BIO 104
Instructor: Avis Gibons
Outline of Transcription (RNA Synthesis)
1) Transcription starts at a specific region of DNA called the promoter, which is located 5’ to the
beginning of the actual gene to be transcribed.
2) Proteins called transcription factors recognize the promoter and bind there.
3) An enzyme called DNA-dependent RNA polymerase binds to the promoter and the transcription
factors.
4) This catalyzes unwinding of a segment of the DNA helix, breaking of hydrogen bonds between
the bases of that segment, and binding of free RNA nucleotides to their complementary bases on
the DNA template strand.
5) RNA polymerase does not need a primer. It hooks RNA nucleotides together in the 5’ to 3’
direction, forming the backbone of the RNA strand.
6) The growing RNA strand does not fully associate with the DNA template, so most of it dangles
off to the side.
7) The process continues until RNA polymerase reaches a DNA sequence called a terminator.
8) The terminator causes RNA polymerase to stop transcription and to release the RNA molecule.
9) The DNA double helix comes back together.
If the immediate product of transcription is mRNA, it is called a primary transcript or heterogeneous
RNA. It undergoes several post-transcriptional modifications before it leaves the nucleus:
1) Its 5’ end is capped with a modified guanine nucleotide. The cap tells the ribosome where to
begin protein synthesis.
2) The 3’ end is capped with a “poly-A” tail, an attachment of about 200 adenine nucleotides. The
tail facilitates transport of the transcript from the nucleus and protects the transcript from
degradation during transport.
3) Genes contain some nucleotide sequences that do not express themselves in the protein product
(noncoding sequences), and other nucleotide sequences that do express themselves in the protein
product (coding sequences). Coding sequences are called exons, and noncoding sequences are
called introns. A third kind of post-transcriptional modification involves removing the introns
from the primary transcript and splicing the exons together. This is performed by a unit called a
spliceosome, a complex of protein, RNA and the relevant intron.
After processing, the now mature mRNA is ready to leave the nucleus.
