Every cell in an organism contains the same genes, but only some of them are expressed. One way eukaryotic cells control which genes are transcribed is by using transcription factors.A transcription factor is a protein that binds to a specific DNA base sequence and controls the transcription of a gene. Each transcription factor has a DNA-binding site with a shape that is complementary to a particular base sequence, usually in the promoter region just upstream of the target gene.In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors move from the cytoplasm into the nucleus. Transcription factors are made by ribosomes in the cytoplasm and enter the nucleus through nuclear pores to reach the DNA.• Activators stimulate transcription. They help RNA polymerase bind to the promoter, so the gene is transcribed and mRNA is made.
• Repressors inhibit transcription. They bind at or near the promoter and stop RNA polymerase binding, so the gene is not transcribed.A transcription factor is often inactive until a signal activates it, for example a hormone binding to it and changing its shape. This lets a cell switch genes on or off in response to its environment. The next lesson shows how oestrogen does this.Transcription factors also control development. Different cells make different transcription factors, so each type of cell expresses its own set of genes. This is how stem cells become specialised.
• Repressors inhibit transcription. They bind at or near the promoter and stop RNA polymerase binding, so the gene is not transcribed.A transcription factor is often inactive until a signal activates it, for example a hormone binding to it and changing its shape. This lets a cell switch genes on or off in response to its environment. The next lesson shows how oestrogen does this.Transcription factors also control development. Different cells make different transcription factors, so each type of cell expresses its own set of genes. This is how stem cells become specialised.