You are interested in studying a gene called CFTR because mutations in this gene in humans cause cystic fibrosis. You have made a line of mice that lack the mouse CFTR gene. These mice are unable to clear bacteria from their lungs, so they get lung disease. You put a normal human CFTR gene into some of these mice and discover that the mice with the human gene are able to clear bacteria from their lungs and no longer get lung disease. From this experiment, you can conclude that: ___

A) The DNA sequences of the mouse CFTR gene and human CFTR gene are identical.
B) The amino acid sequences of the mouse CFTR protein and the human CFTR protein are identical.
C) The mouse CFTR gene and human CFTR gene encode proteins that can serve a similar function.
D) Both answers B and C are true.
E) All of the above are true.

CFTR gene encodes for Cystic fibrosis transmembrane conductance regulator protein. It is ion channel protein and conducts chloride ions across the cell membrane. Its mutation leads to cystic fibrosis which is characterised by presence of thickened mucus in lungs with frequent infections.

In the given experiment, human CFTR gene is able to function in mouse and restore the lost body functions. This is because human and mouse CFTR gene show high level of similarity due to which they code similar proteins which can function in each other's place. However this does not mean that DNA sequence of the genes is identical. There is still some difference between them because they are two different species.

In reality human CFTR gene is around 189 kb in length whereas mouse CFTR gene is around 152 kb in length. The 27 exons have very similar sequence. Most introns of human CFTR gene are larger than the mouse counterpart.