Problem 10 - Biology: The Dynamic Science

Question

Which of the following is not part of the evidence supporting the endosymbiotic theory? Both mitochondria and plastids: a. are each the size of many bacterial cells. b. have inner membranes with a chemical composition similar to that of a bacterial plasma membrane. c. have ribosomes that are similar to bacterial ribosomes. d. contain circular DNA e. have DNA similar to nuclear DNA.

Step-by-Step Solution

Answer

Option (e), stating that mitochondria and plastids have DNA similar to nuclear DNA, is not part of the evidence supporting the endosymbiotic theory.

Step 1: Examine Characteristics of Mitochondria and Plastids

Review the characteristics of mitochondria and plastids that support the endosymbiotic theory. These include being similar in size to bacterial cells, having inner membranes with a chemical composition akin to bacterial plasma membranes, containing circular DNA, and having ribosomes that resemble bacterial ribosomes.

Step 2: Rule Out Valid Evidence

Eliminate options that are known to be true for both mitochondria and plastids and support the endosymbiotic theory. Options (a), (b), (c), and (d) are indeed traits shared with bacteria and support the endosymbiotic theory.

Step 3: Identify Incorrect Evidence

Determine which option does not support the endosymbiotic theory. The DNA within mitochondria and plastids is different from nuclear DNA, indicating that they may have originated from independent bacterial ancestors, which were engulfed by ancestral eukaryotic cells.

Key Concepts

Mitochondria and Plastids Characteristics

Mitochondria and plastids are key organelles within eukaryotic cells, each play essential roles in cellular processes. Mitochondria, often referred to as the 'powerhouses of the cell,' are crucial for cellular respiration, converting nutrients into usable energy in the form of ATP. Plastids, including chloroplasts, are involved in photosynthesis, storage of products like starch, and synthesis of fatty acids, amino acids, and pigments.

Both mitochondria and plastids share several defining characteristics that mirror those of bacterial cells. They are similar in size to typical bacterial cells and have their own DNA, which is circular, just as in bacteria. This DNA carries genes essential for the organelles' functions. Additionally, their inner membranes have a chemical makeup that is similar to that of bacterial plasma membranes, indicating a shared evolutionary origin.

The ribosomes found in mitochondria and plastids are more akin to bacterial ribosomes than to those found in the cytoplasm of eukaryotic cells, suggesting they evolved from an ancient bacterial lineage. These organelles' ability to reproduce independently within the cell via a process similar to binary fission—an attribute of bacteria—further supports their prokaryotic heritage.

Mitochondria and Bacterial Similarities

The similarities between mitochondria and bacteria extend beyond size and internal composition. The relationship is most notably reflected in their genetic makeup. Mitochondria contain their own genome, which is separate from the nuclear DNA of the eukaryotic host cell. This genome is not only circular in structure, reminiscent of bacterial DNA, but it also exhibits a comparable genetic sequence to certain bacteria.

Furthermore, the process of protein synthesis within mitochondria adheres to the bacterial model. The antibiotics that inhibit bacterial ribosomes can also affect mitochondrial ribosomes, implying a common evolutionary origin. Additionally, the lipid composition of mitochondrial membranes is akin to that found in bacterial membranes, and not the eukaryotic cell membrane. These analogies considerably strengthen the argument that mitochondria are of bacterial lineage.

Evidence for Endosymbiosis

The endosymbiotic theory proposes that mitochondria and plastids were once independent prokaryotic organisms that were engulfed by an ancestral eukaryotic cell. Over time, a symbiotic relationship developed, with the host cell providing protection and nutrients, and the engulfed prokaryotes offering additional metabolic capabilities, such as ATP production or photosynthesis.

The presence of double membranes surrounding these organelles provides compelling evidence for this theory. It suggests that the organelles were taken in by the precursor eukaryotic cell through a process similar to phagocytosis, resulting in an outer membrane derived from the host and an inner membrane inherited from the original prokaryote.

Genetic evidence supports endosymbiosis as well; both mitochondria and plastids have retained their own genomes that more closely resemble bacterial DNA than that of the host's nuclear DNA. Phylogenetic studies tracing the genetic sequences of specific organelle genes reveal that they are most closely related to particular groups of bacteria. This genetic affiliation, alongside the structural and functional similarities to bacteria, largely substantiates the endosymbiotic origin of mitochondria and plastids.