Video Transcript
Mitochondria, organelles found in
eukaryotic cells, shown in the figure, have their own circular loops of DNA. According to the endosymbiotic
theory, what does this suggest about the evolution of mitochondria? (A) Mitochondria were previously
separate, prokaryotic organisms before being incorporated in eukaryotes. (B) Mitochondria have evolved to be
separate, parasitic organelles within the cells of a eukaryote. (C) Mitochondria have had many
mutations in their DNA, so they now resemble the structure of bacterial DNA.
This question is asking us about
mitochondria, which are organelles found in eukaryotes that provide the cell with
energy in the form of ATP. How mitochondria came to be is
proposed by the endosymbiotic theory.
This theory states that an early
eukaryotic cell that didn’t have mitochondria ingested a prokaryote that was capable
of aerobic respiration. The eukaryote and prokaryote would
then enter in a symbiotic relationship, where both organisms would benefit. The prokaryote would have provided
energy in the form of ATP, and the eukaryote would’ve provided nutrients to the
prokaryote. Over billions of years of
evolution, the prokaryote would lose many of its genes because the eukaryote could
provide for its survival. And this aerobic prokaryote would
become the mitochondria that we know today.
There’s multiple lines of evidence
for the endosymbiotic theory. Mitochondria have their own DNA
separate from the nucleus of the cell. And this DNA is circular, just like
a prokaryote. Mitochondria perform their own
protein synthesis and have their own ribosomes that are separate from the cell. These ribosomes are very similar to
prokaryotic ribosomes. Prokaryotes and mitochondria are
also similar sizes. There’s additional evidence on top
of this that all point to mitochondria evolving from an aerobic prokaryote.
Now let’s go through the answer
choices to see which one is a correct interpretation of the endosymbiotic
theory.
Answer choice (A) seems to be
correct because it explains that mitochondria were a separate, prokaryotic organism
before being incorporated into eukaryotes. But let’s look at the other options
to be sure.
Answer choice (B) is incorrect
because mitochondria and eukaryotes evolved to have a mutually beneficial symbiotic
relationship, where both the eukaryote and mitochondrial ancestor benefited, and not
a parasitic one.
Answer choice (C) is also incorrect
because it should be the other way around, that bacterial, or prokaryotic, DNA has
had mutations over the course of evolution and it now resembles mitochondrial
DNA.
Therefore, the correct option
regarding the evolution of mitochondria is given by answer choice (A). Mitochondria were previously
separate, prokaryotic organisms before being incorporated in eukaryotes.
