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Control elements

A control element, or regulatory sequence, is a section of DNA that controls gene expression.

Transcription factors bind to control elements initiating or altering the rate of transcription.

Control elements include enhancers, silencers, operators and promoters.

Control elements are present in both prokaryotes and eukaryotes. However, they are typically more complex in eukaryotes, with a more diverse range of promoters, enhancers and silencers.

Many control elements, though not all, lie outside the gene and are therefore classified as non-coding DNA.

A molecule of RNA polymerase is shown assembling mRNA from DNA
A molecule of RNA polymerase is shown assembling mRNA from DNA

Promoters and enhancers are control elements.

  • The promoter region is the site of the start of transcription.

    Transcription factors bind to the promoter sequence and form a transcription initiation complex.

    It is normally only possible for RNA polymerase to bind to the promoter sequence if transcription factors are already present.

  • Enhancer regions are another common type of control element.

    In eukaryotes, these regions may be hundreds of bases away from the gene, but they are physically close to the gene due to the coiled structure of DNA. They work by bending the DNA sequence in order to enhance the rate of transcription.

Transcription factors bound to the promoter region.
Transcription factors bound to the promoter region.

Operators and silencers are control elements that are involved in the transcription of genes.

  • Operators are present in and control the prokaryotic operon.

    Transcription factors bind to operator sequence. These proteins either activate or repress gene expression by controlling whether RNA polymerase is able to bind to the promoter.

  • Silencers are sequences of DNA that repressor proteins can bind to. They inhibit and stop transcription. Silencers are found in both eukaryotes and prokaryotes.

The promoter region has been highly conserved amongst eukaryotes and prokaryotes. This means that even distantly related organisms can share a very similar promoter sequence.

In prokaryotes, the promoter contains two sequences that are important for transcription. Prokaryotes have just one type of RNA polymerase which is able to transcribe all genes.

  • The TATAAT box, which is positioned around 10 bases away from the gene, allows RNA polymerase to bind to the DNA.

  • Around 35 bases away from the gene is a sequence of seven nucleotides TTGACTAT, which alters the rate of transcription.

Transcription factors bound to the promoter region.
Transcription factors bound to the promoter region.

The eukaryotic promoter sequence is more diverse than the prokaryotic sequence. More than ten classes of promoter region have been found in eukaryotes.

Eukaryotes have several types of RNA polymerase. Each of them synthesises a different type of RNA, and binds to a unique promoter sequence. This means that the promoter sequences in eukaryotes vary significantly.

RNA polymerase II (Pol II), which produces mRNA, often carries a TATA box in its corresponding promoter sequence. The TATA box has the core sequence of TATAAA , which is similar to the prokaryotic promoter sequence.

Transcription factors bound to the promoter region.
Transcription factors bound to the promoter region.