Transcription

Transcription is the synthesis of RNA using information in DNA. Eukaryotic genes consist of the following building blocks:

Figure 1. The transcription process

Remember that these building blocks are simply codes of nucleotides. The following are the most important gene components:

• Promoter: DNA sequence located near the 5’ end of the coding region where RNA polymerase and transcription factors bind and initiate the transcription. Generally, a promoter consists of two sequences: a recognition sequence (where RNA polymerase binds) and a TATA box (where DNA starts to denature).

• Enhancer: The transcription control element that is located 1 Mbp away from the gene. It binds the activator (also referred to as transcription factor).

• Poly-A signal: DNA sequence (AAUAAA) located at the 3’ end of coding region. This sequence signals the site of cleavage of the pre-mRNA transcript by an enzyme, after which a poly-A tail is added. Terminators which signal the end of transcription can locate hundreds of nucleotides downstream of the poly-A signal.

• Exon: Expressed regions or coding regions. This part of the gene encodes the mRNA.

• Intron: Intervening regions or noncoding regions. These sequences are spliced out of the mRNA precursor before it leaves the nucleus.

Transcription factors are proteins that mediate the binding of RNA polymerase and the initiation of transcription. Transcription only starts when the appropriate transcription factors are attached and RNA polymerase are bound in the correct orientation. The transcription factors and RNA polymerase on the promoter form a complex called the transcription initiation complex.

The rate of gene expression can be strongly increased or decreased by the binding of specific transcription factors (activator to the enhancer, and repressor to the silencer). Enhancers and silencers can affect the expression of genes located hundreds, or even thousands, of nucleotides away. This is made possible by DNA bends that bring the enhancers/silencers and promoters in close proximity.

Post-transcriptional regulation

Gene regulation can happen right after transcription by processing the RNA. In eukaryotic organisms, RNA processing is performed by alternative RNA splicing and mRNA degradation. Short interfering RNAs are part of a complex mechanism that regulates gene expression by binding to specific mRNAs and inhibiting translation.