A gene is a sequence of DNA which is transcribed into RNA (usually mRNA, but also tRNA and rRNA)

• A gene is composed of three main sections: promoter, coding sequence and terminator
  

Promoter

• The promoter is an upstream non-coding sequence responsible for the initiation of transcription
  

• The promoter functions as a binding site for RNA polymerase (the enzyme responsible for transcription)
  

• The binding of RNA polymerase to the promoter is mediated and controlled by an array of transcription factors
  

  • Transcription factors can either bind directly to the promoter or associate with distal regulatory sequences (enhancers or silencers)
    

  • Transcription factors can either promote the binding of RNA polymerase (increase transcription) or inhibit binding (decrease transcription)
    

Coding Sequence

• After RNA polymerase has bound to the promoter, it causes the DNA strands to unwind and separate
  

• The region of DNA that is transcribed by RNA polymerase is called the coding sequence
  

• The RNA transcript will be complementary in sequence to the coding sequence (with uracil instead of thymine)
  

Terminator

• RNA polymerase will continue to transcribe the DNA until it reaches a terminator sequence
  

• The mechanism for transcriptional termination differs between prokaryotes and eukaryotes
  

Transcription

Transcription is the process by which a gene sequence is copied into a complementary RNA sequence by RNA polymerase

• Free RNA nucleotides within the cell line up opposite their complementary base partner
  

• RNA polymerase covalently binds the RNA nucleotides together via condensation reactions (forming phosphodiester bonds)
  

• The 5’-phosphate is linked to the 3’-end of the growing mRNA strand, hence transcription occurs in a 5’ → 3’ direction