TEACHER NOTES: DRY LAB - EVIDENCE OF DNA AS HEREDITARY MATERIAL
Ministry Expectations:
BIG IDEA
DNA contains all the genetic information for any living organism.
OVERALL EXPECTATIONS
D3. Demonstrate an understanding of concepts related to molecular genetics, and how genetic
modification is applied in industry and agriculture
SPECIFIC EXPECTATIONS
A1.1 formulate relevant scientific questions about observed relationships, ideas, problems, or
issues, make informed predictions, and/or formulate educated hypotheses to focus inquiries or
research
A1.8 synthesize, analyse, interpret, and evaluate qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis and whether
it is consistent with scientific theory; identify sources of bias and/or error; and suggest
improvements to the inquiry to reduce the likelihood of error
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions with reference to scientific knowledge
A1.11 communicate ideas, plans, procedures, results, and conclusions orally, in writing, and/or
in electronic presentations, using appropriate language and a variety of formats (e.g., data
tables, laboratory reports, presentations, de- bates, simulations, models)
A2.2 describe the contributions of scientists, including Canadians (e.g., Evelyn Roden Nelson,
Maude Menten, Albert Juan Aguayo, Kimberley J. Fernie, Michael Archer), to the fields under
study
D3.2 compare the structures and functions of RNA and DNA, and explain their roles in the
process of protein synthesis
D3.7 describe, on the basis of research, some of the historical scientific contributions that have
advanced our understanding of molecular genetics (e.g., discoveries made by Frederick Griffith,
Watson and Crick, Hershey and Chase)
Suggestions for Implementation:
The key concept of the dry lab is for students to understand that DNA is the hereditary material
and is responsible for the variation (transforming principle)that exists amongst us. Students are
presented with an experimental protocol carried out by Frederick Griffith and the observations
are presented in a series of steps. It is anticipated that after each observation students will be
able to gain a better understanding of the experimental results and will be able to come to the
conclusion that DNA is the transforming principle.
Structure: The lab should be done in pairs. In order to consolidate the dry lab the following online visual representation of the results gathered by Griffith, Avery and associates should be
shown to the class: http://www.dnaftb.org/17/animation.html.
Time: 60-75 minutes
Assessment: The dry lab can be used as a form of assessment of learning because students will
demonstrate their inquiry and communication skills through their answers which will be
formulated on the representation of their interpretation of the observations. The dry lab is
broken down into thinking and investigation, and application because it requires students to
use critical and creative thinking skills along with inquiry and problem-solving skills. Lastly,
students are required to make use of their knowledge and skills to make connections between
the results and the observations they are based on.
Source:
Giuseppe, M. (2003). Nelson biology 12. Toronto: Nelson Thomson Learning.
Limitations:
Students may have a difficult time grasping the concept because it is quite abstract (not actually
performing the experiment) but trying to make sense of the observations that are given. Timing
wise students may feel a bit rushed, and therefore, the teacher may wish to split the dry lab
over a period of two days. In addition, it is recommended that this dry lab is done after DNA
replication is covered because students will be able to grasp the concept better.
Modifications for Special Student Needs:
1. The dry lab contains a lot of text and the concept is somewhat abstract and students
with IEPs may have a difficult time grasping it, and the images may not be sufficient.
Therefore, a possible suggestion would be to show these students the animation prior
to completing the dry lab in order to provide them with more guided instruction.
2. Students with an IEP can be given extra time to complete the dry lab (come in during
lunch or after school)
Name:______________________________
TI 11
Dry Lab - Evidence of DNA as Hereditary Material
Introduction:
In the 1920s, Frederick Griffith, an English medical officer, conducted experiments with the
bacterium Streptococcus pneumonia, which is known to cause pneumonia. This bacterium
exists in two forms. One of the forms 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
form contains no coating and is known as the R form because it forms rough colonies on a
culture dish.
Task:
Examine the following qualitative observations that were made by Griffith and answer the
discussion, analysis and extend your thinking questions in pairs. Refer back to the legend
(Figure 1) as needed.
Figure 1. Legend of different strains (cells) used.
1) Mouse A was injected with encapsulated cells (S form), whereas mouse B was injected
with un-encapsulated cells (R form).
Observation #1:
Mouse A contracted pneumonia and died, whereas mouse B continued to survive. Mouse B was
sacrificed and an autopsy was done on both mice. The following results were gathered from the
autopsies: mouse A contained living S cells, whereas mouse B contained living R cells.
A6
Figure 2: Mouse A injected with encapsulated cells (S form).
Figure 3: Mouse B injected with un-encapsulated cells (R form).
Discussion Questions:
a) Based on these results, what conclusions can you draw? ( T/I - 2 marks)
b) Why would Griffith consider repeating this experimental procedure on other mice?
(T/I - 2 marks)
2) Encapsulated (S-form) pneumococcal cells were heated, killed and then injected into mouse C.
Observation #2:
Mouse C continued to live. Mouse C was sacrificed and the autopsy showed that no living S cells
were found in the animal’s tissues.
Figure 4: Mouse C injected with heated encapsulated cells (S form).
Discussion Questions:
c) What does this observation indicate? (T/I - 1 mark)
d) Predict what would happen to the mouse if the R form cells had been heated and then
injected. What would this step have represented in the experimental protocol? (T/I - 2
marks)
3. The heated encapsulated (S-form) cells were mixed with un-encapsulated (R-form) cells. The
mixture was grown on a special growth medium. Cells from the culture medium were injected
into mouse D.
Observation #3:
Mouse D died. An autopsy indicated that the mouse had died of pneumonia; encapsulated (Sform) bacteria and un-encapsulated (R-form) bacteria were isolated from the mouse.
Figure 5: Mouse D injected with heated encapsulated (S-form) cells mixed with unencapsulated (R-form) cells.
Discussion Questions:
e) Would you have predicted this observation? Explain why or why not. (T/I - 2 marks)
Analysis Questions:
Part I:
f)
Scientists studied the dead and live cell mixture (from step 3) and discovered cells with
and without capsules. Therefore, what influence did the heat-destroyed cells have on
the un-encapsulated cells? (T/I - 2 mark)
Part II:
Griffith proposed that a chemical in the dead, heat-treated, encapsulated cells (refer to
step 3) must have been altered by the living un-encapsulated cells and he called this scientific
phenomenon transformation.
In 1944, Oswald Avery, Maclyn McCarty and Colin MacLeod carried out experiments in
test tubes with Streptococcus pneumoniae that led them to determine what the transforming
principle was. This was evident when Avery and his associates discovered 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 seen.
g) What do you think these experiments indicated about the transforming principle? (A - 2
marks)
Extend Your Thinking
h) Predict the experimental results of the following protocols. (A- 4 marks)
 Heated encapsulated bacteria are treated with DNAse, a DNA-digesting enzyme. The
treated bacteria are then mixed with encapsulated pneumonia cells, which are injected
into a mouse.
 All proteins are extracted from the heated encapsulated bacteria. The treated bacteria
are then mixed with un-encapsulated pneumonia cells, which are injected into a
mouse.
Dry Lab - Evidence of DNA as Hereditary Material (ANSWER KEY)
a) Based on these results we can conclude that the living R cells inside of the mouse does not
affect their health while living S cells can kill the mouse.
b) Experiments are always repeated to make sure that they are consistent. This is important
since there could be other reasons that the mice might have died, such as being infected by a
harmful virus. Repeating the experiment several times and obtaining the same results confirms
that the reason that these mice are dying is in fact due to the presence of live S bacteria cells
inside of them.
c) This indicates that the heat killed the S-form cells, and confirmed that the S-cells were
responsible for killing the mice in step 1 of the experiment. In addition, heat killed whatever
was responsible for killing Mouse B in step 1. Also, it indicates that the heat broke down the
polysaccharide coating.
d) If the R-form cells were heated and then injected the mouse would still have survived
because the R cells are harmless regardless, and do not affect the mice. In addition, no living R
cells would be present in the animal’s tissues. Furthermore, this serves as a control in the
experimental protocol because it proves that it’s not the R cells that are resulting in the mice
surviving but rather the fact that they will survive with or without the presence of the R cells.
e) This is an opinionated question and answers will vary.
f) Since the S cells were all heat killed, one would not expect to see any capsules formed. The
fact that there were however, could indicate that the live R cells started producing these
capsules using the information from the surrounding dead S cells.
g) These experiments indicate that DNA is the necessary component needed to transform the R
cells into S cells because the only way R cells were transformed into S cells was when purified
DNA was mixed with the R cells. In all other cases (RNA, proteins and polysaccharide coat) only
R cells were extracted indicating that they were not transformed. Therefore, the transforming
principle has to be DNA.
h)
 The mouse will survive because the DNA is digested by the DNAse, therefore, the R cells
from the un-encapsulated pneumonia cells will not be transformed into S cells because
DNA is the transforming principle.
 The mouse will not survive because the heated encapsulated cells still contain DNA
which means that the R cells from the un-encapsulated cells will be transformed to S
cells and the mouse will end up dying.