Name:
The Structure of DNA
06/08/11
Students will turn in:
1. Assignment 1: DNA Worksheet
2. Assignment 2: Poster – Draw a poster of the ladder structure of DNA, labeled.
3. Assignment 3: The completed DNA model (Use the materials in the supplies basket obtained at
the Course Orientation & tape small pieces of paper to label the model).
Introduction
Human chromosomes are structures composed of two long strands of deoxyribonucleic acid
(DNA) combined with proteins. In 1953, two scientists, James D. Watson and Francis H. Crick,
proposed a model for the structure of DNA. They described the molecule as a double helix (two
spiraling strands), composed of nucleotides. Each nucleotide is composed of a phosphate unit
(PO4) joined to deoxyribose (the five-carbon sugar) along with a nitrogen-containing base. Each
deoxyribose sugar has 5 carbon atoms, and each of the carbons has its own number (see figure
below).
One Nucleotide
#5C
#4C
#3C
#1C
OH
#2C
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Name:
Assignment 1:
DNA Structure Worksheet
Read, label, and answer all of the questions below.
I. Nucleotides: DNA is composed of building blocks called nucleotides
The carbon with
the asterist in
the figure of the
5-C sugar to the
right.
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Below:
1. Write „N-base‟ in the correct position at the end of the correct bond (line).
2. Write „PO4‟ in the correct position at the end of the correct bond (line).
3. Write „OH‟ in the correct position at the end of the correct bond (line).
II. DNA is Composed of Two Polynucleotide Chains
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1. Write “Free 3‟ end” and “Free 5‟ end” at the appropriate ends of the chain below.
2. Cross off all of the #3 C „OH‟ groups- only the „OH‟ groups. Don‟t‟ cross off the
C‟s or the phosphates. Draw a line from the #3 C‟s to the phosphate groups of the
adjacent nucleotides.
3. Note: The OH group bonded to the #3 C in deoxyribose is not removed until the
nucleotide bonds to another nucleotide below it. Therefore, do not cross off the Free 3‟
OH group (since it is not bonded to another nucleotide on that end).
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III. Antiparallel Arrangement of the two Chains
Label the DNA molecule as instructed on the next page.
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1. Number the carbons in all of the deoxyribose sugars.
2. Draw bonds between the sugars and phosphates of adjacent nucleotides (cross off the „OH‟
groups between nucleotides bonded to each other- NOT at the Free 3‟ Ends).
3. Draw the weak bonds between the nitrogen bases (base pairs) as several „dots‟ between them.
4. Identify the Free 5‟end and the Free 3‟ ends of both chains. This will indicate the antiparallel
nature of the two chains with respect to one another.
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ASSIGNMENT 2: Poster
A computer generated illustration is not allowed. You must DRAW the ladder YOURSELF.
Use a small poster to DRAW the structure of DNA in the form of a "ladder". Draw the "ladder"
exactly as illustrated below. The extended ends of each long line represent the Free 5‟ End of
that strand. The ends of the long lines where the line is not extended out is the Free 3‟ end of
each strand. Remember, the strands are antiparallel (upside-down) with respect to each other.
Clearly and completely draw and label the following in your ladder structure. Be sure all of
these words are on your poster:
1. Strong bonds and Weak bonds
2. Deoxyribose
3. Phosphate group (PO4)
4. Base pair
5. Adenine, Cytosine, Guanine, Thymine (Be specific in your drawing)
6. Circle and label one Nucleotide (Be very specific in what you are including in the
nucleotide!).
7. Finally, indicate the Antiparallel nature of the chains by labeling all of the Free 3' and Free 5'
ends (Ends 'sticking out' are the free 5' phosphates). Be sure you have the correct molecules at
the 3' and 5' ends!
Include a legend (key) if using abbreviations.
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ASSIGNMENT 3: DNA Model
Materials and instructions for this lab were adapted from Hubbard Scientific, Inc., DNA Model.
Procedure: First, tally the materials. You must report any missing parts no later than the week
before the assignment is due. The set of parts that you received at the orientation may contain
extra parts. Don't feel that all parts must be used if there are extra. Please return any extra parts
with the completed model.
Do not write directly on the pieces.
Use tape and small pieces of paper to label the model and don't forget your name!
Materials: (Do not use glue or adhesive)
12 - Three prong "deoxyribose centers" (black)
12 - Two prong "phosphates" (red)
22 - Yellow connectors (Strong bonds between the sugar sand phosphates)
3 - Adenine straws (short red)
3 - Guanine straws (short gray)
3- Thymine straws (short blue)
3 - Cytosine straws (short green)
6 - Hydrogen bond-centers (white)
1 - Long gray stand
1 - Four prong center (gray or black)
3 - Medium green straws
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Construct the Model
1. Construct the nucleotides by attaching each deoxyribose (black center) to a phosphate (red
center) with a yellow connector. The prongs (of the phosphates and sugars) should be inserted
into the center of the yellow connectors). Then attach one nitrogen base (red, green, blue, and
gray straws) to each deoxyribose as shown below. Construct 12 nucleotides.
Nucleotide
Yellow Connector
Black
Red, Blue, Gray or Green
Red
2. Assemble one strand of connected nucleotides, consisting of six nucleotides, by connecting the
phosphate (red) of one nucleotide to the deoxyribose sugar (black) of the adjacent nucleotide
using the remaining yellow connectors. Twist the connectors as needed.
See the Figure below. From top to bottom, put the nitrogen bases on one strand in the following
order: adenine, thymine, cytosine, guanine, adenine, cytosine. (ATCGAC)
One connected strand of nucleotides.
3. Repeat the above procedure to build the second strand. Be sure to place the complementary
nitrogen bases in the correct position.
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4. Connect the base pairs using the white hydrogen bonds. See Figure below.
5. Allow the DNA model to remain upright by passing the long gray straw through the holes in
the white hydrogen centers. Attach the three long green straws to the four prong center. Attach
the remaining prong to the bottom of the long gray straw to serve as the stand.
6. Twist the DNA structure gently to form a double helix spiral. With paper and tape (don‟t write
on the model) label the model with your name and BIOL 117HY.
7. ONE MORE THING! Be sure there is a free 3‟ (sugar only) and free 5‟ end (sugar attached to
a phosphate) on both DNA strands ON THE MODEL using the model pieces. Then label all of
the free 3‟ and free 5' ends on the model with tape and a piece of paper.
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