Correct option: b. Mitochondrial rRNA is more similar to bacterial rRNA than to eukaryotic rRNA transcribed from nuclear DNA.​

Introduction

The endosymbiotic theory, championed by Lynn Margulis, proposes that mitochondria evolved from free‑living bacteria that entered into a symbiotic relationship with ancestral eukaryotic cells. One of the strongest pieces of evidence is the similarity between mitochondrial genetic components and those of bacteria, especially mitochondrial rRNA.​

Why option b is correct

Statement b: Mitochondrial rRNA is more similar to bacterial rRNA than to eukaryotic rRNA transcribed from nuclear DNA.

• Mitochondria contain their own DNA and ribosomes, and the small subunit mitochondrial rRNA (such as 12S/18S) shows high sequence and structural similarity to bacterial 16S rRNA.​

• Comparative analyses show that mitochondrial rRNA often shares greater identity with eubacterial rRNA than with cytoplasmic (nuclear‑encoded) eukaryotic rRNA, directly supporting a bacterial ancestry of mitochondria and thus the endosymbiotic theory.​

Why the other options are incorrect

Option a: dependence on host for replication

Statement a: Mitochondria require the internal environment of the eukaryotic cell to replicate.

• Mitochondria divide by a process similar to binary fission, but their replication is tightly controlled by the host cell and occurs only within the cytoplasmic environment.​

• This dependence shows that mitochondria are now integrated organelles, but it does not by itself distinguish endosymbiotic origin from an origin as a purely endogenous compartment, so it is weaker evidence than genetic similarity like that in option b.​

Option c: morphology indistinguishable from bacteria

Statement c: Mitochondria are morphologically indistinguishable from free-living bacteria.

• Mitochondria are indeed similar in size and general shape to some bacteria and have a double membrane, which is consistent with a phagocytosed bacterial ancestor.​

• However, “morphologically indistinguishable” is an overstatement: there are clear ultrastructural differences between modern mitochondria and typical free‑living bacteria, so morphology alone is not decisive and this exact statement is not accurate.​

Option d: mitochondria can form free-living cells

Statement d: Mitochondria that are removed from eukaryotic cells and placed in a growth medium can generate fully functioning free-living cells.

• Isolated mitochondria cannot survive and divide as independent cells; they lack many genes required for autonomous life and depend on nuclear‑encoded proteins and host metabolism.​

• This statement is factually wrong and directly contradicts current understanding of mitochondrial integration into eukaryotic cells, so it cannot be evidence for endosymbiosis.​

Key endosymbiotic evidence overview

For exam preparation, remember the key evidence strongly supporting endosymbiotic theory:

• Mitochondria and chloroplasts have their own circular DNA, similar to bacterial genomes.​

• Mitochondrial rRNA and ribosomal structure are more similar to bacterial ribosomes than to cytosolic eukaryotic ribosomes.​

• These organelles divide by a fission-like process and are transmitted from parent to offspring via the cytoplasm, consistent with descent from once free‑living bacteria.​