Question

Mitochondria and chloroplasts contain some DNA, which more closely resembles prokaryotic DNA than (eukaryotic) nuclear DNA. Use this information to suggest how eukaryotes may have originated.

Short answer

Eukaryotes may have originated from ancient symbiotic relationships between early prokaryotes, leading to the formation of organelles like mitochondria and chloroplasts.

Step-by-step solution

Understanding Mitochondria and Chloroplasts

Both mitochondria and chloroplasts are organelles within eukaryotic cells that contain their own circular DNA, which is similar to prokaryotic DNA.

Comparing DNA Types

Prokaryotic DNA is typically circular and not enclosed in a nucleus, whereas eukaryotic nuclear DNA is linear and housed within a nuclear membrane.

Endosymbiotic Theory

The endosymbiotic theory suggests that eukaryotic cells originated through a symbiotic relationship between early ancestral prokaryotes. These prokaryotes were engulfed by a host cell and developed into mitochondria and chloroplasts.

Evidence Supporting Endosymbiotic Theory

The similarities between the DNA of mitochondria/chloroplasts and that of prokaryotes support the idea that these organelles were once free-living prokaryotes that were incorporated into ancestral eukaryotic cells.

Conclusion

The presence of prokaryotic-like DNA in mitochondria and chloroplasts suggests that eukaryotes may have originated from a symbiotic relationship where ancient prokaryotes became integral parts of early eukaryotic cells.

Key concepts

mitochondria

Mitochondria are essential organelles found in nearly all eukaryotic cells. They are often referred to as the powerhouse of the cell because they generate most of the cell’s supply of adenosine triphosphate (ATP), which is used as a source of chemical energy. Interestingly, mitochondria contain their own genetic material in the form of circular DNA.
Mitochondrial DNA (mtDNA) shares similarities with the DNA of prokaryotes, such as bacteria. For example, mtDNA is circular in shape, unlike the linear DNA found in eukaryotic nuclei. This resemblance to prokaryotic DNA provides significant clues about the origin of these organelles.
Scientists believe that mitochondria originated from free-living bacteria that entered a symbiotic relationship with a host cell. Over time, these bacteria became permanent residents of the host cell, evolving into the mitochondria we see today.
The similarities in DNA structure between mitochondria and prokaryotes support this theory, as well as certain aspects of mitochondrial function, such as their unique methods of protein synthesis and replication, which are more similar to prokaryotic processes than eukaryotic ones.

chloroplasts

Chloroplasts are specialized organelles found in plant cells and certain algae. They are responsible for photosynthesis, the process by which light energy is converted into chemical energy stored in glucose.
Like mitochondria, chloroplasts contain their own circular DNA, which closely resembles the DNA found in prokaryotes. This resemblance extends to their ribosomes and the way they replicate, further supporting the idea that chloroplasts may have evolved from free-living bacteria.
The endosymbiotic theory suggests that an ancestral eukaryotic cell engulfed a photosynthetic bacterium, which then became a permanent resident, eventually evolving into the modern chloroplast. This theory is backed by the fact that chloroplast DNA is more similar to the DNA of cyanobacteria (a type of photosynthetic bacteria) than it is to the nuclear DNA of the plant cells in which chloroplasts are found.
This symbiotic relationship allowed early eukaryotic cells to harness the ability to perform photosynthesis, giving rise to plants and algae and significantly impacting the evolution of life on Earth.

endosymbiotic theory

The endosymbiotic theory is a widely accepted explanation for the origin of eukaryotic cells. This theory suggests that eukaryotic cells originated from a symbiotic relationship between primitive prokaryotic cells.
According to this theory, certain prokaryotic cells were engulfed by a larger host cell. Instead of being digested, these prokaryotic cells formed a symbiotic relationship with the host, greatly benefiting both parties. The engulfed cells provided the host with additional capabilities, such as more efficient energy production in the case of mitochondria or the ability to perform photosynthesis in the case of chloroplasts.
Over time, these engulfed prokaryotic cells became permanent fixtures within the host cell, evolving into the organelles we now recognize as mitochondria and chloroplasts. This endosymbiotic relationship was crucial in the development of complex eukaryotic cells, which gave rise to the diversity of life forms we see today.
The presence of prokaryotic-like DNA in mitochondria and chloroplasts, along with similarities in their ribosomes and reproductive mechanisms, provides strong evidence supporting this theory.

prokaryotic DNA

Prokaryotic DNA refers to the genetic material found in prokaryotic organisms, such as bacteria and archaea. Unlike the linear chromosomes of eukaryotes, prokaryotic DNA is typically circular and not enclosed within a nuclear membrane.
This circular DNA is relatively simple and compact, containing fewer non-coding regions (introns) compared to eukaryotic DNA. Prokaryotic cells lack a nucleus, so their DNA floats freely within the cell's cytoplasm in a region called the nucleoid.
The simplicity and structure of prokaryotic DNA play a crucial role in the endosymbiotic theory. The fact that mitochondria and chloroplasts contain circular DNA, similar to that of prokaryotes, supports the idea that these organelles originated from prokaryotic cells.
Furthermore, certain aspects of mitochondrial and chloroplast DNA replication and protein synthesis are more akin to those of prokaryotes than eukaryotes. This includes the presence of similar ribosomes and the use of similar mechanisms for gene expression and regulation.
These similarities add weight to the endosymbiotic theory, illustrating how complex eukaryotic cells may have evolved from simpler prokaryotic ancestors through a symbiotic relationship.