Question

The endosymbiosis or endosymbiont theory is meant to explain the origin of two specific organelles found in eukaryotic cells.
1. Name the two organelles.
2. Provide two pieces of evidence that support this theory.

Answer 1

Endosymbiosis Theory – Endosymbiosis theory is a leading evolutionary theory that supports the the origin and evolution of eukaryotic cell and its organlles. Endosymbiosis theory states that the organlles of present day eukaryotic cells were once prokaryotic organisms. This theory also elucidates the origin of eukaryotic cells began when a amoeba-like single cell organism comprised of a single nucleus (which basically formed when a piece of cytoplasmic membrane pinched off around the chromosomes) consumed some of the other prokaryotic organisms through phagocytosis, notwithstanding the ingested microorganisms survived within the amoeba-like organism and eventually developed symbiosis with it. After a long period of time and gradual evolution, the whole symbiotic system turns into a single cell like system, known as eukaryotic cell at present day.

1. Name of the two organlles – Mitochondria and Chloroplast.

Endosymbiosis theory suggests that the origin of mitochondria started when an amoeba-like microorganism phagocytosized a bacteria capable of aerobic respiration, but it survived and developed a symbiotic relationship with the organism. Later, with gradual evolution it became an important part of the whole system, known as mitochondria of the eukaryotic cell.

The theory also suggests that the origin of chloroplast began when an amoeba-like bacteria ingested a photosynthetic bacteria which survived there and developed symbiotic relationship with the organism and later became a significant part of the photosynthetic eukaryotic cells known as chloroplast.

2. Two evidences that support the theory –

• Chloroplast and mitochondria both show the characteristics of a prokaryotic cell. The size of both of the organlles are similar to that of a prokaryotic cell. Mitochondria and chloroplast both divide through binary fission, and Fts proteins or homologs are found at the division plate. Mitochondria doesn't have nuclear membrane which is a contrasting factor compared to the fact that most of the primitive eukaryotic cells or microbes used to have nuclear membrane.
• Mitochondria and chloroplast have their own DNA. And like other prokaryotic organisms, the DNA is circular, not linear like eukaryotic cells. Mitochondria and chloroplast also possess their own ribosomes that is comprised 30S and 50S subunits similar to that of the prokaryotic organisms, and not like 60S and 40S subunits which are actually found in eukaryotic cells.

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