Question

What key aspects of chloroplasts (and mitochondria) are consistent with the theory that they may have once been bacteria? List three specific features.

Short answer

Chloroplasts and mitochondria have circular DNA, bacterial-like ribosomes, and replicate via binary fission.

Step-by-step solution

Explore the Endosymbiotic Theory

Understand that the Endosymbiotic Theory suggests that chloroplasts and mitochondria originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. This theory is supported by several key aspects.

Analyze DNA Presence

Consider that chloroplasts and mitochondria contain their own circular DNA, which is similar to bacterial DNA. This suggests a common evolutionary origin with bacteria.

Examine Ribosomes

Both chloroplasts and mitochondria have their own ribosomes that resemble those found in bacteria, rather than those found in the eukaryotic host cell. This is another feature that supports their bacterial past.

Study Reproduction Mechanism

Chloroplasts and mitochondria replicate through a process similar to binary fission, which is the method used by bacteria. This mode of replication is consistent with their proposed evolutionary history as free-living bacteria.

Key concepts

chloroplasts and mitochondria

Let’s start with the fascinating organelles, chloroplasts and mitochondria, which carry out essential functions in cells. Chloroplasts are primarily found in plant cells and are responsible for photosynthesis, converting light energy into chemical energy. Mitochondria, on the other hand, are known as the powerhouse of the cell, generating ATP through cellular respiration.
Both of these organelles have features that hint at their ancient origins as free-living bacteria. This concept is eloquently described by the Endosymbiotic Theory. According to this theory, chloroplasts and mitochondria were once independent bacteria that were engulfed by early eukaryotic cells. Over time, they formed a symbiotic relationship with their host cells.
One striking piece of evidence is that both chloroplasts and mitochondria have double membranes. This suggests an engulfing event, where the outer membrane is derived from the host cell's membrane, and the inner membrane originates from the engulfed bacterium.

bacterial DNA

Another compelling piece of evidence supporting the Endosymbiotic Theory is the presence of bacterial DNA within chloroplasts and mitochondria. Unlike other organelles, these two contain their own small, circular DNA molecules, much like modern-day bacteria. This DNA is distinct and independent from the nuclear DNA of the host eukaryotic cell.
These circular DNA molecules govern the production of essential proteins and enzymes required for the organelles' functions. The similarity in structure to bacterial DNA suggests that these organelles share a common ancestry with bacteria.
Additionally, the DNA replication process in chloroplasts and mitochondria mirrors that of bacteria, further reinforcing the idea of their bacterial origin. They reproduce their DNA through a simple, direct process distinct from the more complex mechanisms found in the eukaryotic nucleus.

ribosomes

Chloroplasts and mitochondria also have their own ribosomes, which are more similar to bacterial ribosomes than to those found in the surrounding eukaryotic cell cytoplasm. Ribosomes are essential cellular machinery responsible for protein synthesis.
In size and structure, the ribosomes within chloroplasts and mitochondria are more akin to the 70S ribosomes of bacteria, rather than the 80S ribosomes typically found in eukaryotic cells. This similarity provides further support for the Endosymbiotic Theory.
The fact that these organelles have maintained their own ribosomes—distinct from their host cells’—implies that they still carry out some level of independent protein synthesis. This characteristic is a strong indicator of their bacterial past.

binary fission

Lastly, the replication mechanism of chloroplasts and mitochondria bears a strong resemblance to bacterial binary fission. Binary fission is the process by which bacteria replicate, involving the duplication of their circular DNA followed by the division of the cell.
Similarly, both chloroplasts and mitochondria reproduce within their host cells through a form of binary fission. During this process, the organelles grow, replicate their DNA, and divide into two, much like bacterial cells.
This method of reproduction is starkly different from the mitotic processes of the host eukaryotic cell and aligns with the theory that these organelles were once free-living bacteria. The preservation of such a fundamental trait through evolution highlights their ancient origins and symbiotic adaptation.