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Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people who ingest the cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration. Infection usually occurs by drinking untreated water that contains cysts.
If the mitosomes of Giardia contain no DNA, yet are descendants of what were once free-living organisms, then where are we likely to find the genes that encode their structures, and what accounts for their current location there?
A) plasmids; conjugation
B) plasmids; transformation
C) nucleus; horizontal gene transfer
D) nucleus; S phase

Question

Quizplus · Accepted Answer

The genes that encode the structures of the mitosomes in Giardia are likely found in the nucleus, and their current location there is likely due to horizontal gene transfer. This is because Giardia is thought to have acquired these organelles through the endosymbiosis of a bacterium, and the transfer of genes from the bacterium's genome to the nuclear genome of the host cell is a common mechanism for the retention of organelles. Plasmids and S phase are not likely to be involved in the structure and location of the mitosomes. Conjugation and transformation may involve the transfer of genetic material, but they are not relevant to the question.

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