DBQ
DP Biology
Is DNA the Hereditary Material?
A virus is a non-living particle composed of a piece of DNA enclosed by a
protein coat. Once attached to the surface of a cell by its tail fibers, the
virus contracts its sheath and injects its DNA into the cell, leaving the
protein coat behind.
Protein coat
Collar
Contractile Sheath
Tail fibers
An experiment was conducted to determine whether it was protein or DNA
which was the hereditary material.
The experimenters, known as Hershey and Chase, grew some viruses in
nutrient medium with radioactive phosphorus. They grew another set of
viruses in nutrient medium with radioactive sulfur. Hershey and Chase
knew that DNA contained phosphorus, but protein did not, while protein
contained sulfur which DNA did not. In other words, they had two batches
of viruses, one batch with radioactive DNA (Batch A) and another batch
with radioactive protein (Batch B).
Separate cultures of the bacterium Escherichia coli were infected with
either the A or B type viruses. After sufficient time for DNA injection had
passed, the empty phage “ghosts” were separated from the bacterial cells,
and the two parts were tested for radioactivity.
In a second experiment, the phages were allowed to complete their life
cycle and the offspring were tested for radioactivity. The results are
presented below:
Phage Type
Bacterial Fraction
Phage Fraction
Phage Offspring
A (32P)
radioactive
non-radioactive
radioactive
B (35S)
non-radioactive
radioactive
non-radioactive
1) What do these results suggest happened to the DNA in “type A”
viruses?
2) Why were the offspring of “Type A” phages radioactive while the
offspring of “Type B” viruses were not?
3) If protein and not DNA were the information carrier in viruses, predict
the results Hershey and Chase would have obtained.
The life cycle of a virus is given below. By the way, Alfred Hershey and
Martha Chase performed their experiment in 1952.