Transcription - DNA to mRNA

Overview of Transcription

Transcription takes place in the nucleus of eukaryotic cells. The enzyme RNA polymerase binds to a specific region of DNA called the promoter, unwinds the double helix, and synthesizes a single-stranded mRNA molecule by reading the template strand in the 3' to 5' direction.

The resulting mRNA is built in the 5' to 3' direction and uses the same base-pairing rules as DNA, except that uracil (U) replaces thymine (T). So where DNA has adenine (A), the mRNA will have uracil (U).

The Three Stages of Transcription

1. Initiation

Transcription begins when transcription factors help RNA polymerase recognize and bind to the promoter region of a gene. In eukaryotes, a common promoter element is the TATA box, located about 25-30 base pairs upstream of the transcription start site. Once bound, RNA polymerase unwinds a short section of DNA, creating a transcription bubble.

2. Elongation

RNA polymerase moves along the template strand in the 3' to 5' direction, adding complementary RNA nucleotides one by one. The mRNA strand grows in the 5' to 3' direction at a rate of roughly 40 nucleotides per second in eukaryotes. As the polymerase advances, the DNA behind it re-winds into its double helix structure.

3. Termination

In eukaryotes, transcription continues past the end of the gene until a polyadenylation signal (AAUAAA) is transcribed. Proteins then cleave the mRNA and add a poly-A tail. RNA polymerase eventually dissociates from the DNA. In prokaryotes, termination may be Rho-dependent or Rho-independent (involving a hairpin loop in the mRNA).

mRNA Processing in Eukaryotes

Before leaving the nucleus, the initial mRNA transcript (called pre-mRNA) undergoes three critical modifications:

• 5' capping - a modified guanine nucleotide is added to the 5' end, protecting the mRNA from degradation and aiding ribosome recognition.
• RNA splicing - non-coding sequences called introns are removed by a complex called the spliceosome. The remaining coding sequences (exons) are joined together. Alternative splicing allows one gene to produce multiple protein variants.
• Polyadenylation - a tail of approximately 200 adenine nucleotides is added to the 3' end, stabilizing the mRNA and facilitating its export from the nucleus.

Template Strand vs. Coding Strand

DNA is double-stranded, but only one strand is used as a template for transcription:

Prokaryotic vs. Eukaryotic Transcription