Science Assessment

Observation: Scientists introduce a copy of the GFP (green fluorescent protein) gene into the DNA of a frog embryo to make a frog that glows green. They notice that only the frog’s muscles but none of the other cells glow green.

Question: Why do only the muscles of the frog glow green?

_{Left: A genetically engineered frog with the GFP gene. Right: The same frog under a light that makes the GFP glow. Image from Lim et al. (2004)}

Claim: Only the muscles glow green because the GFP gene is turned on only in muscle cells and is turned off in all the other cell types.

Evidence: Several of the frog’s different cell types were examined for the presence of the GFP gene and GFP protein. The GFP gene was found in every cell type tested, but GFP protein was detected only in muscle cells. Which of the following statements provides the best reasoning to justify why the evidence supports the claim?

1. The GFP gene is found in every cell because multicellular organisms develop from a single cell. Therefore, in multicellular organisms all of an individual’s cells have the same DNA.
2. The same genes are present in all cell types of an organism, but not all genes are turned on in those different cell types. Different cell types turn on different genes. This means that, if GFP protein is found only in muscle cells, it is because the GFP gene is turned on only in those cells.
3. Not all cells are identical. There are different cell types that are able to perform different functions in the body.
4. Genetic switches are segments of DNA that respond to cellular signals to turn genes on or off.