Endosymbiotic theory — linking cyanobacteria and chloroplasts Genetic and biochemical similarities between modern cyanobacteria and eukaryotic chloroplasts most strongly support which evolutionary interpretation?

Introduction / Context:
The endosymbiotic theory explains the origin of membrane-bound organelles in eukaryotes. Chloroplasts, the sites of oxygenic photosynthesis in algae and plants, share striking features with cyanobacteria. Recognizing this relationship clarifies why chloroplasts have their own genomes and bacterial-like transcription and translation components.

Given Data / Assumptions:

• Chloroplasts contain circular DNA and 70S-type ribosomes.
• Cyanobacterial gene sequences cluster with plastid genes.
• Double membranes around chloroplasts are consistent with engulfment by endocytosis.

Concept / Approach:
According to endosymbiosis, an ancestral eukaryote engulfed a cyanobacterium. Rather than being digested, the cyanobacterium persisted, transferred some genes to the host nucleus, and became the chloroplast. This explains genetic, biochemical, and structural similarities and the retention of a reduced but distinct plastid genome.

Step-by-Step Solution:
Compare chloroplast features (circular DNA, bacterial-type ribosomes) with cyanobacteria.Note phylogenetic analyses that place plastid genes within cyanobacterial lineages.Infer the evolutionary mechanism: endosymbiosis leading to organellogenesis.Choose the option that explicitly states chloroplasts originated from cyanobacteria via endosymbiosis.

Verification / Alternative check:
Modern plastid-targeted proteins are encoded in the nucleus with transit peptides, consistent with gene transfer from the endosymbiont to the host genome after engulfment.

Why Other Options Are Wrong:

• A: Oversimplifies eukaryote origins and ignores the mosaic nature of eukaryotic genomes.
• B: Conflicts with evidence of bacterial contributions (including mitochondria and plastids).
• C: Oxygenic photosynthesis originated in cyanobacteria, not in eukaryotes.
• E: Reverses causality; chloroplasts did not escape to form cyanobacteria.

Common Pitfalls:
Assuming all eukaryotic traits derive from one source; in reality, eukaryotes show chimeric origins with contributions from archaeal and bacterial ancestors.

Final Answer:
Eukaryotic chloroplasts originated from cyanobacteria via endosymbiosis and were retained within early eukaryotic cells