Gene Structure:Polycistronic vs monocistronic

• Cistron: an old name for a gene• Polycistronic: 1 promoter directs

synthesis of 1 mRNA that can be translated to more than one polypeptide– Prokaryotic genes

• Monocistronic: 1 promoter directs synthesis of 1 mRNA that usually translates to only 1 protein– Eukaryotic genes

Monocistronic vs Polycistronic Transcription

• Prokaryotes – Both • Eukaryotes

1. Nucleus – Monocistronic (polycistronic rare)2. Mitochondria

1. Mammals – polycistronic (2 promoters); pre-RNAs processed into smaller species

2. Other – Both

3. Chloroplast - Both

Why does prokaryotes have polycistronic while eukaryotes have

monocistronic?• Eukaryotes don’t need everything so by

using monocistronic, they can regulate what is being created without creating too much of unnecessary resources.

• Prokaryote uses polycistronic because they genome is small and want to make everything in one pathway together.

The lac Operon

P O lacZ lacY lacA

P O lacZ lacY lacA

mRNA 5’ 3’

RIBSOSOME BINDING SITE

Q: How many proteins are made?