Prokaryotic genes are often polycistronic, meaning one mRNA molecule directs the production of multiple proteins, while eukaryotic genes are generally monocistronic, with one mRNA producing a single protein. Eukaryotes regulate gene expression by using separate mRNAs for each gene, avoiding excess resource use, whereas prokaryotes benefit from coordinating multiple genes in an operon from a single mRNA due to their smaller genomes. The lac operon diagram shows a polycistronic mRNA that directs the production of three proteins.

1. 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

2. Monocistronic vs Polycistronic Transcription Prokaryotes – Both Eukaryotes Nucleus – Monocistronic (polycistronic rare) Mitochondria Mammals – polycistronic (2 promoters); pre-RNAs processed into smaller species Other – Both Chloroplast - Both

3. 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.

4. 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?