Hormones and gene expressionEdexcel International A Level Biology: Revision notes
Section 1
Genes are switched on and off
Almost every cell in an organism contains the same DNA, but different cell types make different proteins. This is because only some genes are expressed in each cell type.
Gene expression is controlled at transcription by transcription factors, which are proteins that bind to DNA:
- A transcription factor binds to a specific region of DNA next to a gene, called the promoter.
- An activating transcription factor allows RNA polymerase to bind and transcribe the gene, so mRNA is made and then translated into protein. The gene is switched on.
- Where no transcription factor is bound, RNA polymerase cannot start, and the gene is switched off. Some transcription factors inhibit transcription.
Keep the sequence clear: transcription factor binds DNA, RNA polymerase transcribes, mRNA made, then translation gives protein.
Section 2
Peptide hormones act extracellularly
Peptide hormones, such as insulin, glucagon and ADH, are made of amino acids. They are hydrophilic and cannot pass through the phospholipid bilayer.
- The hormone binds to a specific receptor on the cell-surface membrane. The receptor has a complementary shape to the hormone.
- The hormone does not enter the cell. Binding triggers a second messenger inside the cell, which sets off a cascade that activates enzymes already present.
- The response is fast, because no new protein needs to be made, but it is usually short-lived.
Only target cells that have the receptors respond.
Section 3
Steroid hormones act intracellularly
Steroid hormones, such as oestrogen, testosterone and cortisol, are derived from lipids. They are lipid-soluble, so they diffuse through the phospholipid bilayer into any cell.
- Inside target cells, the hormone binds to a receptor in the cytoplasm or nucleus. This forms a hormone-receptor complex.
- The complex enters the nucleus (if it is not already there) and acts as a transcription factor. It binds to a promoter next to a specific gene.
- This switches the gene on (or off), so the amount of mRNA and then of protein changes.
- The response is slower, taking hours, but lasts longer.
Do not say steroid hormones need a receptor on the cell-surface membrane. They cross the membrane and the receptor is inside the cell.
Section 4
Comparing the two kinds of hormone
Peptide hormones act at the surface and their effect is rapid, working on existing proteins. Steroid hormones enter the cell and change which genes are transcribed, so their effect is slower and longer-lasting.
Because every cell has the same genes but a given hormone only works on cells that have its receptor, hormones can switch genes on in some tissues and not others. For example, a steroid will switch on a gene only in cells that make the right receptor, even though every cell has the gene.
In a comparison question, state for each hormone: lipid- or water-soluble, receptor position, what the receptor does, and how fast the response is.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Hormones and gene expression
- A student compares how two hormones affect their target cells. Insulin is a peptide hormone released by the pancreas, and oestrogen is a steroid hormone released by the ovaries. Both hormones travel in the blood to all parts of the body.Explain why only certain cells respond to insulin, even though every cell in the body is exposed to it.2 marks
- In liver cells, a gene codes for an enzyme that is only made when the cell is exposed to a particular hormone. Researchers find that the gene is switched on only when a transcription factor is bound to a specific region of DNA next to the gene.Explain how the binding of the transcription factor can result in the liver cell making more of the enzyme.2 marks
- Researchers add radioactively labelled hormones to isolated liver cells. After a few minutes, labelled glucagon (a peptide hormone) is found only on the cell-surface membrane. Labelled cortisol (a steroid hormone) is found inside the nucleus. When the cells are also given a chemical that inhibits RNA polymerase, cortisol no longer causes the cells to make a new enzyme, but glucagon still causes the cells to release glucose within seconds.Explain why labelled glucagon is found only on the cell-surface membrane but labelled cortisol is found in the nucleus.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).