Evidence for Endosymbiosis
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You should be familiar with the basic structure of prokaryotic and eukaryotic cells, including the presence of a nucleus and membrane-bound organelles in eukaryotes. You should also understand the concept of DNA replication and protein synthesis.
Why this matters
Understanding the evidence for endosymbiosis reveals how complex eukaryotic cells evolved from simpler prokaryotic ancestors.
The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by a host cell. Rather than being digested, these bacteria formed a symbiotic relationship with the host, eventually becoming permanent organelles. This event was a pivotal moment in the history of life, enabling the evolution of complex multicellular organisms. The evidence supporting this theory is compelling and draws from multiple lines of inquiry, including structural, genetic, and biochemical similarities between these organelles and modern bacteria.
The Core Idea
Imagine a large, primitive cell that accidentally swallowed a smaller, aerobic bacterium. Instead of breaking it down, the host cell kept the bacterium alive. Over time, the bacterium provided energy to the host, and the host provided protection and nutrients. This mutual benefit led to a stable partnership. Eventually, the bacterium lost its independence, transferring some of its genes to the host nucleus and becoming an integral part of the cell. This process, known as endosymbiosis, is the proposed origin of mitochondria and chloroplasts. The key idea is that these organelles were once independent organisms, and their current structure and function reflect their bacterial ancestry.
Structural and Genetic Evidence
One of the strongest pieces of evidence for endosymbiosis is the presence of double membranes in mitochondria and chloroplasts. The inner membrane resembles the plasma membrane of bacteria, while the origin of the outer membrane (the host's engulfing vesicle, or the bacterium's own outer membrane) is still debated. Additionally, these organelles possess their own circular DNA, which is similar in structure to bacterial chromosomes. Unlike the linear DNA found in the eukaryotic nucleus, organelle DNA is circular and lacks histones, a characteristic feature of prokaryotic genomes.
Furthermore, mitochondria and chloroplasts have their own ribosomes, which are of the bacterial type: 70S in chloroplasts, and in mitochondria bacterial in make-up though their size varies (about 55S in mammals). In contrast, the ribosomes in the eukaryotic cytoplasm are 80S. This difference in ribosome size is a clear indicator of a distinct evolutionary origin. The genetic code used by these organelles is also very similar to that of bacteria, with only minor variations. These structural and genetic parallels strongly suggest that mitochondria and chloroplasts were once free-living prokaryotes.
Reproductive and Biochemical Evidence
Mitochondria and chloroplasts reproduce independently of the host cell through a process called binary fission, which is the same method used by bacteria. New mitochondria and chloroplasts only ever arise from existing ones, never from scratch, just as bacteria only arise from bacteria. The rate of organelle division is often correlated with the metabolic needs of the cell, ensuring that there are enough organelles to meet energy demands. This independent reproductive capability is a strong indicator of their bacterial origin.
Biochemically, the metabolic pathways within mitochondria and chloroplasts are similar to those found in bacteria. For instance, the electron transport chain in mitochondria is structurally and functionally analogous to the respiratory chains in aerobic bacteria. Similarly, the photosynthetic machinery in chloroplasts is closely related to the photosystems found in cyanobacteria. These biochemical similarities provide further evidence that these organelles evolved from specific types of bacteria, namely alphaproteobacteria for mitochondria and cyanobacteria for chloroplasts.
Phylogenetic Analysis
Modern molecular biology has provided powerful tools to test the endosymbiotic theory through phylogenetic analysis. By comparing the DNA sequences of organelle genes with those of free-living bacteria, scientists have constructed evolutionary trees that show a close relationship between mitochondria and alphaproteobacteria, and between chloroplasts and cyanobacteria. These phylogenetic trees place mitochondria and chloroplasts within the bacterial domain of life, rather than with eukaryotic organisms. This genetic evidence is one of the most robust pieces of support for the endosymbiotic theory, as it directly links the organelles to their bacterial ancestors.
Additionally, the transfer of genes from the organelles to the host nucleus over evolutionary time has been documented. Many genes that were originally present in the mitochondrial and chloroplast genomes have been moved to the nuclear genome. This process, known as endosymbiotic gene transfer, has resulted in the current state where the organelles rely on the host nucleus for many essential proteins. The direction of this gene transfer, from organelle to nucleus, is consistent with the endosymbiotic model and provides a clear evolutionary trajectory.
The concept
The endosymbiotic theory
Mitochondria and chloroplasts descend from engulfed bacteria.
A common misconception is that the double membrane of mitochondria and chloroplasts implies they were engulfed twice. In reality, the double membrane is a result of the initial engulfment by the host cell, in which the inner membrane is the original bacterial plasma membrane and the outer one is either the host's engulfing vesicle or the bacterium's own outer membrane. This distinction is crucial for understanding the evolutionary history of these organelles. C
Worked example
Problem. The human mitochondrial genome is a circle of 16,569 base pairs carrying 37 genes: 13 encode proteins, 22 encode tRNAs and 2 encode rRNAs. (a) What fraction of the genes encode proteins? (b) What is the average length of genome per gene? (c) How does this compare with the nuclear genome, about \( 3.1 \times 10^9 \) base pairs for roughly 20,000 protein-coding genes?
- (a) Protein-coding fraction: \( 13/37 \approx 0.351 \), about 35%.
- (b) Genome per gene: \( 16\,569/37 \approx 448 \) base pairs.
- (c) Nuclear genome per protein-coding gene: \( 3.1 \times 10^9 / 2.0 \times 10^4 \approx 1.6 \times 10^5 \) base pairs, about 350 times more.
Units check. Base pairs divided by a count of genes gives base pairs per gene. The tightly packed, intron-free mitochondrial genome is one more bacterial trait.
Three quick tests
- What type of bacteria are the ancestors of mitochondria?
Answer
Alphaproteobacteria - What is the size of the ribosomes found in mitochondria and chloroplasts?
Answer
Bacterial-type: 70S in chloroplasts; mitochondrial ribosomes vary (about 55S in mammals) - What process allows mitochondria and chloroplasts to reproduce independently of the host cell?
Answer
Binary fission
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Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.