What is the Structure of DNA?

Transcription

1 ER 1 D: he ereditary Molecule uanine ytosine denine hymine EI What is the tructure of D? hapter 1 Modern enetics for ll tudents 19

2 hapter 1: ection Background E BILIY F D to act as a reservoir of hereditary information resides in four aspects of its structure: 1. Each building block of D (called a nucleotide) contains one of four nitrogenous bases: adenine (), cytosine (), guanine (), or thymine (). 2. Each D molecule consists of two strands, in each of which these four kinds of nucleotides are connected by chemical bonds into long, linear arrays. 3. In principle, the four different kinds of nucleotides may be connected in any possible sequence within a single strand of D. 4. In practice, however, the sequence of nucleotides that is present in one strand of D specifies the sequence of nucleotides that must be present in its partner strand. his is because an on one D strand is always paired with a on the other strand, and a on one strand is always paired with a on the other strand. hese two invariant relationships ( pairs with and pairs with ) are known as the base-pairing rules. "But why," your students are likely ask, "is always paired with? nd why is always paired with? Why can t pair with or? r why couldn t pair with or?" ood questions! he answer is that the base-pairing rules are a direct consequence of the chemical structures of,,, and as well as the nature of the chemical bonds that hold the two strands of a D molecule together. his is explained to some extent in the excerpt from he artoon uide to enetics that your students will read in ection.2, but the concepts are so fundamental that it is worthwhile to spell them out in a bit more detail here. he D structure diagram on page 22 illustrates four features of D structure that are relevant to understanding the base-pairing rules: 1. wo of the nitrogenous bases present in D ( and ) contain two rings and thus are much larger than the other two ( and ), which have only one. itrogenous bases with two rings (such as and ) are known as purines, whereas nitrogenous bases with a single ring (such as and ) are known as pyrimidines. 2. stable D structure is formed only when the two strands are a constant distance apart, and this only happens when a purine on one strand is paired with a pyrimidine on the other strand. he strands would have to move much further apart to permit a purine to pair with another purine and would have to move much closer together to permit a pyrimidine to pair with a pyrimidine. o - and - base pairs would not be compatible with a stable D structure. hapter 1 Modern enetics for ll tudents 20

3 3. he two strands of D are held together by a series of hydrogen bonds, which are the weak bonds formed when a single hydrogen () atom is positioned between (and thus "shared" by) two nitrogen () atoms or one nitrogen and one oxygen () atom. he arrangements of atoms in the four kinds of nitrogenous bases is such that two hydrogen bonds are formed automatically when and are present on opposite D strands, and three are formed when and come together this way. ote also on the diagram that the locations of the atoms that participate in formation of hydrogen bonds are such that the hydrogen bonds are the same length in the - and the - base pairs. - or - pairs would not be able to form similar sets of hydrogen bonds. 4. he hydrogen bonds described above and shown in the diagram can be formed only when the two D strands are antiparallel (run in opposite directions from one another) and are coiled into a pair of helices. stable two-stranded molecule could not be formed if the two strands ran in the same direction or were stretched out straight. he latter features may not be obvious from looking at a two-dimensional drawing of D like the one shown on the next page, but they become very obvious if one tries to build a D model, starting with realistic three-dimensional scale models of the nucleotides. It was when Watson and rick tried to build such a model that they discovered the double-helical nature of D. In exercise.1, your students will construct a flat model of D using pieces of colored paper to solve a puzzle. his should get them started on the path toward understanding D structure, even though the puzzle (being flat) will not illustrate the helical nature of D. First, it will allow them to discover the base pairing rules for themselves. econd, it will allow them to discover that the base-pairing rules mean that each D molecule contains all of the information required for constructing a second molecule of the very same sort (i.e., the information required for D replication). In.2, they will learn how the structure of the D double helix was deduced, by reading a rather charming excerpt from he artoon uide to enetics. In.3, they will read a short segment on the importance of model building in biology; and then in.4, they will build a simple model of the D double helix. aving done so, they will be challenged to consider the strengths and weaknesses of the two kinds of D models they have now constructed. In.5, they will label all the parts of a drawing of a short piece of D. his exercise should reinforce their understanding of the differences in size between purines and pyrimidines, of the base-pairing rules, and of the fact that the two strands of a double helix run in opposite directions. Finally in.6, they will have the fun of using a D Word earch" to reinforce their understanding of certain terms and concepts that they should have mastered in this unit. hapter 1 Modern enetics for ll tudents 21

4 D RUURE he nitrogenous bases 5' ydrogen bonds between paired bases 3' 3' he sugar-phosphate backbones of the D double helix 5' hapter 1 Modern enetics for ll tudents 22

5 uanine ytosine denine hymine.1 he uzzle of D tructure and Replication UDE E LE VERVIEW his activity provides students with the opportunity to discover for themselves the important ideas of base-pair complementarity and the role that this plays in D replication. MERIL For each student or pair of students: puzzle pieces (see advance preparation below) one half of a file folder per student DVE RERI uzzle pieces will need to be prepared before the first class, but then they can be reused. hotocopy each of the puzzle pages on a different color of paper. Each sheet contains 20 puzzle pieces of one type. You will need 10 puzzle pieces of each type for each bag. o for a class of 20, for example, you would need 200 pieces of each type and thus would need to make ten copies of each page. he pieces will hold up much longer, of course, if you laminate them before cutting them out. IMELIE art of this activity requires about 10 minutes. hen students answer the first four questions on base-pairing rules before going on to art B. art B requires approximately 5 minutes. tudents then answer Questions 5 and 6 about D replication. hapter 1 Modern enetics for ll tudents 23

6 WER LYI D LUI QUEI UDE E Do you see any consistent relationship between the D bases (puzzle pieces) in one strand of your puzzle and the bases with which they are paired in the other strand? If so, state the nature of the relationship(s) you see. Yes. is always paired with, and is always paired with. 2. alf of the puzzle pieces that you were given (the s and s) were much larger than the other pieces (the s and s). Did this size difference cause your D model to be significantly wider in some parts than in others? If not, why not? o. Because one of the large bases ( or ) is always paired with one of the small ones ( or ), a double-stranded D molecule always has a constant width. 3. Is there any consistent difference in the way that the puzzle pieces in the right-hand strands and the left-hand strands of your model are oriented? If so, what is the difference? Yes. he two strands run in opposite directions. 4. ow can you account for the fact that no matter which bases were selected for the lefthand strand of a D molecule, everyone had just the right pieces left over to assemble a matching right-hand strand? We all started with equal numbers of each kind of nitrogenous base. Because always pairs with and always pairs with, you always can build a two-stranded molecule if you start with as many s as s and as many s as s. 5. re the two D puzzles that you now have the same or different? ow can you account for this? hey are identical. Because of the base-pairing rules, each individual strand of a twostranded D molecule contains all of the information required to build a new twostranded molecule that is just like the starting molecule. 6. What do you suppose biologists call this process of making two identical doublestranded D molecules from one when it occurs in cells? hey call it D replication. EMLE uzzle piece templates follow on pages hapter 1 Modern enetics for ll tudents 24

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11 uanine ytosine denine hymine.2 he piral taircase UDE E LE VERVIEW he piral taircase is an excerpt from he artoon uide to enetics by Larry onick and Mark Wheelis, which can be ordered from arper ollins ublisher, 10 East 53rd treet, ew York, Y (IB ). ortions of the text are reprinted with permission of the authors. his reading is a user-friendly way to introduce students to the basics of D structure. IMELIE his reading requires about 5-10 minutes and may be used as a homework assignment. uanine ytosine denine hymine.3 What is a Model? nd What is it ood For? UDE E 26 LE VERVIEW his reading is to be used as an introduction to Lesson.4 Building a hree-dimensional D Model. IMELIE his reading requires only a couple of minutes. You may want the students to read it as a homework assignment before building the three-dimensional models in class. hapter 1 Modern enetics for ll tudents 29

12 uanine ytosine denine hymine.4 Building a hree-dimensional D Model UDE E LE VERVIEW In this lesson, the students will build a second model. his one will be made from three-dimensional plastic pieces. he students will then be asked to observe the three-dimensional model (B) and compare its qualities to the flat, paper model () that they made at the beginning of this section. IMELIE his model-building exercise can be completed in 50 minutes if students are prepared and are on task. It is often difficult to convince students to take the model apart to be used for the next class immediately after they have constructed it. o, either prepare them for this or have another set available for the next class period. MERIL For each student or pair of students: 1 D Model Kit he kits can be ordered from: Frey cientific 100 aragon arkway.. Box 8101 Mansfield, (800) D Model Kit- atalog # DVE RERI If your D Model Kits have been used before, it is a good idea to check them to be sure that no parts are missing. here is also a list of kit materials on the tudent ages so the students can do this. his would add to the time necessary for completion. hapter 1 Modern enetics for ll tudents 30

13 WER LYI D LUI UDE E What feature or features of a double-stranded D molecule are represented better in model than in model B? In model, we clearly see that two kinds of nitrogenous bases ( and ) are larger than the other two kinds ( and ) and that there appears to be a physical basis for the fact that pairs only with and pairs only with. In model B, the nitrogenous bases all appear to be the same size and shape, and thus there is no obvious reason why could not pair with - or even with another. lso, in model we could see more clearly that the two chains were running in opposite directions, whereas in model B the only difference between the two chains was in the position of the red phosphate groups at their ends. 2. What feature or features of a double-stranded D molecule are represented better in model B than in model? Well, of course, model B shows the three-dimensional (double-helical) shape of D much better than model does. In addition, the straws and connectors used in model B represent the sugar-phosphate backbone of each chain much better than the pencil lines in model did. 3. What feature or features of a double-stranded D molecule that you read about in the excerpt from the artoon uide to enetics are not well represented in either model or model B? either model shows clearly that the nitrogenous bases all lay flat and parallel to one another, like the surfaces of stairs in a spiral staircase. hapter 1 Modern enetics for ll tudents 31

14 uanine ytosine denine hymine.5 D Model Questions UDE E 31 LE VERVIEW his exercise uses a different approach to reinforce students understanding of basic aspects of D structure. It emphasizes a feature that is also emphasized in your BKRUD information for ection (20) that the two chains of a D molecule must run in antiparallel (opposite) directions in order for the bases to pair properly and form the hydrogen bonds that hold the two strands together. o reinforce this concept, suggest to the students that they should turn the page 180 before labeling the right-hand half of this double-stranded molecule. hus, the labels should end up in the orientations shown on the next page, which shows that the two strands are identical in basic structure but are oriented in opposite directions. IMELIE his exercise requires about 15 minutes. hapter 1 Modern enetics for ll tudents 32

15 WER QUEI 1. Label each sugar group on the diagram with a letter. ee diagram 2. Label each phosphate group with a letter. ee diagram 3. ne adenine () and one guanine () have already been labeled. Label the rest of the nitrogenous bases. ee diagram 4. ircle one nucleotide. What three things go together to make a nucleotide? sugar, a phosphate and a nitrogenous base 5. he sides of the D ladder are made up of alternating sugar and phosphate groups. 6. he rungs of the D ladder are made up of base pairs. 7. is always paired with. 8. is always paired with. 9. aired bases are held together by weak bonds called hydrogen bonds. 10. When the D ladder twists the way it normally does, the shape of the molecule is called a double helix. hapter 1 Modern enetics for ll tudents 33

16 uanine ytosine denine hymine.6 D Word earch UDE E 32 LE VERVIEW his is a fun exercise, in which students use the ever-popular word-search format to reinforce their learning of basic terminology and concepts related to D structure. IMELIE his exercise requires about 15 minutes. It could be assigned for homework. WER I J Y R L W X B V X M I U R W Y Q W D U Y J W L V Q E B K D I I B E L D K E Z E L U E E D Y W B L Q U K B E I Y E X I F X Q Y J R E B D Z K M R R Q L I D D D B R K R M E R D I U W M J I X L L U L E Z L W I V Y B V B F L D V L D U U K E E W I I Q E E R X Y U I L Q E B E L F B Q I V E B Q E I D E B J E I M Y B X X R I I W L B D Q Q R W B R D U B J Z Z Y F V I I Y M R U X V E D X B E I R X Y Z Y Y D E Y E Y W X I L E E L B U D Q I E J E I E Q V W Q X M L B X V D X F M K X I I F R R M Y 1. he nitrogenous base adenine 2. he nitrogenous base cytosine 3. he nitrogenous base guanine 4. he nitrogenous base thymine 5. he genetic material inside all cells D (abbreviation) 6. he full name for D deoxyribonucleic acid (two words) 7. he scientific name for the shape of the D molecule double helix (two words) 8. he arrangement of two bases in the D molecule forms a base pair (two words) 9. he name of the bonds that hold the two strands of D together (between the bases) hydrogen bonds (two words) 10. airs of these molecules form the steps or rungs in the D molecule (two words) nitrogenous bases 11. his subunit of D has three parts: a phosphate, a sugar and a nitrogenous base nucleotide 12. he long backbones of the D molecule are made of alternating sugar and phosphate groups 13. his process occurs when D makes a copy of itself replication hapter 1 Modern enetics for ll tudents 34