Outline the steps required for a gene originally present in the endosymbiont genome to be transferred to the nuclear genome and be expressed, and for its product to be targeted back to the organelle of origin.

Key Concepts

Endosymbiotic Theory

The endosymbiotic theory posits that certain organelles, such as mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This theory explains the presence of their own DNA, which is distinct from nuclear DNA, and suggests a complex evolutionary relationship between these organelles and their host cells.

Gene Transfer Mechanisms

Gene transfer from the endosymbiont genome to the nuclear genome involves several mechanisms, including horizontal gene transfer and the integration of endosymbiont DNA into the nuclear genome. This process typically requires the DNA to be transcribed and translated in the nucleus, followed by the transport of the resulting protein back to the organelle, often involving specific targeting signals.

Protein Targeting and Import

Once a gene product is synthesized in the cytoplasm, it must be correctly targeted to the organelle of origin. This process involves signal peptides that direct the protein to the appropriate organelle, where it is imported through specialized translocons. Proper folding and post-translational modifications may also occur to ensure the protein functions correctly within the organelle.