Transcription

Transcription involves the transfer of genetic information from the 'template' strand of DNA into a complementary RNA strand. It consists of three phases: initiation, elongation, and termination. Initiation occurs when RNA polymerase binds to a promoter region (e.g., the TATA box) facilitated by transcription factors. RNA polymerase then unwinds the DNA and synthesizes RNA in the 5' to 3' direction, matching U (uracil) with A (adenine). Unlike DNA replication, transcription does not require a primer. In eukaryotes, the initial transcript (pre-mRNA) must undergo processing before leaving the nucleus: this includes 5' capping (addition of 7-methylguanosine), 3' polyadenylation (poly-A tail), and splicing (removal of non-coding introns and joining of coding exons). Alternative splicing allows a single gene to code for multiple proteins, significantly increasing proteomic diversity. Regulation of transcription is the mechanism by which hormones (like steroids) and drugs exert their long-term effects.

Regulation of transcription is complex. Enhancers and silencers are DNA sequences that increase or decrease the rate of transcription, often located far from the gene. Transcription factors (TFs) bind to these sites. Eukaryotic RNA Polymerase II is the main enzyme for mRNA. During elongation, the DNA double helix is temporarily unwound to form a transcription bubble. In termination, specific sequences signal the RNA polymerase to detach. Post-transcriptional modification is vital: the 5' cap is essential for ribosome recognition during translation, and the poly-A tail provides stability. Splicing is performed by the spliceosome (composed of snRNPs). Defects in splicing can lead to diseases such as Beta-thalassaemia. RNA interference (RNAi) is a natural mechanism where small RNA molecules inhibit gene expression by causing the destruction of specific mRNA molecules, a process now being harnessed for therapeutic use (e.g., siRNA drugs).

Rifampicin, a key drug in tuberculosis treatment, acts by inhibiting bacterial RNA polymerase. Death cap mushrooms (Amanita phalloides) contain alpha-amanitin, which inhibits RNA polymerase II, leading to fatal liver failure. Steroid hormones (Glucocorticoids, Estrogen, etc.) enter the nucleus and bind to nuclear receptors which act as transcription factors, explaining their delayed onset of action. Many cancers are driven by dysregulated transcription factors (e.g., MYC). Beta-thalassaemia often results from mutations at splice sites, leading to incorrect mRNA processing and non-functional haemoglobin.

Investigations related to transcription include: RT-PCR (Reverse Transcription PCR) to measure mRNA levels (gene expression), and FISH (Fluorescence In Situ Hybridization) to identify gene translocations. mRNA-based diagnostics are becoming more common in oncology to determine tumor subtype and prognosis (e.g., Oncotype DX for breast cancer).

Therapeutic management involves using drugs that modulate transcription. For example, Tamoxifen targets the Estrogen Receptor (a transcription factor) in breast cancer. In cases of alpha-amanitin poisoning, management is supportive, often requiring liver transplantation. Recent medical advances include mRNA vaccines and siRNA therapies (e.g., Inclisiran for hypercholesterolaemia) which bypass or target the transcription/translation pathway.