The structure of a gene

Genes are short segments of DNA that form the basic unit of heredity. Genes are the section of DNA that controls the production of polypeptides primarily. A gene’s structure is a very important factor to its function.

Genes in eukaryotes contain coding (exons) and non-coding sequences of bases (introns), in prokaryotes only coding regions exist.

Different flanking regions of genes

Upstream region

  • The upstream region is the region of DNA that is before the coding region of the DNA.
  • The upstream region usually contains a promotor sequence which is a section of DNA that binds RNA polymerase and indicates where to start transcribing RNA.
  • Mutation of the upstream region can affect the activity of the coding regions that follow.

Downstream region

  • This region is located after the end of the coding region of the gene.
  • It includes an “end transcription signal” which terminates transcription.
  • Mutation to the downstream region can cause gene action to be altered.


  • Promotor sequence is the control region of the upstream region that exists to switch the gene on or off.
  • The promotor sequence is a non-coding sequence that RNA polymerase binds to, initiating transcription.
  • Promotor sequences often include what is known as a TATA box. The TATA box is sequence of DNA that is  highly conserved between species and is recognized by RNA polymerase as part of the promotor region.
  • In eukaryotes RNA polymerase cannot simply bind to the promotor and initiate transcription without a group of regulatory proteins known as transcription factors that facilitate the RNA polymerase binding.