Transcription is the first stage of protein synthesis where a segment of DNA is copied into mRNA by RNA polymerase. However, not all genes are transcribed all the time. The cell regulates which genes are expressed in different conditions or cell types. This regulation is controlled by transcription factors and other molecular signals.

1.  Transcription Factors (TFs)

• Definition: Proteins that bind to specific DNA sequences to control the rate of transcription.
• Each transcription factor has a specific target gene and binds near the gene at a promoter region.
• Two main types:
  • Activator transcription factors: Increase transcription by helping RNA polymerase bind.
  • Repressor transcription factors: Inhibit transcription by blocking RNA polymerase binding.

2. Promoter Regions

• Located just upstream (before) the coding region of a gene.
• Serve as binding sites for transcription factors and RNA polymerase.
• Without proper binding at the promoter, transcription cannot begin.

3. Hormonal Control of Transcription – Oestrogen and Testosterone

Some transcription factors are activated by hormones, such as oestrogen and testosterone, which act as signalling molecules to regulate gene expression.

Example: Oestrogen

• Oestrogen is lipid-soluble and can pass through the cell membrane by simple diffusion across the phospholipid bilayer.
• Inside the cytoplasm, it binds to an oestrogen receptor, forming a hormone–receptor complex.
• This complex acts as a transcription factor and moves into the nucleus.
• It binds to promoter regions of specific genes and stimulates transcription by helping RNA polymerase bind.

The region labelled A is the promoter region.

Example: Testosterone

• Works similarly to oestrogen — binds to androgen receptors, forming a hormone–receptor complex.
• The complex then enters the nucleus and modifies gene expression in target cells like muscle and reproductive tissue.