The Processes of Protein Synthesis
- assembly of messenger RNA (mRNA) from a DNA template.
- RNA processing
- modification of pre-mRNA before transport out of nucleus.
Eukaryotic Transcription Step-by-Step
- A DNA strand contains a promoter site.
- Promoter site is special sequence of nucleotides (often a TATA box).
- RNA polymerase binds to the promoter site.
- RNA polymerase unwinds DNA.
- Just past promoter site, RNA polymerase assembles RNA from nucleotides in nucleus.
- RNA Polymerase moves along DNA in 3' to 5' direction.
- The RNA builds from 5' to 3' (the complement remember).
- RNA strand elongates.
- Several RNA polymerases may chug along DNA helix making several RNA strands.
- Termination sequence in DNA stops RNA transcription.
- RNA polymerase release RNA strand and DNA template.
- The RNA strand is called pre-mRNA.
- Eukaryotic cells modify pre-mRNA.
- A 5' cap is added to the 5' end.
- A poly-A termination tail is added to the 3' end.
- Some sections of RNA (and DNA) are not used for encoding polypeptides.
- The sections that are expressed in a polypeptides are called exons.
- The intervening sections are called introns.
- Spliceosomes cut out the introns, paste together the exons.
- Exons may allow a single gene to make several polypeptides.
- Exon shuffling may create beneficial genes.