DNA Activity Model. For complete technical support call Objectives: Use models to demonstrate complementary base pairing

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1 Neo/SCI Teacher s Guide DNA Activity Model Objectives: Use models to demonstrate complementary base pairing Learn the components of nucleotides. Model the double helix structure of DNA # For complete technical support call Neo/SCI PO Box Rochester, NY Website: neosci.com support@neosci.com Copyright Neo/SCI Corporation. All rights reserved. For complete technical support call Neo/SCI PO Box Rochester, NY Website: neosci.com support@neosci.com

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3 Neat Websites Science Concepts Nucleotide structure DNA structure Complementary base pairing The genetic code Background If you were asked to describe the features that make you a unique individual, what would you list? You would probably share some of these characteristics with other people in your family, but unless you have an identical twin, those features would not be exactly the same as those of your relatives. The characteristics that make each of us N base ourselves are coded for by a sequence of chemical building blocks in all of the cells of our bodies. This chemical is deoxyribonucleic acid, or DNA for short. The building blocks of DNA, called nucleotides, can be assembled in almost endless combinations so, Phosphate 5-C sugar organisms as different as an elephant, a fern, an amoeba, and you, all share the same DNA it s just arranged differently. Nucleotide structure DNA nucleotides are composed of three smaller components which are covalently bonded: a five-carbon (pentose) sugar, a phosphate group, and a nitrogen-containing base. There are four types of DNA nucleotides. The only difference between them is the identity of the nitrogen-containing base. The phosphate group and deoxyribose (the five-carbon sugar) are always the same. By connecting nucleotides endto-end in specific sequences, a code is developed which allows the cell to make proteins either for use within the cell or to be shipped outside the cell. 9 2

4 G C T A Purines Biotechnology: Discuss the human genome project with your students. Ask them to think of ways in which this type of information may be useful to scientists, physicians, lawyers. Assign groups to study different techniques used in studying DNA and/or medical uses of information from the human genome project. Figure 1 Pyrimidines Going Further The four nitrogen-containing bases are adenine, guanine, cytosine, and thymine. Two of these, cytosine and thymine, are single-ring structures and are called pyrimidines. The other two bases, adenine and guanine, are double-ring structures (Figure 1) and are called purines. When nucleotides link together to form DNA, it is the sugar and phosphate groups which link the bases together in a long chain. The nitrogen-containing bases stick out to one side of the growing DNA chain, with the sugar and phosphate groups forming an unbroken strand on the opposite side (Figure 1). Bases on adjacent strands of DNA can pair up to form one, double-stranded piece of DNA. This can only happen if each strand of DNA is complementary to the other. In order for bases to pair up, one must be a purine and the other must be a pyrimidine in fact, in DNA, adenine only pairs with thymine (A-T), and cytosine only pairs with guanine (C-G). In other words, cytosine (C) is complementary to guanine (G), and thymine (T) is complementary to adenine. When complementary strands of DNA match up, hydrogen bonding occurs between complementary bases to hold the entire structure together. Because of the shapes of the molecules involved, the entire double-stranded DNA doesn t lie flat like a ribbon. Instead it forms a twisted structure, often called the double helix. Although this structure is well known today, it was one of the great mysteries of the last century. This mystery was solved by a pair of scientists, James Watson and Francis Crick, who successfully assembled a model of DNA using nucleotides. Before then, most scientists believed that proteins, not nucleotides, carried the genetic code. Watson and Crick demonstrated to the world that the genetic diversity we see in all living creatures is a result of different arrangements of four nucleotides linked together into long strands of DNA called chromosomes. Research the following questions: Why isn t it possible for adenine to pair with cytosine or guanine? What type of chemical bond is found between the phosphate group, sugar group, and base of a nucleotide? What type of chemical bond is found between the strands of the DNA molecule? What is the other major difference between RNA and DNA? What is RNA used for within a cell? How many types of RNA are there? Learn and Read More About It Ulf Lagerkvist. DNA Pioneers and Their Legacy. Yale University Press, Lev Liapin. Unraveling DNA : The Most Important Molecule of Life. Perseus Press, Paul Strathern. The Big Idea : Crick, Watson, and DNA (Strathern, Paul, Big Idea.) Anchor Books, James D. Watson. The Double Helix : A Personal Account of the Discovery of the Structure of DNA. New American Library Press,

5 Ask the students to perform the following: Describe and/or use the model to illustrate the following: Nucleotide Nucleic Acid Gene Chromosome Pentose sugar Nitrogen-containing base Base pairing Complementary strand Assessment Use the model to demonstrate a gene which is 6 base pairs in length. Use the model to demonstrate a complementary strand of DNA for the 6 base pair gene. Cross Curricular Integration Language arts: One of the major differences between RNA and DNA is the type of sugar used to build them. Ask the students to speculate about this difference based on what they know about suffixes and root words. History: Ask the students to research the discovery of the structure of the genetic material, including the competing proposals of protein versus nucleic acids, and the competing international teams of scientists working on the puzzle. Activity Assembly of the DNA double helix model DNA stand Metal base with metal rod 2 Single DNA strands, 12 base pair Step 1 First, examine the four nitrogen containing bases. Notice that adenine (red) can only combine with thymine and that cytosine (yellow) can only combine with guanine (blue) C A 1 What you need What to do... Step 2 Align the first set of bases (thymine and adenine nucleotides) on the stand as shown, so that hydrogen bonds can form between the bases. Bring the bases together so that they snap in around the metal rod. G T 7 4

6 D Bases D C A Metal rod Step 3 Align the next set of base pairs (cytosine and guanine) and snap into place. Continue to connect the remaining complementary base pairs until the entire double helix is assembled. T G D D Metal base Questions 1. The full name for DNA is deoxyribonucleic acid. What does the deoxyribo- part of the name stand for? Deoxyribo refers to the sugar component of DNA. 2. The full name for the other type of nucleic acid is RNA, or ribonucleic acid. Based on what you know of DNA, what do you think the ribopart of RNA s name stands for? Ribo refers to the sugar component of RNA. These sugars are not the same, although ribo (meaning ribose) is a shared component of DNA and RNA. 3. What does the term complementary mean in base-pairing? It means that like bases don t pair together no A-A or T-T or C-C or G-G pairs occur. Complementary pairing means that each base pairs with another that complements its structure. 4. What is a stretch of DNA which functions as a section of the genetic code called? A stretch of DNA on a chromosome which functions as a unit of the genetic code is called a gene (codes for a polypeptide). 5. What makes one gene different from the next gene within a chromosome? The linear sequence order of bases determines what the gene codes for; for example, a DNA section of AGTCTAACG may code for a different product than a DNA section with AGTATCCGA. 6. What do you think must happen to the double-strand of DNA when a cell makes a copy of itself? It must unzip along the hydrogen bonds to make copies of itself. 5 6

Chapter 11: Molecular Structure of DNA and RNA

Chapter 11: Molecular Structure of DNA and RNA Student Learning Objectives Upon completion of this chapter you should be able to: 1. Understand the major experiments that led to the discovery of DNA as