A student repeats the work of Griffith with the S and R strains of Streptococcus pneumoniae…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student repeats the work of Griffith with the S and R strains of Streptococcus pneumoniae. For each injection, say whether the mouse lives or dies, and whether live S cells can be found afterwards.
- Live R cells only.
- Heat-killed S cells only.
- Heat-killed S cells mixed with live R cells.
- Heat-killed R cells mixed with live R cells.
- An extract of heat-killed S cells, treated with DNase, then mixed with live R cells.
Need a reminder? The key ideaShow
Griffith found that heat-killed S cells change live R cells into S cells. Avery, MacLeod and McCarty found that only DNase stops this change. Hershey and Chase tagged DNA with 32P and protein with 35S. The 32P went into the bacteria, and the 35S stayed outside.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) R cells have no capsule and are harmless. The mouse lives, and no S cells appear.
Step 2. (b) Dead S cells cannot cause the disease. The mouse lives, and no S cells appear.
Step 3. (c) The transforming principle of the dead S cells changes some R cells into S cells. The mouse dies, and live S cells are found.
Step 4. (d) There is no S strain at all, so nothing can transform the R cells. The mouse lives, and no S cells appear.
Step 5. (e) DNase destroys the DNA, which is the transforming principle. The R cells stay R cells. The mouse lives, and no S cells appear.
Answer: only (c) kills the mouse, and only in (c) are live S cells found.
Check. The mouse dies only where the S cells, or the DNA to make them, are present with living cells.
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