<P> Flanking the open reading frame, genes contain a regulatory sequence that is required for their expression . First, genes require a promoter sequence . The promoter is recognized and bound by transcription factors and RNA polymerase to initiate transcription . The recognition typically occurs as a consensus sequence like the TATA box . A gene can have more than one promoter, resulting in messenger RNAs (mRNA) that differ in how far they extend in the 5' end . Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at a high rate . Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently . Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters . </P> <P> Additionally, genes can have regulatory regions many kilobases upstream or downstream of the open reading frame that alter expression . These act by binding to transcription factors which then cause the DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site . For example, enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter; conversely silencers bind repressor proteins and make the DNA less available for RNA polymerase . </P> <P> The transcribed pre-mRNA contains untranslated regions at both ends which contain a ribosome binding site, terminator and start and stop codons . In addition, most eukaryotic open reading frames contain untranslated introns which are removed before the exons are translated . The sequences at the ends of the introns, dictate the splice sites to generate the final mature mRNA which encodes the protein or RNA product . </P> <P> Many prokaryotic genes are organized into operons, with multiple protein - coding sequences that are transcribed as a unit . The genes in an operon are transcribed as a continuous messenger RNA, referred to as a polycistronic mRNA . The term cistron in this context is equivalent to gene . The transcription of an operon's mRNA is often controlled by a repressor that can occur in an active or inactive state depending on the presence of certain specific metabolites . When active, the repressor binds to a DNA sequence at the beginning of the operon, called the operator region, and represses transcription of the operon; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon). The products of operon genes typically have related functions and are involved in the same regulatory network . </P>

What is possible because each single cell of a plant contains the same information