Transcription

The Processes of Protein Synthesis

Transcription

assembly of messenger RNA (mRNA) from a DNA template.

RNA processing

modification of pre-mRNA before transport out of nucleus.

Translation

assembly of polypeptide using mRNA template, ribosomes and tRNA.

Eukaryotic Transcription Step-by-Step

1. A DNA strand contains a promoter site.
2. Promoter site is special sequence of nucleotides (often a TATA box).
3. RNA polymerase binds to the promoter site.
4. RNA polymerase unwinds DNA.
5. Just past promoter site, RNA polymerase assembles RNA from nucleotides in nucleus.
6. RNA Polymerase moves along DNA in 3' to 5' direction.
7. The RNA builds from 5' to 3' (the complement remember).
8. RNA strand elongates.
9. Several RNA polymerases may chug along DNA helix making several RNA strands.
10. Termination sequence in DNA stops RNA transcription.
11. RNA polymerase release RNA strand and DNA template.
12. The RNA strand is called pre-mRNA.

mRNA Processing

1. Eukaryotic cells modify pre-mRNA.
2. A 5' cap is added to the 5' end.
3. A poly-A termination tail is added to the 3' end.
4. Some sections of RNA (and DNA) are not used for encoding polypeptides.
5. The sections that are expressed in a polypeptides are called exons.
6. The intervening sections are called introns.
7. Spliceosomes cut out the introns, paste together the exons.
8. Exons may allow a single gene to make several polypeptides.
9. Exon shuffling may create beneficial genes.