DNA Model Instructional Guide

HUBBARD'· scientific

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Published by Hubbard Scientific, Inc. Chippewa Falls, Wisconsin

Copyrlght@HubbardSclentific, Inc., Chippewa Falls, Wisconsin All rights reselVed. The Student Guide portion of this instrucUonal material may be reproduced by photoropy. C<lples of the guide must not be for resale or for classroom use other than by the purcl1aser. Additional reproduction Is prohibited without permission from the publisher: Hubbard Sc/entlfic, Inc .. P.O. Box 760, Chippewa Falls, WI 54729 Printed in the U.S.A.

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 F.H.C. Crick, proposed a model of the structure of the complex DNA. They described the molocuJe as a double helix or spiral, composed of nuc1eotides. Each nucleotide is composed of a phosphate unit joined to deoxyribose, a five-carbon sugar and a nitrogen­containing base. The DNA molecule is a double strand of posSlbly thousands of nucleotides bonded by their bases.

There are four types of bases in DNA. Two arc purines, either adenine or guanine. TIle other two are pryimidiIJes. with thymine or cytosine. They have the code letters A. G, T. and C. which are part of the code system controlling protein synthesis in a cell.

One of the amazing things about DNA is that it can build an exact duplicate of itself. The process is known as replication. During 'this process, the cell passes on its genetic code from one generation to the next. During replicaoon, the DNA "unzips" its two halves, the two bases attached to each other detach, and a new set of nuc1eotides with complimenting bases, attach to the free, newly detached bases. Two duplicate strands of DNA have now formed. When the cell undergoes mitosis or meiosis. the duplicate strands migrate to opposite ends of the cell. Upon completion of mitosis or meiosis, two celJs with identical chromosomes have formed.

A similar process, called transcription, occurs in the foonation of nearly exact portions of the DNA molecule. The molecule fanned by this process is ribonucleic acid (RNA). There are slight differences in the nucleotide structure of RNA. One is the sugar ribose, which contains one more oxygen atom than does deoxyribose. The other is a replacement of one of the nitrogen bases. In RNA, uracil replaces thymine, thus the base codes for RNA, which is a single strand, is A. G, U, and C. DNA controls protein synthesis, but does so by sending messenger RNA out of the nucleus into the cytoplasm of the cell to participate in protein synthesis.

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The four bases of DNA, A. G, T, and C fonn the "alphabet" from which the DNA code "words" are formed. As mentioned before, there may be thousands ,of baseS on a DNA strand, which may occur in any order however, it is not a single base that is important, rather a group of three called triplet codons. This makes 64 different three-Jetter words possible. Each of the coded words (triplet codons) determines the sequence of amino acids making up a protein. Each DNA code word always,codes the same amino acid. In different organisms, the arrangement of the code words may be in different order, thus giving rise to many different proteins.

There are thousands of triplet codes on the DNA strand. The DNA "unzips" and exposes only portions of its bases in making RNA. If the . code of DNA is TAG, TAA, TAT, the RNA code bases will be AUC, AUU. and AUA. Guanine always binds with cytosine and uracil always binds with adenine. (See table below.)

- I Amino Acid Triplet Code alanine GCA, GCG, GCC, GCU arginine CGA, CGG, CGC, CGU, AGA, AGG asparagine MC,AAU aspartic acid GAC,GAU cystine UGC, UGU glutamiC acid GM, GAG ' , glutamine CAA,CAG glycine GGC, GGU, GGA, GGG histidine CAC,CAU isoleucine AUC, AUU, AUA leucine CUC,CUU,CUA,CUG,UUA,UUG lysine AM, AAG methionine AUG .

. -: phenylalanine UUU,UUC proline CGA, GGG, GGG, GGU serine UGA, UGG, UCC, UCU, AGU, AGG threonine AGA, AGG, ACC, ACU tryptophan UGG tyrosine UAC,UAU valine GUA, GUG, GUC, GUU

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After 1ranscription, the me.~'reIlger RNA (mRNA) may act as a pattern, or template for the building of amino .aeids into proteins. The mRNA attaches to the ribosome in the cytoplasm of a cell at a point where" a protcin chain will start. Amino acids in the cytoplasm needed "0 make proteins at the ribosome sites are carried there by another type of R."NA, trallsfer RNA (tRNA). In this manner, the mRNA serving as a template is supplied with thc proper. sequence of amino acids to make a specific protein. Thus the DNA molecule controls the physical make-up .of cells by determining the PJ;'Oteins and enzymes in a cell, as well as the transfer of genetic information in a celL Since DNA controls "physical make-up, chemical activity,'and heredity, it is called the "Master Molecule"_

Note: Each packet of parts contains three white straws representing ·uracil. Students can now conStruct messenger RNA as suggested in Part ill.

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DNA Model Kit Student Guide

Student Name ________ ~_,Class _____ _

Introduction Deoxyribonucleic acid (DNA) is found in the chromosomes of aU living thins. It is the chemical of which genes are composed. With an understanding of this all important molecule, scientist mow how chromosomes can duplicate during division and transfer genetic infonnation to new cells. They also understand the functioning of DNA instructions sent out to direct the activity of protein synthesis within the cell.

The DNA molecule is tile shape of a double helix (spiral). The molecules making up DNA are deoxyribose. phosphate, aod nitrogen base of which there are four: adenine, thymine, cytosine, and guanine. These arc called nucleotides. Adenine and guanine are purines and cytosine and thymine arc pyrimidine. They arc know by their code letters A, G, T, and C. There is a specific manner in which they bond, A only to T and Conly to G.

Purpose In this investigation, you will examine the structure of DNA by building. your own model.

Materials 12 ...:. 3 prong "deoxyribose" centers (black) 12 - 2 prong ·phosphate~ centers (red)

3 - "AdenineM straws (red) 3 - "Guanine- straws (gray) 3 - ''Thymine" straws (blue) 1 - 4 prong center 3 - -Cytosine" straws (short green) 6 - Hydrogen bondcenters (white)

24 - connectors (yellow) 1 - long strand (gray) 3 - long posts (green) 3 - "Uracil" (white)

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Procedure

Part 1 - Structure of DNA

1. In order to construct your segment of the DNA model, you will need 12 black centers, 10 red centers, 3 red straws, 3 blue straws, 3 gray straws, 3 short green straws, 6 white centers, 20 yellow straws, and the stand (one long gray straw, three green posts, and a four prong center).

2. Construct a backbone by attaching a deoxyribose (black center) to a phosplbilte (red center) with a yellow straw (figure 1).

~Deoxyribose SU9ar

~~~nitrOgen base ____ phosphate

figure 1

3. Construct a chain composed of 6 deoxyribose (black centers) and five red centers with the yellow straws to construct one backbone.

4. Attach one of the nitrogen bases (red, blue, green, or gray) to the open deoxyribose bond in any sequence you desire. You now have one side of the DNA model.

5. Repeat steps 2 and 3 to construct the other side of the DNA model (figme 2).

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figuro2

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6. ' Use the six white bond centers which represent hydrogen bonds between bases, to attaeh the left side of the model , making sure to attaeh only a red straw (adenine) to a blue straw (thymine) and a gray straw (guanine) to green straw (cytosine). Note: you will still have picces left. •

7. Place the constructed segment of the DNA model on the stand by passing the long gray straw tluougb the holes in the white hydrogen centers. Attach the four-prong center to SctVC as a stand. Twist the DNA structure gcntly to fonn a doublc helix spiral (figure 3).

figure 3

8. On a separate piece of paper, answer questions one through six in the Discussion .Question section of this guide.

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Part II - DNA Replication

Your DNA model represents only a short portion of DNA in a chromosome. which is usually composed, of thousands of nucleotidcs. Although your model is oilly a short portion, its replication is the same as that of an entire chromosome during mitosis and meiosis:

I. Find another group to work with. "Unzip" your DNA chain, that is, separate one sidc of the DNA strand from the other by removing one set of nuclcotidcs from the white centers. Have the other group do the same with their model.

2. Observe your two separate strands. Note that when the DNA molecule splits in tow, new bases, complimentary to the original ­ones, now attach to the two free halves creating two strands of DNA. Using the other team's model, create two strands of DNA. Be sure the bases that attach arc complementary to each other, A to T, and C toG.

3. Note that a replicating DNA strand does not take its new parts from an existing DNA strand. It would receive these new deoxyribose sugars. phosphate and nitrogen bases from the nucleQplasm. The new pieces arc constructed in the cytoplasm of the cell and move into the nucleus of the cell where DNA replication takes place.

4. Now answer questions 7 through 9 in the Discussion Questions· section ofthis guide.

Part 111 - Structure of RNA

DNA ensures the exact replication of chromosomes, which arc genetic "codes" of instruction for thc entire cell. 'lbcse instructions arc sent out of the nucleus of a cell by another nucleic acid, ribonucleic acid. or RNA. It is similar to DNA in that it is also composed of nueleotides, however deoxyribose and thymine are not found in RNA. Deox.yribose is replaced by ribose and thymine is replaced by ura(:r"i. Other than those two replacements the RNA molecule contains the same molecu1es as DNA.

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1. Discuss question 10 with your group.

2. Allow the other leam to have their original pieces back, that were created in Part I. Replace the thymine (blue strnws) with uracil (white straws). '

3. The structure of RNA is similar to DNA with severnl exceptions. Answer questions 10 through 16 in the Discus."ion Question sections of this guide.

Discussion Questions

1. What is the geneml structure of the DNA molecule?

2. Namc the two molecules which alternate to make the side portions or "backbone" of a DNA model.

3. What is the name of the spccific molecule to whieh each nucleotide is attached?

4. Name the molecules or parts of a nucleotide, which join by a hydrogen bond to attach to the double strnnd of DNA.

5. If there arc four thymine bases on your model, how many adenine bases will there be?

6. What arc the bases on the left side of the molecule you constructoo? The right side?

7. If you were to open the entire molecule along the hydrogen bonds, what bases would the left side attach to? The right side?

8. Would the two new DNA molecules contain the same base pairs? 9. Would the two DNA molecules be exact copies of each other?

Explain.

10. Based upon this infonnation what are the messenger RNA molecuJes that the left side of your DNA molecule would construct? 1b.e right side?

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11. A base sequence of Adenine, Adenine, Adenine (A, A, A) in mRNA would only join in what sequence oftRNA?

Using the table below, answer questions 12 - 14.

-Amino Acid 1·- tRNACode --Serine ACU Proline----'---- - - - 1--' GGG ---Leucine AAU Glutamine --1-- -----GUU--- --

-;;c,'------- ------Phenylalanine AAA Valine I ' eAA

--,. -----+- ---Lysine I . UUU - --

12. What specific amino acid is brought to the mRNA by a tRNA molecule with the following sequence - Po, A, U?

13. If a protein molecule consists of the following amino acid sequence: leucine, glutamine, tyrosine, serine, and leucine what would the sequence of tRNA molecule be. .

14. A ribosome receives the following mRNA message: AAA, GUU, GAA, and CGA, what is the sequence of the tRNA that would join themRNA.

15. What is the function ofmRNA? tRNA?

16. Itow can a mutation occut1

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