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DNA Structure
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James Watson and Francis'Crick made some important discoveries in the studv of DNA. There were other
scientists whose work was instrumental in understanding the structure of DNA, and Watson and Cricx used careful
logic to integrate evidence from several experiments to unravel the mystery of the structure of DNA.
Purpose: The purpose of this investigation is to show how a double strand of DNA is constructed following the
basic logic used by Watson and Crick.
Preparation: Cut out the nucieotide models found on the separate sheet of colored paper. You will notice there
are two copies of each of the four 4 types of nucleotides, adenine, cytosine, guanine, and thymine. Use the dotted
lines as your cutting guide.
Decide how a single strand of DNA might be constructed. The pentagon-shaped structure on your cut-outs is a 5carbon sugar called "deoxyribose". The small circular structure on the model represents a "phosphate". A bond
forms between the phosphate of one nucleotide and the sugar of the next nucieotide.
1. Place 4 of the nucleotides (one set) in a vertical column and determine where the bonds must go.
2. You will now form a SINGLE strand of DNA by taping or gluing these 4 nucleotides together.
3. Draw links between the phosphate of one nucieotide and the sugar of the next to indicate a bond.
4. You now have a model of a chain of four different nucieoltdes heid together by phosphate-sugar bonds.
Questions:
e
On which side of the model are the bases?
•
Why should all of the sugars in the chain be turned in the same direction?
The data indicated to Watson and Crick thai DNA is made of a "double strand" that has a uniform diameter.
Arrange the remaining 4 nucieotides in a column parallel to the first chain you made, but DO NOT glue or tape
them in place.
»
How can the models be arranged so that the width of the total structure is always the same?
»
Record your observations below about how the nucleotides might "fit" together. What characteristics of
the models determine your answers to this question?
»
Use the same argument to explain why certain nucleotides do NOT "fit" together.
So far you have found that some bases fit together and others do not. You know what some possibilities, far "fit"
are, but you have not determined what the case is in the actual DNA moieeuie. From observations of shape and
size ainnc, you are confident that adenine could pair with either
or
. Does it
pair with one, the other or both in a real DNA molecule?
Questions:
*
If adenine always pairs with cytastne, the amount of adanine in a DNA motecule shauk! aixvsys be
to the amount of cytosine.
*
if adnnino always pairs with thymino, the amount of adenine should always be
to the
amount of thymine.
*
!f adenine pairs with cytosine haif the time arid rhymins half the time, tfiert- .shnukl bs- __
times
as much .idonine as there is cylosine and uiyrnins in a DNA molecule.
Here is where the work or another scientist, Erwin Chargaff, was enormously important. Chargaff, and American
biochemist, showed that DNA in human liver cells is 30.3% adenine, 19.5% guanine, 30.3% thymine and 19.9%
cytosine.
•
Review the possible pairing described in the three questions above. Which seems most likely in iight of
Chargaff's data?
•
What does the data suggest about which nucleotide pairs with guanine? Explain your answer.
Now consider more of the Chargaff data. The following table shows the percentages of the four bases found in
DNA from different kinds of cells.
Percentages of Four Bases Found in DNA of Different Cells
Adenine
Thymine
Guanine
Cytosine
Thymus Cells
Human
Sheep
Pig
Spleen Cells
Human
Sheep
'
Pig
Liver Cells
Human
Sheep
Pig
Sperm Cells
Human
Sheep
Yeast
30.9
29.3
30.9
19.9
21.4
19.9
29.4
28.3
29.4
19.8
21.0
19.8
29.2
28.0
29.6
21.0
22.3
20.4
29.4
28.6
29.2
20.4
21.1
20.8
30.3
29.3
29.4
19.5
20.7
20.5
30.3
29.2
29.7
19.9
20.8
20.5
30.7
26.8
31.3
19.3
22.0
18.7
31.2
27.2
32.9
18.8
21.0
17.1
Questions:
•
Look closely at the data for the four different kinds of human cells. What do you notice about the percentages of any
single base in the different kinds of cells?
•
Which bases pair together in human DNA? Use examples from Chargaff s data to explain your answers.
•
Position your models for your second DNA strand and tape the entire model together forming a double strtiad that is
equal in width with vertical parallel nucleotide bases.
Chargaff s data supported the idea-that adenine always pairs with thymine and that cytosine always pairs with guanine. These
molecules form weak chemical bonds called hydrogen bonds.
•
Adenine and thymine always form two hydrogen bonds between them.
•
Cytosine and guanine always form three hydrogen bonds between them (cee + gee = three)
Draw the correct number of hydrogen bonds between the bases in your Double-stranded DNA molecule.
Congratulate your partner and yourself fora job well done!