Question

when muscles are relaxed, troponin i attaches to the myosin - binding site on the actin filament. this attachment blocks myosin from binding to actin. in response to chemical signals, troponin i changes its position and allows myosin to bind. when myosin attaches to actin, the myocyte contracts.
in people with a heart condition called restrictive cardiomyopathy, troponin i has a reduced ability to attach to the myosin - binding site. which of the following is likely to be observed in the tnni3 gene, troponin i proteins, and myocytes in individuals with restrictive cardiomyopathy?
a. tnni3: normal nucleotide sequence, troponin i: altered structure and function, myocyte: reduced ability to contract
b. tnni3: abnormal nucleotide sequence, troponin i: incorrect amino - acid sequence, myocyte: reduced ability to contract
c. tnni3: normal nucleotide sequence, troponin i: altered structure and function, myocyte: reduced ability to relax
d. tnni3: abnormal nucleotide sequence, troponin i: incorrect amino - acid sequence, myocyte: reduced ability to relax

Explanation:

In restrictive cardiomyopathy, troponin I has a reduced ability to attach to the myosin - binding site. A gene mutation (abnormal nucleotide sequence in the TNNT3 gene) would lead to an incorrect amino - acid sequence in the troponin I protein. Since troponin I is involved in muscle contraction, a defective troponin I would result in a reduced ability of myocytes to contract.

Answer:

B. abnormal nucleotide sequence, incorrect amino acid sequence, reduced ability to contract