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Student Study and .Analysis Slreet
lntroduction
Deox¡nibonucleic acid, DNA, is respons¡ble for loring information concem¡ng the sEucture of proteins and the
genetic profile of an organ¡sm. ln order to pass this information along to future generations of the organísm,
the DNA must be perfectly replicated in both amount and content lf the nucleic acid were not reproduced to
such an efficient degree, characteristics of heredity would be disrupted. and an organism's identity would be
devastated.
Only frve elements are involved in the formation of a nucleic acid: carbon, hydrogen. oxygen, nitrogen, and phosphorus. Theycombine in
three dist¡nct subunits to form a nrrcleot'de [F¡gure 1], thesmallest p¡ece
of a.nucleic acid strand. The first subunit is one of two sugars. Ribose is
a five carbon sugar with hydroxyl (-OH) functional grorJps extending
from the first, second, third, and fifth carbon. Ribose is only found in
ríóonucleic acids (RNA). lf the hydroxyl group is absent from the second carbon, the ¡ibose is said to be deoryfied (oxygen removed), and
the sugar ís now called deoryribose. Deoxyribosè is only found in deoxynbonucleic acids (DNA).
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The second subun¡t is a phosphate r¿dical group. Derived from phosphoric acid, a phosphate contains a centralized phosphorus atom surrounded by four orygen atoms. The phosphate bonds with the sugar in an alternating pattem. Two of these sugar-phosphate strands make up a DNA molecule, and the strands ¡un parallel to
each other like railroad track.
These -tracks" are linked at every sugar section by the third subunit, called a nigogen base. Only four possible
bases exist for DNA. C¡osrhe añd úrymine are tñe two possibilities which havè sinaller, single-iing stiuctures
(categorized as pyrimidines), while ødenine and guonine are the larger, doublevjng structures (catçorized as
purines). RNA contains the same possible bases as DNA" with the except¡on of ghymine, which is replaced by
the base urocít When two nucleotides ioin at their nitrogen bas6, they do so inia specific manner. Because of
the constrictions on hydrogen bonding, cytosine can only partner with guaninþ, and thymine only with adenine. When the single strands of DNA are hydrogen bonded, the finished molécule takes on the shape of a
double helix [Figure 2]. This resembles a ladder that has been twisted into a spiral,staircase structure. Before a
cell can dMde, tt5
¡t5 DNA.must-be r€plicated- -The process by which DNA in ch¡omosomes ¡s copied before cell division is called replicotion. During replication, the
hydrogen bonded nitrogen baser ¡n the DNA double helix molecule spl¡L The
molecule appears to unzip at this section. An en4yme calléd DNA polymerase
moves along the unz¡pped strands and adds the cone<t complementary nucle.
otide to the strand. Complementary nucleotides bond to each side of the unzipped DNA forming two new DNA strands which are identical to the odginal.
Since the single stnnds which make up the DNA are minor images, both can be
used to rebuild the opposing single strand- This allovvs for the exactness in
replication.
OÞjective
Módels w¡ll be used to visualize the double helix structure of Dt¡A" They will
emphasize the hydrogen bonding which connects nitrogen base subun¡ts, and
how those bonds break and reform to replicate the DNA stand.
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Replication Modgl Packeg
containing:
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16 blaclç deoryribose motecule píeces
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14 wh¡te, phosphat€ molecule tubes (-O
4 red, guanine base tubes (--O
4 blue, thymine base tubes (--------¡
4 green. adeninebasetubes:
4 gray, cytosine base tubes (-n
8 hydrogen bond¡ng plugs l-T-h
'l Student Study and Anaþis Sheet (per súdent)
Procedure
Part l - Buitding a DNll molecule
l. Laying down the track
A O-btain seven phosphate model tubes and eight deoryribose sugar molecule pieces. Connect thesè in
a stra¡ght chain, so that the third, open bonding site on-each sugar is facing thè same direction. óbta¡n
seven mote Phosphate tubes with eight sugar pieces, and form ãsecond chãin with each extra bondino
s¡te fac¡ng the same dkêction. Set the chains parallelto each other ôn the table so the empty bondin!
sites face each other and form a structure thât resembles.railroad traiìcki [Figure 3].
B.
Phosphate and sugar altemately bond to form the backbone of allDNA molecules.
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Figure 3 - Phosphate-sugar chains
Making the rungs
A
The open bonding s¡tç on each nucleotíde's sugar's filled by a n¡trogen base- DNA has four possible
nitrogen bases, and they have been color coded for. this-ínvestigation{see-ú.¡e mater¡als{¡st at thè beginning of the study and añLaþis sheet). Sta.t¡ng at the top of oneãf youi phosphate-sugar track, bond-the
following n¡trogen bases, ¡n order, to the strand: guaning cytosine, adenine, thymine, thymine. guaning
adenine, and cytosine.
B.
The second chain of phosphate-sugar moleculei also conta¡ns n¡trogen bases, and they are aligned in
a specific arra-ngement dêpenäent on ihe first st¡and3 pattem. The nitrlgen bases guaniñe and
{tosine
must alwa)¡s line up across from each other. Likewise, àdenine and thymine must añnrays match up.
C
s¡nce the top n¡trogen base on the fißt chain was guanine, the top nitrogen base on the second str¿nd
must be grtosine.- Place ac)^osine on the top deoryri-bose [Figure 4]. F¡n¡slithe second strand by placing
the appropdate nitrogen base on each sugar to cor¡ectbr complement its opposing base.
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F¡gure 4 - Part¡alb/ <ompleted DNA model
D.. Nitrogen base pairs ioin via hydrogen bonding to create the "rungs' which run between the phosPhatssugar'track." Obta¡n e¡ght hydrogen bonding¡lugs, and connect the two strands at their coinplementary nitrogen bases. This model is a portion of a DNA molecule.
3.
The shape of DIVA
A
DNA does not exíst as the stra¡ght ladder you now have ¡n Íiont of you. lnstead, the molecule twists
around an imaginary vertical ax¡s centered on ihe hydrogen bonds. Holding the DNA model at each end,
twist the molecule to make a structure that resembles a spiral staircase.
B.
This structure is called a double helix, and
initructor to observe your model.
Refer to problems
Part
4.
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¡s a
trademark characteristic of DNA strands. Allow your
through 3 on your Questions sheeL
- Replicet¡ng e DNA molecule
Forming an orig¡nàl DNA rnolecule
A.
Disassemble the DNA model formed in
Partl. Construct
a DNA model as shown in Figure 5.
b-adenine-ttrymine-f
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- guanine--<ytostne -
-.ytorin"-iuunin"
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Figure 5 - DNA example for reptication procedure
g. Conn!¿ttat¡nienrafuúpg,'¡ägiv¡¡luql ¡iùþgen base to a sugar piece, and add one phosphate Ube to
that sugar as well. Remenlber that a phosphate-sugar-n¡trogen base's called a nucleotide. You should
have eight nudeotides when ¡ou are ftnished.
5.
Start¡ng the replication process - -unzipping"
A
A DNA molecule beg¡ns to replícate itse.lf by breaking the hydrogen bonds between the nitrogen
bases. Once several consecutive bases have been spliç the DNA strand gives the appearance of being
'unzipped.'
B. Break the bottom three hydrogen bonds of the model you have iust formed, leaving the first connection together.
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6. Replication
A.
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Free nudeotides are able to hydrogen bond
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tothe unzipped DNA sinqle strands, followind the pa¡ring
.*'ln.*e individ, rat model ñircreotioes aõm you, seíection, jò'in a cömpõ
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mentary nudeot¡de to each
side of the bottom three unzipped segmentJ-
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B.
Unzip the final, originat hydrogen bond.
c
complete the replication process by joining the remaining two (2) nucleotide motecules.
Dupl¡c¡ty
A.
Compare the two DNA stands with vour o¡ioinal tigure 51. Was the replication process exact? The
efficiency of this synthesis allows DNA io be pãsed from generat¡on to generat¡on of organisrn with
negligible margin of enor.
Refer to problems 4 ond S on the
euations
sheet below.
Ouestions
1.
Draw a diagram of your DNA molecule. Kèep the illust¡ation in a straight, 'ladder. form,
to draw the double heiix shape.
Figure 5; do not attempt
as
seen in
2.. A partial strand of DNA has the nitrogen base pattern shown below. ln the spaces provided, ¡ndicate
what n¡trogen base would be needed to ¿omplete the DNA"
-
' :-
adenine-
- -...--.--.----:-
- c)¡tosine
-adenine.--.- guanine -- guanine
- thyníne
- adenine
-
--
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3.
The shape of a DNA motecule is called a "double helix.,, Describe this shape in your own words.
4.
Howisa DNAmoteculerepticated?
5.
Heredity ¡nformation is carried in thè DNA of an organism, and gerietic traits are passed from genera-
-
(nrlte
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tion to.generation through reproduction. How does äre Ott repiication process ehsure informãtion is
transm¡tted accurately?
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