Eukaryotes and their Origins Flashcards
Eukaryotic Origin
Evidence suggests that Eukaryotes evolved mid-Proterozoic eon. Before this, all life on Earth was prokaryotes (lacking nucleus and membrane-bound organelles:
- Endosymbiotic Theory
Endosymbiotic Theory
leading hypothesis for the origin of eukaryotes; eukaryotes arose as a result of the fusion of archaea cells with bacteria after the archaea cell engulfed an ancient, AEROBIC bacterial cell
- the endosymbiotic bacterial cell remained in the archaea cell in a mutualistic relationship
- the bacterium allowed the host Archaean cell to USE OXYGEN to release energy stored in nutrients and the host cell protected the bacterium from predators
- the descendants are present in all eukaryotic cells today as MITOCHONDRIA
Second Endosymbiosis Event
a secondary endosymbiosis event occurred in a group of green algae that engulfed photosynthetic cyanobacterium, leading to the origin of CHLOROPLASTS. it happened at least TWICE with different endosymbionts
1. common ancestor of supergroup Archaeplastida
2. Ancestor of small amoeboid rhizobia taxon
Evidence for Endosymbiotic Theory
- Size
- DNA
- Binary Fission
- Replacement of mitochondria + chloroplast
- Membranes
Evidence: Size
Mito + Chloro are approximately the SAME SIZE as prokaryotic cells, but are located inside larger eukaryotic cells instead of free-living
Evidence: DNA
Mito + Chloro each have their own DNA, organized in a circular chromosome like typical prokaryotic genomes, and their genomes contain genes similar to genes found in prokaryotic genomes
Evidence: Binary Fission
Mito + Chloro reproduce by BINARY FISSION, a process prokaryotes use to reproduce.
- Eukaryotic cells reproduce by mitosis
Evidence: Replacing Mito and Chloro
If mito + chloro are removed from a eukaryotic cell THE CELL HAS NO WAY TO PRODUCE NEW ONES
- genetic instructions to make new mito and chloro are not present in the eukaryotic nuclear genome as they are present in mito and chloro genomes
Evidence: Membrane
The membrane composition of mito + chloro are similar in composition to prokaryotic membranes than to eukaryotic membranes
Parsimony + The Theory
Since all eukaryotes have mitochondria but only photosynthetic eukaryotes have chloroplasts:
- FIRST, an ancestral eukaryote engulfed the bacteria
- SECOND, only in the plant/algae lineage a later descendant of this ancestral eukaryote then engulfed a cyanobacteria-like species
Unique Eukaryote Adaptations/Traits
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores
- Mitochondria
- Mitosis
- Meiosis + Sex
Eukaryotes: Nuclei
This trait is necessary and sufficient to define an organism as eukaryotic. All extant eukaryotes have cells with a nucleus.
Eukaryotes: Mitochondria
Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have mitochondria
Eukaryotes: Mitosis
a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton.
- Universally present in eukaryotes
Eukaryotes: Meiosis and Sex
Process of genetic recombination unique to eukaryotes in which diploid nuclei undergo meiosis to yield haploid nuclei and subsequent karyogamy, a stage where 2 haploid nuclei fuse together to create a diploid zygote nucleus
Sexual Reproduction
- sexual reproduction with meiosis is a defining feature in eukaryotes
- offspring get half of their DNA from 2 different parents
- always involved 2 changes in ploidy (# of copies of each chromosome
Sexual Reproduction: First Ploidy Change
- occurs via meiosis
- a cell division reduces ploidy by 1/2 from diploid to haploid (2n -> 1n)
- n: number of copies of each chromosome
Sexual Reproduction: Second Ploidy Change
doubling from 1n -> 2n by fertilization, or the joining of gametes (sperm and egg)
Haplontic Life Cycle
sexual life cycle where organisms have a multicellular haploid stage and the diploid stage only exists as a single cell (the fertilized egg)
- EX: fungi
Diplontic Life Cycle
sexual life cycle where organisms have a multicellular diploid stage and the haploid stage exists only as a single cell (the gametes)
- EX: animals
Haplodiplontic Life Cycle
- AKA alternation of generations
- organisms have both multicellular diploid and haploid stages
- EX: plants
Protist
any eukaryote that is not a plant, fungus, or animal (any eukaryote that falls outside these 3 monophyletic groups)
- they do not refer to a monophyletic group
Single-Celled Eukaryotes: Humans + Environment
- Human pathogens + parasites
- Plant pathogens and parasites
- Photosynthesis
- Decomposition
Single-Celled Eukaryotes: Photosynthesis
phytoplankton, single-celled photosynthetic organisms, are the primary producers of aquatic food webs:
1. Diatoms
2. Dinoflagellates
Diatoms
single-celled photosynthetic algae that generate ~20% of O2 produced on the planet and half the organic material in the ocean
Dinoflagellates
single-celled species like algae that are endosymbionts of corals and other animals
- play an essential part in coral reefs
Single-Celled Eukaryotes: Decomposition
organisms need to be broken down after they die to make inaccessible nutrients into simpler organic molecules
- Oomycetes (fungus-like protists) play an essential role in returning inorganic nutrients to soil and water to foster new plant growth
