Question

Why was finding a close relationship between mitochondrial DNA and bacterial DNA considered particularly strong evidence in favor of the endosymbiosis theory?

Short answer

The close relationship between mitochondrial DNA (mtDNA) and bacterial DNA provides strong evidence for the endosymbiosis theory because it indicates that mitochondria share a common ancestor with bacteria, rather than being native to eukaryotic cells. This supports the idea that mitochondria were once independent prokaryotic organisms engulfed by another cell, forming a symbiotic relationship and eventually becoming an essential part of the eukaryotic cell. If the endosymbiosis theory was not valid, mitochondrial DNA would be more closely related to nuclear DNA of eukaryotic cells, which is not the case.

Step-by-step solution

Brief introduction to the endosymbiosis theory

The endosymbiosis theory is a widely accepted theory that suggests that some organelles in eukaryotic cells, such as mitochondria and chloroplasts, were once independent prokaryotic organisms that were engulfed by another cell, forming a symbiotic relationship. Over time, the engulfed organism lost its autonomy and became a part of the host cell, providing specific functions to the cell.

Understanding mitochondrial DNA and bacterial DNA

Mitochondrial DNA (mtDNA) is a small circular DNA molecule found in the mitochondria of eukaryotic cells. Bacterial DNA is also in the form of circular molecules. Mitochondrial DNA is inherited separately from the nuclear DNA and is only passed down from the mother. Bacterial DNA, similar to mitochondrial DNA, is not contained within a nucleus.

Finding the relationship between mitochondrial DNA and bacterial DNA

When researchers started sequencing the genomes of different organisms, they found that mitochondrial DNA sequences were more closely related to bacterial DNA sequences than to the nuclear DNA of eukaryotic cells. This finding indicates that mitochondria share a common ancestor with bacteria, rather than being native to the eukaryotic cell.

The importance of this relationship in supporting the endosymbiosis theory

The close relationship between mitochondrial DNA and bacterial DNA provides strong evidence for the endosymbiosis theory because it suggests that mitochondria originated from an engulfed prokaryotic organism. If the endosymbiosis theory was not valid, then we would expect mitochondrial DNA to be more closely related to the nuclear DNA of eukaryotic cells. The fact that the relationship between mitochondrial DNA and bacterial DNA is closer implies that they share a common ancestry, supporting the idea that mitochondria were once independent prokaryotic organisms that were engulfed by another cell, eventually becoming an essential part of the eukaryotic cell.

Key concepts

Mitochondrial DNA

Mitochondrial DNA (mtDNA) is a unique type of genetic material found in the mitochondria, an essential organelle within eukaryotic cells responsible for energy production. Unlike nuclear DNA, which is inherited from both parents and housed within a cell's nucleus, mtDNA is only inherited from the mother. This is because the mitochondria in sperm are typically not involved in fertilization.

MtDNA exists as a small, circular molecule, which interestingly resembles the genetic material found in many bacteria. This resemblance is one of the fascinating aspects that supports the Endosymbiosis Theory. The presence of mtDNA within mitochondria suggests that these organelles may have once been free-living prokaryotic organisms that were incorporated into early eukaryotic cells. Over time, these organisms became an integral part of the cell, remaining as mitochondria that perform critical functions in cellular respiration and energy conversion.

• Small and circular structure similar to bacterial DNA
• Exclusively maternally inherited
• Suggests a common ancestry with bacteria

Prokaryotic Organisms

Prokaryotic organisms are simple, single-celled life forms that lack a defined nucleus. Their genetic material is not enclosed within a membrane-bound nucleus but instead exists in a single, circular strand of DNA. This characteristic is shared among all bacteria and archaea, the two main domains of life classified as prokaryotes.

The simplicity of these organisms makes them extraordinarily adaptable and capable of surviving in diverse environments. Bacterial cells, for instance, were likely among the first forms of life on Earth. Their structural similarities to mitochondrial DNA raise intriguing questions about the origins of eukaryotic cells, which are far more complex.

• Simple cell structure without a nucleus
• Contains circular DNA
• Includes bacteria and archaea
• Highly adaptable to environmental changes

Symbiotic Relationship

A symbiotic relationship refers to a long-term interaction between two different biological species. These relationships can be mutually beneficial, where both organisms gain advantages from each other. The Endosymbiosis Theory illustrates this type of cooperation perfectly, suggesting that mitochondria and chloroplasts within eukaryotic cells originated from prokaryotic organisms.

In this relationship, the host cell provided the engulfed prokaryote with protection and a controlled environment, while the prokaryote contributed essential metabolic processes. For mitochondria, these processes include energy production through cellular respiration, which is fundamental for the host cell's survival. Over millions of years, this symbiotic relationship led to a merging of the two entities into a single, more complex cell.

• Interaction between different biological species
• Can be mutually beneficial
• Key component of the Endosymbiosis Theory

Eukaryotic Cells

Eukaryotic cells are complex cells characterized by a defined nucleus and internal compartments called organelles, each performing specific functions. These cells make up all complex organisms, including animals, plants, fungi, and protists.

The presence of organelles such as mitochondria distinguishes eukaryotic cells from prokaryotic cells. The Endosymbiosis Theory suggests that these organelles originated from free-living prokaryotes. Eukaryotic cells are integral to the structure and function of larger life forms, providing a platform for more complex biological processes.

• Complex internal structure with organelles
• Contains a nucleus with DNA
• Makes up all complex life forms