An endocrine axis describes:
A. the direction of hormone transport within the body.
B. the signal transduction cascade that occurs when a hormone binds to a receptor.
C. the molecule that forms the backbone of a hormone.
D. a hormonal signaling pathway between endocrine glands and tissues.
E. the pattern of genes that are expressed in response to a hormone.

D. A hormonal signaling pathway between endocrine glands and tissues

Explanation:

Our body is formed by a complex organization of various tissue, organs and organ-systems. It has specific mechanisms to regulate the functions of all its cell, tissue and organs. Besides neural system, the endocrine system also plays a crucial role in the regulation of body functions. This system consists of many pathways that originate from the hormone secreting glands called endocrine glands and reaches to the target cell or tissue or organ. These pathways are collectively named as the endocrine axis.

before it leaves the nucleus of a eukaryotic cell, mrna is altered in the following ways: - a cap is added to the 5' end of the mrna molecule; - a poly a tail is added to the 3' end; - introns are removed and exons are spliced together.