Hormones and transcription factors

Respiration, Muscles and the Internal Environment · Homeostasis, hormones and the kidney · note 3 of 7

Spec 7.22

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Hormones and transcription factorsSpec 7.22

In short

Genes are switched on and off by transcription factors, which are proteins that bind to specific DNA sequences and increase or decrease transcription. Steroid hormones are lipid-soluble, enter the cell and bind to intracellular receptors that act as transcription factors. Peptide hormones bind to receptors on the cell surface and act extracellularly through a second messenger.

Transcription factors are proteins that bind to specific base sequences on DNA, near a gene. Some (activators) help RNA polymerase bind and so switch the gene on; others (repressors) block transcription and switch the gene off. Whether a gene is expressed, and how much protein is made, depends on which transcription factors are active in a cell.

Two ways in which hormones affect gene expression
Feature	Steroid hormones (e.g. oestrogen)	Peptide hormones (e.g. insulin, glucagon)
Solubility	Lipid-soluble	Water-soluble
Entry to the cell	Pass through the phospholipid bilayer by diffusion	Cannot pass through the membrane
Receptor	Inside the cell (cytoplasm or nucleus): acts intracellularly	On the cell surface membrane: acts extracellularly
How gene expression changes	The hormone–receptor complex acts as a transcription factor: it binds to DNA, switching a gene on (or off), so mRNA and a protein are made	Binding activates a second messenger (e.g. cyclic AMP) inside the cell, which triggers a cascade that activates or inactivates transcription factors and enzymes
Speed of response	Slower: protein must be made	Often quicker, because enzymes already present are altered

Exam tip:

Peptide hormones act without entering the cell because they cannot cross the phospholipid bilayer. Steroid hormones can, so their receptor is inside the cell.

A steroid hormone crosses the cell surface membrane, binds an intracellular receptor, the hormone–receptor complex enters the nucleus and acts as a transcription factor binding DNA, so mRNA is made and translated into protein. — Steroid hormones enter the cell and act as transcription factors.

A peptide hormone binds a receptor in the cell surface membrane, which activates a second messenger in the cytoplasm; this activates an enzyme or a transcription factor. — Peptide hormones cannot cross the membrane, so they work through a second messenger.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between negative and positive feedback?

In negative feedback the response reverses the change, so a factor such as temperature stays near its set point. In positive feedback the response increases the change and moves the factor further away, as with oxytocin in childbirth. Homeostasis relies on negative feedback.

How does the body regulate temperature during exercise?

Respiration in muscles releases heat, raising the temperature of the blood. Thermoreceptors in the hypothalamus detect this and trigger vasodilation of skin arterioles and sweating. These transfer heat to the surroundings, so core temperature returns to the set point by negative feedback.

How does the kidney filter blood in ultrafiltration?

The afferent arteriole is wider than the efferent arteriole, so blood in the glomerulus is at high pressure. Water and small solutes are forced through pores in the capillary endothelium, the basement membrane and the podocytes into the Bowman's capsule, while blood cells and large proteins stay in the blood.

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