Smallpox has been eliminated through the use of vaccines.

A. Mitochondria and chloroplasts formed spontaneously within early eukaryotic cells: Spontaneous formation of complex organelles within primitive eukaryotic cells is not supported by molecular or fossil evidence. Organelles like mitochondria and chloroplasts have their own genomes and double membranes, indicating an origin separate from the host cell.

B. Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells: The endosymbiotic theory proposes that ancestral eukaryotic cells engulfed aerobic bacteria (mitochondria) and photosynthetic cyanobacteria (which became chloroplasts). Evidence includes organelles containing their own circular DNA, ribosomes similar to prokaryotes, double membranes, and reproduction by binary fission within the host cell. This explains the integration of energy-converting organelles into eukaryotic metabolism.

C. Eukaryotic cells evolved directly from modern bacteria: Modern bacteria are descendants of ancient prokaryotes but are not direct precursors of eukaryotic cells. Eukaryotic cells arose through complex evolutionary processes, including endosymbiosis and the development of a nucleus, cytoskeleton, and membrane-bound organelles.

D. Organelles developed from infoldings of the nuclear membrane: Some organelles, such as the endoplasmic reticulum and nuclear envelope, likely from membrane invaginations. Mitochondria and chloroplasts have independent genomes and structural characteristics indicating a prokaryotic origin rather than development from nuclear membrane infoldings.

E. Chloroplasts evolved independently of mitochondrial ancestry: While chloroplasts and mitochondria both originated from prokaryotic endosymbionts, they have separate evolutionary lineages. Chloroplasts derive from photosynthetic cyanobacteria, whereas mitochondria derive from aerobic proteobacteria. Both however follow the principles of endosymbiosis.

A. A: Immediate host cell lysis after infection characterizes the lytic cycle, not the lysogenic cycle. In the lytic pathway, viral DNA hijacks host machinery, replicates rapidly, and leads to destruction of the host cell. Lysogeny instead involves a dormant phase without immediate lysis.

B. B: Release of viral particles through budding from the host membrane is typical of enveloped animal viruses, not bacteriophages. Bacteriophages infect bacterial cells and do not use membrane budding as a release mechanism.

C. C: Viral RNA remaining free in the cytoplasm without replication does not describe lysogeny. In bacteriophages, the viral genome is DNA, and during lysogeny it integrates into the bacterial chromosome rather than remaining free and inactive in the cytoplasm.

D. D: In the lysogenic cycle, bacteriophage DNA integrates into the bacterial host genome and becomes a prophage. The viral DNA replicates passively along with the host chromosome during cell division without causing immediate cell death. This allows the virus to persist in a dormant state until conditions trigger entry into the lytic cycle.

E. Both A & E are correct: Immediate host cell death following attachment or infection describes the lytic cycle, not lysogeny. In lysogeny, the host cell survives and continues to replicate while carrying the integrated prophage within its genome.