Name: __________________________________________________ Date: ______________ Period: _______
DNA History and Model Activity
In the 1950’s, the race was on. At this time in history, we were unsure of how DNA was constructed. Three teams of scientists
were trying to unravel the molecular structure of DNA. Team 1 was a team at CalTech in Pasadena lead by Linus Pauling.
Team 2 included an American named James Watson and an English scientist named Francis Crick, who were working
together at Cambridge University in England. Team 3 was lead by an English female scientist named Rosalind Franklin, who
was working at King’s College in England.
Linus Pauling
James Watson and Francis Crick
Rosalind Franklin
Deoxyribonucleic acid (DNA) is a large polymer made from smaller units called nucleotides.
From the picture to the right, you can see that every nucleotide is made from three smaller
parts: (1) phosphate molecule, (2) sugar molecule, (3) nitrogen base. When combined, these
three parts make a single nucleotide. When nucleotides are combined, they create a larger
molecule called a nucleic acid. DNA is one type of nucleic acid. You can see a nucleic acid below
made from 4 nucleotides. The picture below is very simplistic because in real life it is common
for DNA to be made from millions of nucleotides bonded together. This simple model below is
only 4 nucleotides in size.
1.
Cut out strip 1, strip 2, strip 3, and strip 4.
2.
Tape strip 2 to the bottom of strip 1, thus making a longer strip.
3.
Tape strip 4 to the bottom of strip 3, thus making another longer strip.
Prior to the race in the 1950’s, scientists were unsure whether DNA was a double helix
or a triple helix. This is because the technology in the 1950’s was not as advanced as it is
today and it made obtaining pictures of DNA very difficult. Rosalind Franklin’s team at
King’s College was producing the best images of DNA during this race to discover the
structure of DNA.
4.
Cut out the circle shaped “phosphate” molecules and tape/glue them inside the dotted
circles of your two strips.
Prior to the DNA race, a scientist named Erwin Chargaff noticed an important
observation when studying DNA. Chargaff noticed that the amount of the nitrogen base
called “adenine” was always equal to the amount of the nitrogen base called “thymine.”
He also discovered that the amount of the nitrogen base called “guanine” was always equal to the amount of the nitrogen base
called “cytosine.” For a while, no one knew why that was.
5.
Cut out the pentagon shaped “sugar” molecules and tape/glue them inside the dotted pentagons of your two strips.
The answer was finally discovered by team 2, James Watson and Francis Crick. As mentioned before, Rosalind Franklin was
producing the best images of DNA with the help of X-Ray technology. But she was working at a rival college of Watson and
Crick so they didn’t share their work. Without Franklin’s knowledge, some of the photographs that Franklin had taken
wound up in the hands of James Watson. Together with Crick, the photos would be the clue they needed to identify the
structure of DNA as a double helix. Erwin Chargaff’s observations now made sense. The reason the amount of adenine (A)
was equal to thymine (T) was simple. They were connected/bonded together! If there was a “T” on the left side of the DNA
molecule, then an “A” would be on the right side. The same was true of guanine (G) and cytosine (C). The following are known
as Chargaff’s Rules:
Rule 1: Adenine always pairs with Thymine.
Rule 2: Cytosine always pairs with Guanine.
6.
On the combined strip 1 and strip 2 of paper….
a. cut out and then tape/glue the nitrogen base “A” onto the X of the first sugar. Once taped/glued, the “A” should be
sticking out to the right of the sugar molecule.
b. cut out and then tape/glue the nitrogen base “G” onto the X of the second sugar.
c. cut out and then tape/glue the nitrogen base “A” onto the X of the third sugar.
d. cut out and then tape/glue the nitrogen base “T” onto the X of the fourth sugar.
e. cut out and then tape/glue the nitrogen base “T” onto the X of the fifth sugar.
f. cut out and then tape/glue the nitrogen base “C” onto the X of the sixth sugar.
7.
On the combined strip 3 and strip 4 of paper….
a. cut out and then tape/glue a blank nitrogen base onto the X of all six sugars. Once taped/glued, the blank box should
be sticking out to the left of the sugar molecules.
On the day of their discovery on February 28, 1953, Watson and Crick walked into the Eagle Pub, which was located across
the street from their laboratory, and interrupted the other customers who were eating lunch. In that pub, they proclaimed
“We discovered the secret of life!” Shortly after that, their research paper was printed in the journal Nature.
In 1962, James Watson and Francis Crick were awarded the Nobel Prize in Medicine. Sadly, Rosalind Franklin had passed
away in 1958 due to cancer, which many believed she obtained during to her experiments with X-Ray technology as she took
photos of DNA and other molecules. Had she survived, she likely would have been awarded the Nobel Prize as well since her
work help Watson and Crick uncover the mystery of DNA.
8.
Line up the nitrogen bases (the boxes) of combined strip 1 and 2 to the nitrogen bases (boxes) of combined strip 3 and 4.
Tape/glue the nitrogen bases together. This should create one large molecule of DNA, a double helix.
9.
Follow Chargaff’s rules and handwrite the missing nitrogen bases into the blank nitrogen base boxes.
Analysis Questions:
A) In which country were Watson, Crick, and Franklin working? __________________________________________________
B) What is the monomer of DNA called? _____________________________________________________________________
C) The shape of DNA is often called a ladder. Look at your final model.
I. Which molecules make up the outside of a DNA ladder? _________________________________________________
II. Which molecules make up the inside of the DNA ladder? _________________________________________________
D) What is a combination of one sugar, phosphate, nitrogen base called? ____________________________________________
E) How many different types of nitrogen bases does DNA have? ___________________________________________________
F) The nitrogen base of a DNA nucleotide is only connected to the sugar/phosphate molecule. (circle the answer)
G) What type of energy was Rosalind Franklin working with? _____________________________________________________
H) A pairs with T … C pairs with G … are rules named after which scientist? ________________________________________
I)
Where did Watson and Crick first announce their discovery? ___________________________________________________
J)
Using a colored marker/highlighter/crayon, color one single nucleotide on your paper DNA model.
K) How many nucleotides are in your finished DNA model? ______________________________________________________