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Learning Goals
By the end of this reading you should be able to:
- List and describe the key characteristics of Eukaryotes
- Correlate energy production in prokaryotes to the changing conditions on early Earth
- Explain the evidence that supports the theory of endosymbiosis
- Describe the evidence to support the second endosymbiosis of plastids
Introduction
Living things fall into three large groups: Archaea, Bacteria, and Eukarya. The first two have prokaryotic cells, and the third contains all eukaryotes. A relatively sparse fossil record is available to help discern what the first members of each of these lineages looked like, so it is possible that all the events that led to the last common ancestor of extant eukaryotes will remain unknown. However, comparative biology of extant organisms and the limited fossil record provide some insight into the history of Eukarya. Data from these fossils have led comparative biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor.
Characteristics of Eukaryotes
Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor because these characteristics are present in at least some of the members of each major lineage. Some characteristics have been lost in some lineages over time.
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores.
- Mitochondria. Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria.
- A cytoskeleton containing the structural and motility components called actin microfilaments and microtubules.
- Flagella and cilia, organelles associated with cell motility. Some extant eukaryotes lack flagella and/or cilia, but they are descended from ancestors that possessed them.
- Chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them.
- Mitosis, a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton. Mitosis is universally present in eukaryotes.
- Sex, a process of genetic recombination unique to eukaryotes in which diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid nuclei and a subsequent stage where two haploid nuclei fuse together to create a diploid zygote nucleus.
The Endosymbiotic Theory
Eukaryotic cells are differentiated from prokaryotic cells by a variety of features, one of which is the presence of internal membrane-bound structures (ex: nucleus, mitochondria, chloroplasts). How did these internal structures evolve? Before we can delve into that we need to talk about energy because it is one of the driving forces of life and there are some key points related to energy metabolism in prokaryo