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Biology Unit 5
Topic 7: Run for your life
17. Explain how genes can be switched on and off by
DNA transcription factors including hormones
Some drugs such as anabolic steroids are closely related to natural steroid hormones. They can pass
directly through cell membranes and be carried into the nucleus bound to a receptor molecule. These
hormone/receptor complexes act as transcription factors. They bind to the promoter region of a
gene allowing RNA polymerase to start transcription. As a result more protein synthesis takes place
in the cells. E.g. Testosterone increases protein synthesis in muscle cells, increasing the size and
strength of the muscle tissue. Peptide hormones do not enter cells directly, but they bind with
receptors on the cell surface membrane. This starts a process that results in the activation of a
transcription factor within the nucleus. E.g. erythropoietin (EPO) stimulates the production of red
blood cells. This means that the blood can carry more oxygen which is an advantage for an athlete.
Anabolic steroids enter the cytoplasm and bind to a receptor molecule. They are carried into the
nucleus and enter it through nuclear pores. Once inside the cell's nucleus they can modify gene
expressions. The receptor complex acts as a transcription factor, allowing it to bind to DNA and
genes linked to protein synthesis are switched on. The protein is affected by a change in RNA.
Peptide hormones bind to a receptor in the cell surface membrane. This complex then leads to the
activation of a second messenger in the cytoplasm. This causes a protein kinase cascade. Several
proteins are activated until the final product enters the nucleus. It acts as a transcription factor.
Protein synthesis of enzymes for red blood cells has genes which are switched on due to this
transcription factor. Erythropoietin doesn't enter the cell, but binds to a receptor in the cell surface
Text Book: p. 180 -181