Hormones

Folie 1

• Hormones are chemical messengers, released directly into the blood from endocrine glands. It's important that the cells in the endocrine glands that make hormones are not themselves affected by their products.
• For this reason, most hormones are produced either in an inactive form or packaged within secretory vesicles by the Golgi apparatus. The vesicles fuse with the cell surface membrane, releasing their contents by exocytosis.
• Each hormone affects only specific target cells, modifying their activity. They either enter the target cells or bind to the complementary receptor molecules on the outside of the cell membranes.

Folie 2

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• Most hormones fit into one of two main classes: Peptide hormones & Steroid hormones, according to their chemical structure & their mode of action.
• Peptide hormones consists of small protein molecules. They cannot pass through cell membranes but attach to receptors on cell surfaces. This activates a second messenger molecule inside the cell, which brings about a series of reactions resulting in a gene being transcribed.

Folie 3

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• Steroid Hormones are lipids, similar to cholesterol in their basic structures. They diffuse across the cell membrane & are picked up by receptor molecules & carried into the nucleus where they act as a transcription factor & directly affect transcription of genes.

Folie 4

• How transcription factors work
• RNA polymerase, the enzyme involved in synthesis of RNA from a gene on DNA, needs to be activated by a transcription factor before it can work. The complex of a steroid hormone with its receptor molecule is one example of a transcription factor.
• The transcription initiation complex binds to the promoter region at the beginning of a gene on a DNA strand. This initiates transcription of that gene.
• A gene is not transcribed if a protein repressor molecule is attached either to its promoter region or to the transcription factors themselves.