Gene control by transcription factorsEdexcel A-Level Biology A: Revision notes

Almost every cell in the body contains the same genome, but each cell type makes only some of the proteins coded for. This is called gene expression. A liver cell and a muscle cell therefore differ because different genes are transcribed, not because they carry different DNA.

Not making unneeded proteins saves energy and amino acids. Gene control also lets cells respond to changes, for example to hormones.

In transcription, the enzyme RNA polymerase makes an mRNA copy of a gene using the DNA template strand. The enzyme binds to a promoter region near the start of the gene. If RNA polymerase cannot bind, the gene is switched off. If it can bind, mRNA is made, which is then translated into protein at a ribosome.

Transcription factors are proteins that bind to specific base sequences in DNA, usually in or near the promoter, to control whether a gene is transcribed.

• Activators help RNA polymerase to bind, so transcription increases.
• Repressors (inhibitors) prevent RNA polymerase binding, so the gene is switched off.

Each has a shape complementary to its DNA sequence, so it controls only particular genes.

Steroid hormones such as oestrogen and testosterone are lipid-soluble, so they diffuse through the phospholipid bilayer into target cells.

1. Hormone binds to a specific receptor protein inside the cell.
2. The hormone-receptor complex changes shape and becomes active.
3. It enters the nucleus and binds to the promoter of a specific gene as a transcription factor.
4. RNA polymerase binds, transcription increases and more mRNA is made.
5. mRNA is translated into more of the protein.

Only cells with the receptor respond. Anabolic steroids work in a similar way by switching on genes for muscle proteins.