Download Activity 2: The elegant experiment of Matthew Meselson

Class Notes: DNA Structure
Activity 1: Practice
Close your notes and do your best to complete the following. On the image below:
a. Label the 5’ and 3’ end
b. The strand is 5’ TACG 3’. Label this strand appropriately.
c. What will be the 3’ to 5’ order of the opposite strand? Write those bases below the bases shown.
c. Indicate the location of phosphodiester and hydrogen bonds
d. Draw an arrow to indicate the direction of DNA replication.
e. If this is a new strand, circle the location of the next nucleotide binding site. Hint – are new
nucleotides added to the 3’ or 5’ end?
Activity Wrapup:
A segment of DNA is 20% guanine. Which must also be true?
a. It is 30% adenine
b. It is 80% cytosine
c. It is 60% purines
d. It is 60% pyrimidines
Activity 2: The elegant experiment of Matthew Meselson and Franklin Stahl
Unknown: Is DNA copied conservatively or semiconservatively?
A. Draw the difference between conservative replication and semiconservative DNA replication.
Use two different colors to indicate the initial DNA and the copied strands.
Conservative
Semiconservative
(old double-strand of DNA remains, new
(Two new double-strands, each contain one
copied double-strand is completely new)
old strand and one new copied strand)
Experimental design: Meselson and Stahl used bacteria as the organism. First, they grew the
bacteria for several generations in a heavy isotope of nitrogen 15N (this has eight neutrons
instead of the normal seven).
B. Label the nitrogen atoms in the image on the first page with asterisks.
Then they moved the bacteria for a single generation
to a growth mixture that contained a lighter isotope
(14N).
After the bacteria were allowed to go through a single cell cycle, they were removed from the
medium and placed in a centrifuge tube containing a salt gradient. When spun in a centrifuge,
more dense bacteria move farther down the salt gradient than less dense bacteria.
C. There were two control tubes – one with all 14N bacteria and one with all 15N bacteria.
Label which control tube is which, and explain why:
D. Which of the tubes below represents the results after one round of replication if
semiconservative replication is occurring? Circle one and explain why.
Controls
Possible results: A
B
E. The image below is the photograph of the bands in Generation 0, 1, 2 and 5. Be able to
explain these results, including the label, position and thickness of each band.
Indicate which are true:
A. DNA replication requires the breaking of phosphodiester bonds
B. Energy for building a new strand comes from a hydrolysis of phosphates from a nucleotide
C. The number of pyrimidines and purines in DNA is equal
D. The number of AT pairs and CG pairs in DNA is equal
E. A researcher who wanted to radioactively label DNA should use radioactive sulfur rather than
radioactive phosphorus