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 their own DNA, a double-membrane, and replicate via binary fission.

Step-by-step solution

Define the theory

The theory that chloroplasts and mitochondria may have once been bacteria is known as the endosymbiotic theory. This theory suggests that chloroplasts and mitochondria were once free-living prokaryotic organisms that were engulfed by a host cell, leading to a symbiotic relationship.

Identify three key features

According to the endosymbiotic theory, identify three specific features of chloroplasts and mitochondria that are consistent with this theory.

Feature 1 - Own DNA

Both chloroplasts and mitochondria contain their own DNA, which is separate from the host cell’s nuclear DNA. This DNA is circular, similar to bacterial DNA.

Feature 2 - Double Membrane

Both of these organelles have a double-membrane structure. The inner membrane is believed to be derived from the bacterial membrane, while the outer membrane is from the host cell membrane.

Feature 3 - Reproduction

Chloroplasts and mitochondria replicate through a process similar to binary fission, which is characteristic of bacteria.

Key concepts

chloroplast DNA

Chloroplast DNA is an essential component of the endosymbiotic theory. Chloroplasts are the structures within plant cells where photosynthesis takes place. They contain their own DNA, which is separate from the cell's nuclear DNA. This DNA is circular, which is typical of bacterial DNA, unlike the linear DNA usually found in the nuclei of eukaryotic cells.

The circular structure and independent nature of chloroplast DNA support the idea that chloroplasts were once free-living bacteria. Before being engulfed by a host cell, these prokaryotes must have had their own genetic material, just like modern bacteria. This supports the key notion of the endosymbiotic theory: that chloroplasts originated from bacteria and then formed a symbiotic relationship with a host cell.

mitochondrial DNA

Mitochondrial DNA (mtDNA) is another strong piece of evidence for the endosymbiotic theory. Mitochondria are often termed the powerhouses of the cell, generating the energy a cell needs to function. Like chloroplasts, mitochondria also contain their own circular DNA, separate from the host cell's nuclear DNA.

This circular mtDNA is similar to bacterial DNA, which indicates a shared ancestry with bacteria. Mitochondria likely originated from free-living bacteria that entered into a symbiotic relationship with a host cell. Over time, these bacteria evolved into mitochondria, which now function as integral parts of eukaryotic cells.

double membrane structure

The double membrane structure of both chloroplasts and mitochondria is a significant indicator of their bacterial origins. They each have two membranes:

• The inner membrane is thought to have come from the engulfed bacteria itself.
• The outer membrane is believed to be derived from the host cell's membrane.

The presence of these two membranes supports the idea that these organelles were once separate entities - bacteria - that were engulfed by another cell.

According to the endosymbiotic theory, the host cell's membrane wrapped around the bacteria when it engulfed them, resulting in a double membrane as seen today. This structure is significant evidence for their evolutionary history.

binary fission reproduction

Reproduction through binary fission is another critical feature tying chloroplasts and mitochondria to their bacterial origins. Binary fission is a simple method of cell replication used by bacteria in which a single cell divides into two identical cells.

Both chloroplasts and mitochondria replicate using a process remarkably similar to binary fission. During this process, the DNA within these organelles is duplicated, and the organelle splits into two, ensuring that each daughter cell gets a copy of the DNA. This method of replication aligns with the endosymbiotic theory, further supporting the idea that these organelles were once free-living bacteria. Their ability to reproduce independently within a host cell highlights their ancient origins.