Download Dry Lab Exercise 4.1.1. - Evidence of Hereditary Material

SBI4U–MolecularGeneticsUnit
Date:_________________________
DryLabExercise4.1.1:EvidenceofHereditaryMaterial-ANSWERS
Background Information
In the 1920s, Frederick Griffith, an English medical officer,
started experimenting with Streptococcus pneumoniae. This
bacterium, which causes pneumonia, exists in two forms.
One form is surrounded by a polysaccharide coating called a
capsule and is known as the S form because it forms smooth
colonies on a culture dish. The second harmless form has no
coating and is known as the R form because it forms rough
colonies on a culture dish (Figure 1).
Figure 1
The following is an abbreviated summary of Griffith’s procedures and results:
1. Mouse A was injected with encapsulated cells (S form), while mouse B was injected with unencapsulated cells
(R form).
Observation
Mouse A contracted pneumonia and died, while mouse B continued to live. Mouse B was sacrificed, and an autopsy
was conducted on both mice. The autopsies revealed living S cells in mouse A’s tissues and living R cells in mouse
B’s tissues.
(a) What conclusions can you derive from the experimental results?
The S form of Streptococcus pneumoniae (encapsuled cells) is virulent. Mouse A contracted
Streptococcus pneumoniae and died.
(b) Why might a scientist decide to repeat this experimental procedure on other mice?
A scientist might repeat this experiment with other mice to ensure that the experimental
results and conclusions were valid. Repetition ensures that Mouse A did in fact die of
pneumonia from Streptococcus pneumoniae and that the unencapsulated (R form) of
Streptococcus pneumoniae, given to Mouse B who survived, is harmless.
2. Encapsulated (S-form) pneumococcal cells were heated, killed, and then injected into mouse C (Figure 2).
Observation
Mouse C continued to live. Mouse C was sacrificed and the autopsy revealed that no living S cells were found in the
animal’s tissues.
(c) What is the significance of this result?
The significance of this result is that the encapsulated (S form) of Streptococcus pneumoniae
could be killed, and therefore inactivated, using heat. Also, since no encapsulated cells
were found in the tissue of the mouse, the capsulated cells had no means of rendering
themselves active.
(d) Predict what would have happened to the mouse if the unencapsulated (R-form) cells had been heated and
then injected. What would this step have represented in the experimental protocol?
If unencapsulated (R form) cells were heated and then injected into a mouse, the mouse would
live. The unencapsulated cells would have been killed. This represents the control. The
purpose of the control is to illustrate that the heating of the encapsulated cells is the cause
of the previous mouse living and not something to do with how the unencapsulated form
functions. This conclusion is corroborated by the observations that no encapsulated or
unencapsulated cells were found in any of the tissues of the mice.
3. The heated encapsulated (S-form) cells were mixed with unencapsulated (R-form) cells. The mixture was
grown on a special growth medium. Cells from the culture medium were injected into mouse D (Figure 2).
Figure 2
Observation
Mouse D died. An autopsy indicated that the mouse had died of pneumonia; encapsulated (S-form) bacteria and
unencapsulated (R-form) bacteria were isolated from the mouse.
(e) Would you have predicted this observation? Explain why or why not.
(e) Answers will vary. Some may predict that the mouse would have lived since the heat
should have killed the encapsulated form of cells. Others may suspect that the mouse
would have died because the DNA may have been transferred from the treated
encapsulated form to the unencapsulated form.
Analysis
(f) A microscopic examination of the dead and live cell mixture (step 3) revealed cells with and without capsules.
What influence did the heat-destroyed cells have on the unencapsulated cells?
(f) The heat-destroyed cells had the ability to transform some of the unencapsulated cells
into the virulent encapsulated form.
(g) Griffith hypothesized that a chemical in the dead, heat-treated, encapsulated cells (step 3) must have altered
the living unencapsulated cells and he dubbed this chemical phenomenon transformation. In 1944, Oswald
Avery, Maclyn McCarty, and Colin MacLeod conducted experiments in test tubes with Streptococcus
pneumoniae that led them to conclude that DNA is the transforming principle, as they called it, and not proteins,
as was widely believed. In their experiments, what must have happened to the DNA when the cells divided?
(g) When the cells divided, the DNA must have been incorporated into the unencapsulated
cells changing them from harmless to virulent cells.
Extension Question
(h) To discover the identity of the transforming principle, Avery and his associates ruptured heat-killed,
encapsulated cells to release their contents. RNA, DNA, protein, and purified polysaccharide coats were
isolated and were tested for transforming activity. Avery and his associates found that only R cells mixed with
purified DNA isolated from dead S cells were transformed to S cells. When R cells were mixed with purified
RNA, with the polysaccharide coat, or with protein extracted from dead S cells, only R cell colonies were
isolated. Do these results support their hypothesis? Explain.
(h) The results do support Avery, McCarty, and MacLeod’s hypothesis. They hypothesized
that DNA was the transforming principle. Unencapsulated cells were transformed to
encapsulated cells when mixed with purified DNA isolated from encapsulated cells. RNA,
protein, and the polysaccharide coat were not able to transform unencapsulated cells into
encapsulated cells, illustrating that DNA was the transforming principle.
(i) Predict the experimental results of the following protocols. Support your
prediction with a hypotheses.
•
Polysaccharide-digesting enzymes are used to digest the encapsulated polysaccharide coat of the heated S form of
the bacteria. The treated bacteria are then placed with unencapsulated pneumonia cells, which are then injected
into a mouse.
•
Heated encapsulated bacteria are treated with DNAase, a DNA-digesting enzyme. The treated bacteria are then
mixed with unencapsulated
pneumonia cells, which are injected into a mouse.
•
All proteins are extracted from the heated encapsulated bacteria. The treated bacteria are then mixed with
unencapsulated pneumonia cells, which are injected into a mouse.
(j) Based on the information provided, suggest improvements to the experimental protocols.