DNA Model INSTRUCTIONAL GUIDE R310000 Copyright © 2013 American Educational Products, Chippewa Falls, Wisconsin All rights reserved. The Student Guide portion of this instructional material may be reproduced by photocopy. Copies of the guide must not be for resale or for classroom use other than by the purchaser. Additional reproduction is prohibited without permission from the publisher: American Educational Products. Printed in the USA. 12. A base sequence of Adenine, Adenine, Adenine (A, A, A) in mRNA would only join with what sequence on a tRNA molecule? U U U would be the tRNA sequence to match the mRNA sequence A A A. 13. What specific amino acid is brought to the mRNA by a tRNA molecule with the following sequence: A, A, U? The amino acid carried by the AAU tRNA molecule would be leucine. 14. A ribosome receives the following mRNA message: AAA, GUU, GAA, and CGA. Determine the complementary tRNA nucleotide triplets that would join the mRNA. The complementary tRNA triplets would be UUU, CAA, CUU, and GCU. 15. What is the sequence of amino acids that those tRNA molecules would bring to the mRNA? Those tRNA molecules would bring, in order, the amino acids lysine, valine, glutamate and arginine. 16. What is the function of mRNA? Of tRNA? The function of mRNA is to make a copy of a gene that can move to a ribosome so the encoded protein can be made. The function of tRNA is to deliver to the ribosome the amino acid encoded by an mRNA sequence. 17. A mutation is a change in the DNA of an organism. How would a mutation affect the protein made by the cell? If the DNA is changed, then the mRNA is changed. Then a different tRNA may be needed, which would deliver a different amino acid to the ribosome. If a different amino acid is in the protein, the shape of the protein would change. Depending on where the change occurred, the protein might function poorly or not at all. BACKGROUND INFORMATION Chromosomes are structures in the nucleus of a cell and are composed of long strands of deoxyribonucleic acid (DNA). In 1953, two scientists, James D. Watson and Francis H. C. Crick, proposed a model of the structure of DNA. They described the molecule as a double helix or spiral, composed of two chains of nucleotides. Each nucleotide is composed of a molecule of the five-carbon sugar deoxyribose joined to a phosphate unit and to a nitrogencontaining base. Both strands of the DNA double helix consists of hundreds to millions of nucleotides. Bonds between the nucleotides hold the two strands together. There are four types of nitrogen bases, and therefore four types of nucleotides in DNA. Two are the purines, adenine and guanine. The other two, thymine and cytosine, are pyrimidines. These are usually represented by their first letters: A, G, T and C. The bonds that hold the strands of the helix together involve an attraction between adenine and thymine (A-T) and between guanine and cytosine (C-G). The attraction between these nitrogen bases also plays a role in replication, the process by which chromosomes are duplicated exactly in preparation for cell reproduction. During replication, the DNA strands are “unzipped”; that is, the bonds between the A-T and C-G nucleotide pairs are broken. Then unattached nucleotides attach to the exposed nitrogen bases and are sealed into new “complementary” strands. As a result each of the original two strands of the double helix has now been paired up with a new strand that is identical to the one from which it had been “unzipped”. When a cell reproduces through mitosis, each “daughter” cell will receive one of the newly-formed double helixes and will be genetically identical to each other and to the original cell. The order in which the four nucleotides occur in sections of DNA called genes determines the sequence of amino acids in the proteins the cell can make. Sets of three nucleotides specify specific amino acids in a code that is consistent among the various organisms on Earth. However, the DNA sequence is not directly involved in making proteins. A related molecule, ribonucleic acid (RNA) is made in the first stage of protein synthesis, transcription. There are slight differences in the nucleotide structure between DNA and RNA. RNA has the sugar ribose instead of deoxyribose, has the nitrogen base uracil (represented by the letter U) instead of thymine, and occurs as a single strand rather than as a double strand. During transcription, a strand of RNA is copied from a segment of DNA (a gene). As with replication, the attraction between the bases (A-T, U-A, G-C and C-G) is used to make the RNA strand that is complementary to the DNA sequence. In eukaryotic cells, such as plant or animal cells, this strand of RNA is able to leave the cell’s nucleus, while the much larger chromosomes cannot. This RNA acts as a messenger and brings the coded nucleotides to a ribosome, a structure in the cell’s cytoplasm where the specified amino acids are assembled into a chain called a polypeptide or protein. 4 The triplet sets of RNA nucleotides are called “codons”. There are 64 possible codons formed from the four RNA nucleotides (A, G, U and C). These correspond to 20 amino acids 1 and to three “stop” codons which signal the end of a polypeptide or protein chain. The table below illustrates codons associated with the amino acids. Amino Acid alanine arginine asparagine aspartic acid cystine glutamic acid glutamine glycine histidine isoleucine leucine lysine methionine phenylalanine proline serine threonine tryptophan tyrosine valine Triplet Code GCA, GCG, GCC, GCU CGA, CGG, CGC, CGU, AGA, AGG AAC, AAU GAC, GAU UGC, UGU GAA, GAG CAA, CAG GGC, GGU, GGA, GGG CAC, CAU AUC, AUU, AUA CUC, CUU, CUA, CUG, UUA, UUG AAA, AAG AUG UUU, UUC CCA, CCG, CCC, CCU UCA, UCG, UCC, UCU, AGU, AGC ACA, ACG, ACC, ACU UGG UAC, UAU GUA, GUG, GUC, GUU At the ribosome, the second stage of protein synthesis, translation, takes place. The messenger RNA (mRNA) acts as a pattern or template for the assembling of amino acids into the polypeptide or protein chain. The mRNA attaches to the ribosome. Other, nontemplate strands of RNA called transfer RNA (tRNA) bind to amino acids in the cytoplasm. The tRNA molecule which has both a triplet of nucleotides complementary to the mRNA codon (the “anticodon”) and the corresponding amino acid lines up with the mRNA codon in the ribosome. When another tRNA molecule binds to the adjacent codon, the two amino acids are bonded together. As the ribosome moves along the mRNA chain, the amino acid chain lengthens. When a “stop” codon enters the ribosome, the completed polypeptide or protein chain is released. Thus, the DNA molecule indirectly controls the physical make-up of cells by determining the structure of the proteins, such as enzymes, in the cell. The action of these enzymes and other proteins, in turn, determines the functions of the cell. Since DNA controls the function of the cell by encoding proteins and determines heredity, it is often called the “Master Molecule”. 2 ANSWER KEY FOR DISCUSSION QUESTIONS 1. What is the general structure of the DNA molecule? The DNA molecule’s structure is a double helix, two twisted chains of nucleotides, held together by hydrogen bonds. 2. Name the two parts of a DNA nucleotide which alternate to make the side portions, or “backbone” of the DNA molecule. The two parts that make up the DNA “backbone” are the deoxyribose sugar and the phosphate group. 3. Name the part of the DNA nucleotide to which a nitrogen base is attached. The nitrogen base is attached to the deoxyribose sugar. 4.Name the parts of a nucleotide which join by a hydrogen bond to form the double strand of DNA. The nitrogen bases of a pair of nucleotides join by a hydrogen bond 5. If there were four thymine bases on your model, how many adenine bases would there be? There would be four adenine bases, because the adenine base attaches to a thymine base by a hydrogen bond. 6. From top to bottom, what are the bases on the left side of the molecule you constructed? On the right side? Answers will vary. One possible answer is that the left side would be adenine-guanine-thymine-guanine-cytosine-adenine and that the right side would be thymine-cytosine-adenine-cytosine-guanine-thymine. 7. When you opened the entire molecule along the hydrogen bonds, to what bases did the left side attach? The right side? Answers will vary. Using the possible answer provided for #6, the answer would be thymine, cytosine, adenine, cytosine, guanine and thymine would attach to the left side, and adenine, guanine, thymine, guanine, cytosine and adenine would attach to the right side. 8. Do the two new DNA molecules contain the same base pairs? Yes. 9. Are the two DNA molecules you now have exact copies of each other? Explain. Yes, the pairings of the nitrogen bases ensures this. 10. Based upon this information, what are the bases of the messenger RNA molecule that the left side of your DNA would construct? The right side? Answers will vary. Using the possible answer provided for #6, the left side would form the RNA molecule uracil-cytosineadenine-cytosine-guanine-uracil and the right side would form adenine-guanine-uracilguanine-cytosine-adenine. 11. To build a messenger RNA molecule, in what way would you modify the materials of this kit? To build a messenger RNA molecule one would need different colored parts to replace deoxyribose with ribose and different colored parts to replace thymine with uracil. 3