thymine(A-T) and cytosine always pairs with guanine(C-G). These paired, or complementary, bases fittogether like two pieces of a puzzle.

The order of the nucleotides in DNA is acode that carries information. The DNA code is

read like a book. Genes are segments ofDNA thatrelate to a certain trait. Each gene has a startingpoint and an ending point, with the DNA codebeing read in one direction. The bases A, T, C,and G form the alphabet of the code. The codestores information about which proteins the cellsshould build. The types of proteins your bodymakes help to determine your traits.

10 Apply Place boxes around the bases that pairwith each other.

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11 Devise The bases are often referred to simplyby their initials—A, T, C, and G. The phrase"all tigers can growl" may help you rememberthem. Think of another phrase that useswords starting with A, T, C, and G that couldhelp you remember the bases. Write yourphrase below.

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How are copies of DNA made?The cell is able to make copies ofDNA molecules through aprocess known as replication. During replication, the two strands ofDNA separate, almost like two threads in a string being unwound.The bases on each side of the molecule are used as a pattern fora new strand. As the bases on the original molecule are exposed,complementary nucleotides are added. For example, an exposedbase containing adenine attaches to a nucleotide containingthymine. When replication is complete, there are two identicalDNA molecules. Each new DNA molecule is made of one strand of

old DNA and one strand of new DNA.

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When are copies of DNA made?Before a cell divides, it copies the DNA so that each new daughtercell will have a complete set of instructions. Our cells can replicateDNA in just a few hours. How? Replication begins in many placesalong the DNA strand. So, many groups of proteins are working toreplicate your DNA at the same time.

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What are mutations?Changes in the number, type, or order of baseson a piece ofDNA are known as mutations.Sometimes, a base is left out. This kind of changeis known as a deletion. Or, an extra base might beadded. This kind of change is an insertion. Themost common mutation happens when one basereplaces another. This kind of change is known asa substitution.

How do mutations happen? Given the largenumber of bases in an organisms DNA, it is notsurprising that random errors can occur duringreplication. However, DNA can also be damagedby physical or chemical agents called mutagens.Ultraviolet light and the chemicals in cigarettesmoke are examples ofmutagens.

Cells make proteins that can fix errors inDNA. But sometimes a mistake isn't corrected,and it becomes part of the genetic code.Mutations to DNA may be beneficial, neutral,or harmful. A genetic disorder results frommutations that harm the normal function of a

cell. Some of these disorders, such as Tay-Sachsdisease and sickle-cell anemia, are inherited, orpassed on from parent to offspring. Other geneticdisorders result from mutations that occurduring a person's lifetime. Most cancers fall intothis category.

Th'a Snake has olbiniym, a condition in^hich the body connotwake tJie pig»»ientythat give color to t^e ykin ond eye?.

14 Explain Albinism is an inherited geneticdisorder. Explain what is meant by "inheritedgenetic disorder."

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13 Apply Place a check mark in the box to indicatewhich type of mutation is being shown.

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What is the role of DNA and RNA inbuilding proteins?Imagine that you are baking cookies. You have a big cookbook thatcontains the recipe. If you take the book with you into the kitchen,you risk damaging the book and losing important instructions.You only need one page from the book, so you copy the recipe ona piece of paper and leave the cookbook on the shelf. This processis similar to the way that the cell uses DNA to build proteins.First, some of the information in the DNA is copied to a separatemolecule called ribonucleic acid, or RNA. Then, the copy is used tobuild proteins. Not all the instructions are needed all the time. Ineukaryotes, the DNA is protected inside the cell's nucleus.

Like DNA, RNA has a sugar-phosphate backbone and thebases adenine (A), guanine (G), and cytosine (C). But instead ofthymine (T), RNA contains the base uracil (U). Also, the sugarfound in RNA is different from the one in DNA. There are threetypes ofRNA: messenger RNA, ribosomal RNA, and transfer RNA.Each type ofRNA has a special role in making proteins.

Active Reading15 Identify As you read, number the sentences thatdescribe the steps of transcription.

Transcription: The Information in DNA IsCopied to Messenger RNAWhen a cell needs a set of instructions for making a protein, it firstmakes an RNA copy of the necessary section ofDNA. This processis called transcription. Transcription involves DNA and messengerRNA (mRNA). Only individual genes are transcribed, not thewhole DNA molecule. During transcription, DNA is used as atemplate to make a complementary strand ofmRNA. The DNAopens up where the gene is located. Then RNA bases match up tocomplementary bases on the DNA template. When transcription iscomplete, the mRNA is released and the DNA molecule closes.

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Translation: The Informationin Messenger RNA Is Used toBuild ProteinsOnce the mRNA has been made, it is fedthrough a protein assembly line within aribosome. A ribosome is a cell organellemade ofribosomal RNA (rRNA)and protein. As mRNA passesthrough the ribosome, transferRNA (tRNA) molecules deliveramino acids to the ribosome.

Each group of three bases onthe mRNA strand codes for

one amino acid. So the geneticcode determines the orderin which amino acids are

brought to the ribosome. Theamino acids join together toform a protein. The processof making proteins fromRNA is called translation.

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Tcan^cciptfon DDNA DmRNA DtRNA Dribosome

Tcanylation DDNA DmRNA DtRNA Dribosome

Lesson 6 DNA Structure and Function 155

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21 Explain How could a mutation in the DNA affect what proteins aremade by the cell?

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