The endosymbiotic theory provides a compelling explanation for the origin of eukaryotic cells, which are the complex cells making up plants, animals, fungi, and protists. This theory postulates that mitochondria and chloroplasts, two organelles within eukaryotic cells, were once free-living prokaryotic organisms. These prokaryotes entered into a symbiotic relationship with an ancestral host cell, eventually integrating and evolving into the organelles we recognize today. Lynn Margulis played a significant role in advocating for and compiling evidence supporting this transformative theory during the 20th century.
Genetic Similarities
Mitochondria and chloroplasts possess their own genetic material, which is distinct from the DNA found in the eukaryotic cell’s nucleus. This organellar DNA is circular, resembling the circular chromosomes characteristic of bacteria rather than the linear DNA found in eukaryotic nuclei. This structural similarity in DNA organization strongly suggests a prokaryotic ancestry for these organelles.
Further genetic evidence comes from the ribosomal RNA (rRNA) sequences within these organelles. Both mitochondrial and chloroplast rRNA sequences show a closer evolutionary relationship to bacterial rRNA than to the rRNA found in the eukaryotic cell’s own cytoplasm. For instance, mitochondrial rRNA sequences are particularly similar to those of alpha-proteobacteria, while chloroplast rRNA sequences align closely with those of cyanobacteria.
The ribosomes within mitochondria and chloroplasts are of the 70S type, composed of 50S and 30S subunits. This type of ribosome is found in bacteria, whereas the ribosomes in the cytoplasm of eukaryotic cells are larger, 80S ribosomes. The presence of bacterial-like ribosomes within these organelles supports the idea that they originated from free-living bacterial ancestors.
Structural Resemblances
Mitochondria and chloroplasts share notable physical and structural characteristics with bacteria. Both organelles are comparable in size to typical bacteria, generally ranging from 0.5 to 10 micrometers in length for chloroplasts and 0.5 to 1.0 micrometers in diameter for mitochondria. This size correspondence hints at their potential past as independent microorganisms.
A distinctive feature of both mitochondria and chloroplasts is their double membrane. The inner membrane of these organelles is thought to be derived from the original bacterial cell membrane, while the outer membrane is believed to have originated from the host cell’s membrane during the engulfment process.
The inner membrane of mitochondria is highly folded into structures called cristae, which increase surface area for metabolic processes, a characteristic resembling the infoldings found in some bacterial membranes. Similarly, chloroplasts contain internal membrane systems called thylakoids, where photosynthesis occurs. These thylakoid membranes bear resemblance to the photosynthetic membranes found in cyanobacteria, the presumed ancestors of chloroplasts.
Reproductive Independence
Mitochondria and chloroplasts exhibit a unique mode of reproduction within the eukaryotic cell, which mirrors how bacteria multiply. These organelles do not arise de novo from the eukaryotic cell’s components. Instead, they grow and divide through a process known as binary fission.
Binary fission is the primary method of reproduction for bacteria, where a single cell divides into two identical daughter cells. The fact that mitochondria and chloroplasts employ this same division mechanism, independently of the host cell’s mitotic division, suggests their ancestral autonomy.
Biochemical Parallels
Mitochondria and chloroplasts display metabolic pathways and enzyme systems that are characteristic of bacteria. Both organelles carry out their own protein synthesis using their bacterial-like ribosomes. This process can be inhibited by certain antibiotics, such as chloramphenicol and streptomycin, which specifically target bacterial protein synthesis without affecting the protein synthesis machinery in the eukaryotic cytoplasm. This selective sensitivity indicates a shared biochemical similarity with bacteria.
The mechanisms for energy production within these organelles closely parallel those found in prokaryotes. Mitochondria utilize electron transport chains and ATP synthesis pathways that are remarkably similar to those in aerobic bacteria. Similarly, chloroplasts perform photosynthesis using electron transport chains and ATP synthesis mechanisms that closely resemble those found in cyanobacteria. These biochemical commonalities provide additional support for the endosymbiotic theory, highlighting the bacterial heritage of these essential eukaryotic organelles.