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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 e nds 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
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
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.)
GCA, GCG, GCC, GCU
CGA, CGG, CGC, CGU, AGA, AGG
GM, GAG '
GGC, GGU, GGA, GGG
AUC, AUU, AUA
CGA, GGG, GGG, GGU
UGA, UGG, UCC, UCU, AGU, AGG
AGA, AGG, ACC, ACU
GUA, GUG, GUC, GUU
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
DNA Model Kit
Student Name_ _ _ _ _ _ _ _~_,Class _ _ _ _ __
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
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
In this investigation, you will examine the structure of DNA by building.
your own model.
12 ...:. 3 prong "deoxyribose" centers (black)
12 - 2 prong · phosphate~ centers (red)
3 - "Adenine 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)
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
2. Construct a backbone by attaching a deoxyribose (black center) to a
phosplbilte (red center) with a yellow straw (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
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
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).
8. On a separate piece of paper, answer questions one through six in the
Discussion .Question section of this guide.
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:
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
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
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
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.
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.
What is the name of the spccific molecule to whieh each nucleotide
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?
10. Based upon this infonnation what are the messenger RNA molecuJes
that the left side of your DNA molecule would construct? 1b.e right
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.
Proline- - - -'- ---
' ------- -----Phenylalanine
----- GUU--- - -
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
15. What is the function ofmRNA? tRNA?
16. Itow can a mutation occut1