Lecture 24: Gene Structure and Evolution

(version 15 April 2003)

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Gene Structure and Evolution

A deep understanding of evolution requires an understanding of how genes are structured and regulated.

Prokayrotic Gene Structure

Bacteria, cholorplasts, mitochondrial genomes

Basic Gene Structure

Key features of prokayrotic gene structure and regulation:

Eukaryotic Gene Structure

Three basic types of eukaryotic genes

Pol II genes

Key differences from prokayrotic genes:

Very important control elements for regulation may be very far upstream (5' of gene) or downstream (3' of gene).

Alternative Splicing of exons allows one gene to make several different mRNAs, depending on which exons are included in the final message. Hence, one "gene" may code for a large number of different products. Some genes are known to make close to 100 different transcripts based on splicing patterns.

Pol I genes

For ribosomal RNA (rRNA) genes

Pol III genes

Four Families of Introns

Nuclear tRNA introns

Group I introns

Group II introns

Pol II introns

This is the group that most thing of when speaking about "introns". These are best viewed as a degenerate case of Group II introns.

Evolution of (Pol II) Introns: Introns Early vs. Introns Late

Big debate as to whether Pol II introns are early or late --- where they were initially present in bacteria but later lost (Introns early) or did they only arise in the eukaryotes (introns late).

Support for the "early" view: exons roughly correlated with protein domains.

Support for Introns late

Very clear evidence of exon shuffling in many (higher) eukarytoic-specific genes. Hence, higher eukarytoes seem to indeed use some exons as functional units.