6.3 Transcription and RNA Processing

RNA polymerase binds the promoter (often with a TATA box and transcription factors), unwinds DNA, and builds RNA 5′→3′ by reading the template strand 3′→5′. The RNA matches the coding strand (with U for T).

Before leaving the nucleus, pre-mRNA gets a 5′ cap (GTP) and a poly-A tail — both protect from degradation and help ribosomes bind.

Splicing by spliceosomes removes introns and joins exons. Alternative splicing joins exons in different combinations, so one gene can code for several proteins.

Prokaryotes have no nucleus, so transcription and translation happen at the same time.

Template strand 3′-TAC GGA-5′ → mRNA 5′-AUG CCU-3′.

Worked example: Why can ~20,000 human genes make 100,000+ proteins? → Alternative splicing.