Question

The endosymbiotic theory states that mitochondria and chloroplasts evolved from prokaryotes living within other prokaryotes (see p. 116). What aspects of the eukaryotic genome support this theory? What aspects do not support the theory?

Short answer

The endosymbiotic theory is supported by the fact that chloroplasts and mitochondria have their own, prokaryote-like DNA, and can reproduce independently within the cell, behaving like bacteria. However, the theory is refuted by evidence of gene transfer, indicating that over time, many organelle genes have moved into the cell nucleus.

Step-by-step solution

Understanding Endosymbiotic Theory

The endosymbiotic theory suggests mitochondria and chloroplasts evolved as result of a symbiotic relationship between two free-living prokaryotes, one moving inside the other, then evolving over time into the modern day organelles we see today. This happened through a process called endosymbiosis.

Analysis of Eukaryotic Genome As A Proof

Some aspects of the eukaryotic genome support the endosymbiotic theory. Chloroplasts and mitochondria have their own DNA, which is circular and similar to prokaryotic DNA. This means they can reproduce independently within the cell through a process that looks a lot like binary fission, a method used by modern bacteria. Ribosomes of mitochondria and chloroplasts are also more similar to prokaryotic ribosomes than the rest of the eukaryotic cell's.

Analysis of Eukaryotic Genome As A Disproof

Some aspects of the eukaryotic genome do not support the theory. Over millions of years, many genes presumably originally present in the prokaryotic symbionts have been transferred to the nuclear genome of the host. This is called gene transfer. It explains why mitochondria cannot live independently today, contradicting the theory indirectly.

Key concepts

Mitochondria

Mitochondria are fascinating organelles found in most eukaryotic cells. They are renowned as the "powerhouses of the cell" because they generate the energy cells need in the form of ATP. One striking feature of mitochondria is that they carry their own genetic material. This mitochondrial DNA (mtDNA) is circular, resembling the DNA found in prokaryotic organisms such as bacteria. This similarity supports the endosymbiotic theory.

• Mitochondria have two membranes, similar to prokaryotic cells.
• Their production involves a unique form of division reminiscent of bacterial binary fission.
• Mitochondrial ribosomes more closely resemble those of bacteria than those in the eukaryotic cytoplasm.

This organelle's features provide a window into primitive endosymbiotic events that shaped today's complex cells.

Chloroplasts

Chloroplasts are another key player in the endosymbiotic theory. Predominantly found in plant cells and some algae, chloroplasts enable photosynthesis, converting sunlight into usable energy for the cell. Chloroplasts, like mitochondria, possess their own DNA, further indicating that they originated as independent prokaryotic organisms.

• Their DNA is also circular and similar to that of prokaryotes.
• Chloroplast ribosomes resemble those found in bacteria.
• The process of photosynthesis occurs in stacked membranes called thylakoids, resembling internal membranes in certain bacteria.

These traits underline the ancient symbiotic relationship between host cells and the ingested prokaryotes that eventually became chloroplasts.

Prokaryotic DNA

Prokaryotic DNA is a significant piece of the puzzle in understanding the endosymbiotic theory. Consisting mainly of circular DNA, it differs greatly from the linear DNA found in eukaryotic nuclei. The presence of circular DNA within mitochondria and chloroplasts strengthens the argument that these organelles were once independent prokaryotic organisms.

• Circular DNA enables a simpler, more efficient replication process.
• The inheritance of mitochondrial and chloroplast DNA is mostly maternal, further echoing traits seen in prokaryotic replication.
• This type of DNA is packaged without proteins involved in eukaryotic DNA structuring, like histones.

These aspects of prokaryotic DNA are evidence supporting the endosymbiotic theory, painting a vivid picture of how cellular evolution might have occurred.

Eukaryotic Genome

The eukaryotic genome provides both supporting and contradictory evidence for the endosymbiotic theory. It is comprised mostly of linear DNA found in a true nucleus. Yet, some eukaryotic features, such as their RNA processing mechanisms, differ significantly from those in prokaryotes.

• Mitochondria and chloroplasts contain their own genomes, separate from the nuclear DNA.
• Evidence supports that over time, much of the original symbionts' DNA was transferred to the host's nucleus.
• Eukaryotic cells have evolved to become highly compartmentalized, a feature not seen in simpler prokaryotic cells.

These clues provide insight into how eukaryotic genomes display remnants of ancient endosymbiotic events, while showcasing unique developments in complexity and specialization.

Gene Transfer

Gene transfer is a crucial concept that both strengthens and complicates the endosymbiotic theory. Over evolutionary time, genes have moved from the genome of symbiont organelles like mitochondria and chloroplasts to the host cell’s nuclear genome. This gene transfer process has allowed these organelles to integrate more fully into the host cell system.

• Horizontal gene transfer is a method where genes from the symbionts were incorporated into the host genome over time.
• This transfer explains why some proteins necessary for organelle function are now encoded by nuclear DNA.
• Such gene migration suggests a complex evolutionary relationship between host and symbiont.

Understanding gene transfer helps clarify the blurred lines of independence between organelles and their host cells, revealing the interconnectedness resulting from evolutionary processes.