Acetylcholine binds to a GPCR on heart muscle, making the heart beat more slowly. The activated receptor stimulates a G protein, which opens a K channel in the plasma membrane. Which of the following would enhance this effect of the acetylcholine

(a) addition of a high concentration of a non-hydrolyzable analog of GTP

(b) addition of a drug that prevents the α subunit from exchanging GDP for GTP

(c) mutations in the acetylcholine receptor that weaken the interaction between the receptor and acetylcholine

(d) mutations in the acetylcholine receptor that weaken the interaction between the receptor and the G protein

The correct answer is option (b) "addition of a drug that prevents the α subunit from exchanging GDP for GTP".

Explanation:

The G protein is a name given to a family of proteins that act as a switch in diverse cellular processes, transmitting signals from outside to inside of the cells. When a G protein receptor is activated by a neurotransmitter such as dopamine, the alpha subunit exchanges its GDP for a GTP and disassociates G-protein itself. An addition of a drug that prevents this action would prevent  G protein disassociation, which would enhance the effect of stimulating G protein that acetylcholine does.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.