International A Level Biology / Control of the internal environment
How hormones switch genes on and off
Edexcel International A Level Biology (YBI11), chapter 22: Control of the internal environment, specification point 7.22. Free lesson.
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Hormones can affect whether genes are transcribed by changing transcription factors, which bind to gene promoters. Peptide and steroid hormones reach these factors in different ways, so it matters which part of the pathway binds to the DNA.
What a transcription factor does
Most cells have the same genes, but many are switched off. A transcription factor is a protein that binds to the promoter, the DNA just before a gene. It affects whether RNA polymerase can transcribe that gene. Some transcription factors help RNA polymerase bind, so the gene is transcribed and switched on. Others stop it binding, so the gene is not transcribed and is switched off. In an answer, name the transcription factor and say whether the gene is transcribed.
How peptide hormones affect genes
Peptide hormones are chains of amino acids. Insulin and glucagon are examples. They are water soluble, so they cannot cross the hydrophobic middle of the cell surface membrane. Instead, they bind to a matching receptor in the membrane and start a chain of events inside the cell. The hormone is the first messenger. Receptor binding leads to a second messenger being made inside the cell. For glucagon and adrenaline, this can be cyclic AMP. Adrenaline is not strictly a peptide, but it is water soluble. The second messenger activates enzymes in the cytoplasm. These activate a separate…
How steroid hormones affect genes
Steroid hormones, such as oestrogen and testosterone, are made from cholesterol, so they are lipids. They are lipid soluble and diffuse through the cell surface membrane without needing an outer receptor. A steroid binds to a matching receptor inside the cell, in the cytoplasm or nucleus. The two form a hormone-receptor complex. If the complex forms in the cytoplasm, it moves through a nuclear pore into the nucleus. The complex itself acts as the transcription factor and binds to the promoter. RNA polymerase transcribes the gene and the mRNA is translated into protein. For example,…
How the two routes compare
Both routes end with a transcription factor binding to a promoter, followed by gene transcription and protein production. The key difference is how the factor is produced. Peptide hormones are water soluble, bind to cell surface receptors and use a second messenger to activate a separate transcription factor. Steroid hormones are lipid soluble, bind to receptors inside the cell, and use the hormone-receptor complex as the transcription factor. Growth hormone is a peptide, not a transcription factor. It binds to a surface receptor and triggers a second messenger. A separate…
Applying the pathway to cell division
To explain how oestrogen could make some breast cancer cells divide faster, follow its route to the gene and link the gene products to cell division.