Lipid-soluble signaling molecules, such as testosterone, cross the membranes of all cells but affect only target cells because .

This question lacks options, the options are:

A) only target cells retain the appropriate DNA segments

B) intracellular receptors are present only in target cells

C) only target cells possess the cytosolic enzymes that transduce the testosterone

D) only in target cells is testosterone able to initiate the phosphorylation cascade leading to activated transcription factor

The correct answer is B.

Explanation:

Receptors are proteins that can be found in the nucleus, cytoplasm, or cytoplasmic membranes. Receptors bind to a ligand with high specificity and high affinity. In some fat-soluble signal molecules, they bind to specific protein receptors located on the surface of the target cells they affect. These surface protein receptors bind the signal molecule (or ligand) with a high affinity, and transform this extracellular event into one or more intracellular signals, which alter the behavior of the target cell. Testosterone needs to be transported in the blood through proteins, which is why only 3% is in free form, while the remaining 97% is bound to proteins, among which the sex hormone transporter globulin and the albumin, it enters cells very easily and binds to androgen receptors (intracellular receptors generally found in the cytoplasm), significantly stimulating protein synthesis, among many other processes.

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.