Biology 1010 Lecture Notes

Unit 2. Origin and Early Evolution of Eukaryotes

These notes were last updated September 28, 2001

Some key words, phrases, and ideas :

• autogenous vs endosymbiotic
• prokaryote vs eukaryote
• eukaryotic nucleus, chromosome, nucleolus
• eukaryotic endomembrane system:  endoplasmic reticulum, Golgi complex, transport vesicles
• eukaryotic cytoskeleton: microtubules, microfilaments, flagella and cilia, amoeboid movement
• eukarytotic mitochondria and chloroplasts; primary vs secondary chloroplasts
• asexual vs sexual reproduction; alternation of generations and haploid vs diploid stages
• gene pools, alleles, allele frequency; dominant vs recessive alleles
• Hardy-Weinberg equilibrium
• directional vs stabilizing vs disruptive selection; genetic bottlenecks and founder effects; genetic drift

1. Introduction--the status of life at the end of the Archean
1. rise of cyanobacteria
1. increased oxygen in the atmosphere
2. selective advantage for aerobic respiration
3. obligate anaerobes in retreat
2. Structure and origins of eukaryote life (late Archean to early Proterozoic
1. autogenous development of more complex internal structures
1. endomembrane system with numerous compartments--localization and specialization
1. membrane-bound nucleus containing DNA packaged into chromosomes (mixtures of DNA and special proteins called histones)
2. endoplasmic reticulum--associated with some protein synthesis (membrane enclosed and/or bound proteins--rough ER) and lipid metabolism (smooth ER)
3. Golgi complex--associated with processing material for export
4. lysosomes--often associated with digestion and phagocytosis
2. ribosomes of a different form than those found in bacteria and archaeans
3. cytoskeleton
1. microtubules of tubulin formed at microtubule organizing centers
1. help provide shape and framework to the cell
2. serve as road-way for structures attached to motor molecules (dynein)
3. provide the basis for movement of the cell (flagella and cilia)
2. microfilaments of actin
1. help provide shape and framework to the cell
2. serve as road-way for structures attached to motor molecules (myosin)
3. provide a basis for movement of the cell (pseudopodia and amoeboid movement and the change from a gel to a sol state)
2. endosymbiotic acquistion and development of mitochondria and plastids
1. mitochondria probably acquired once; evidence includes
1. mitochondrial structure: ribosomes, DNA form, position of electron transport chains, double membranes
2. DNA sequences
2. plastids acquired as primary plastids (from bacteria) or secondary plastids (from eukaryotes)
1. evidence for primary plastids
1. basic structure:  ribosomes, DNA form, double membranes
2. intermediates with peptidoglycan
3. DNA sequences
2. secondary plastids
1. plastids with three or four membranes
2. intermediates with reduced nuclei in the plastid
3. DNA sequences
1. by middle of the Proterozoic, probably at least five distinct lineages (kingdoms?)
1. the alveolates
1. ciliates--highly developed protists characterized by the presence of numerous cilia and two types of nuclei; many contain endosymbiotic algae
2. dinoflagellates--a major group in the phytoplankton characterized by the presence of two types of flagella (one ribbon-like in a girdle, the other normal) and a cell wall often taking the form of armor plates; many contain secondary brown or green plastids, tertiary plastids are also known; some are capable of bioluminescence; some form red tides, which may be toxic
3. sporozoans--intracellular parasites of other eukaryotes; Plasmodium species are the cause of malaria in humans
2. the sarcomastigotes, including most of the zooflagellates and amoeboid forms; eventually leads to multicellular animals
1. foraminiferans--major group in the marine zooplankton, characterized by an amoeboid body with a calcium carbonate shell; their numerous shells provide important deposits that allow us to date geological events
2. acellular slime molds--characterized by the ability to form a multinucleate, acellular, amoeboid mass; in bad times the mass forms sporangia and spores
3. the euglenoids and a few zooflagellate groups (Trypanosoma, Leishmannia)--euglenoids are characterized by the presence of one (two) flagella, a prominent eyespot, and a proteinaceous pellicle (wall); many forms also contain secondary green plastids
4. the stramenopiles, defined in part by a unique flagellar arrangement (in cells with flagella) in which one of the two flagella is decorated with small hair-like structures (tinsel flagellum); the group includes the chromistan algae (brown algae, yellow-green algae, diatoms) and water molds
1. golden-brown algae--common flagellated phytoplankton with brown secondary plastids
2. diatoms--important group of phytoplankton characterized by the presence of a siliceous cell wall and brown secondary plastids; large deposits of diatom shells are mined and used for a variety of purposes
3. brown algae (kelp)--mostly multicellular, mostly marine seaweeds with brown secondary plastids; common brown seaweeds; some are harvested for chemicals (alginates, iodine, mannitol) or for food
4. yellow-green algae--common algae with yellow-green secondary plastids
5. water molds (oomycetes)--heterotrophic group, usually forming filamentous masses; common decomposers (saprophytes) and plant parasites
5. a lineage that will eventually lead to the plants, but including red algae, glaucophytes, and green algae
1. glaucophytes--small group characterized by bluish primary plastids that appear to be intermediate between cyanobacteria and true plastids
2. red algae--large group of mostly marine, mostly multicellular seaweeds with reddish primary plastids; widely grown for chemicals (carrageenan, agar) and food
3. green algae--large and diverse group including many microscopic algae and macroscopic green seaweeds, all with green primary plastids; some of the macroscopic forms are important in reef-formation; a few of the microscopic forms are grown for food and/or chemicals (the pinkish color in salmon); included in the green algae is the base group of all higher plants
6. a lineage that will eventually lead to the fungi
3. Variation in unicellular eukaryotes
1. asexual cell cycles
1. basic phases and stages
1. interphase--usually divided into three stages; these can be monitored using microspectrophotometers
1. G1--growth/normal metabolism
2. S--DNA replication
3. G2--preparation for division
2. nuclear division (mitosis)--usually similar to the basic mitosis taught in high school, but many lineages have their own unique brand
3. cytokinesis--division of the cytoplasm into separate cells
2. expected results--two (or, in some cases, more) offspring genetically identical to the original cell
3. sources of variation
1. mutation (mistakes in DNA replication)--more common than in bacteria because the genome is larger, less common because the cell cycle is longer
2. transduction
3. endosymbiosis(?)
2. sexual cycles
1. What is sex?
2. life cycles of simple eukaryotes with sexual stages
1. haploid stage (sometimes called the gametophyte stage)
1. vegetative cells are capable of reproducing by asexual cell division; sometimes forms multicellular structures
2. in bad conditions some of the cells are transformed into gametes (cells capable of fusing with other cells)
2. diploid zygote--results from the fusion of gametes
1. in some cases zygotes undergo meiosis to produce haploid vegetative cells
2. in some cases zygotes undergo asexual cell division to form diploid cells (sometimes called the sporophyte stage); under certain conditions the diploid cells undergo meiosis to produce haploid cells
3. meiosis in sexual life-cycles
1. requires a diploid cell with two copies of every chromosome (homologous pairs)
2. basic phases and stages
1. interphase I--follows the same basic scheme as interphase in asexual cell division
2. nuclear division I/cytokinesis I--follows the same basic scheme as mitosis in asexual cell division except that:
1. during prophase the homologous chromosomes are first linked to each other, then they exchange pieces (crossing over)
2. at anaphase, the pairs are separated
3. expected result--two haploid cells, each with a mixture of chromosomes from the haploid parent, each with twice as much DNA as the haploid parent
3. interphase II--no DNA replication
4. nuclear division II--follows the same basic scheme as mitosis in asexual cell division; expected result--four haploid cells with the same amount of DNA as the haploid parent (half as much as the diploid cell that started it all)
3. sources of variation
1. mutation
2. independent assortment of chromosomes into haploid cells--new mixture of chromosomes and genes
1. examples
3. fusion of haploids into diploids--yet another mixture of chromosomes and genes
1. examples
3. variation in sexual life-cycles
1. haploid dominant cycles:  the main stage is haploid; zygotes undergo meiosis forming new haploids
2. diploid dominant cycles: the main stage is diploid; meiosis results in haploid gametes which fuse to form diploid zygotes
3. alternating generations
4. Are sexual life-cycles beneficial?
4. Sexual life-cycles and evolution--a brief introduction to population genetics
1. some terminology
1. populations
2. genes and alleles
3. allele frequencies
2. natural selection in asexual populations
1. haploid stages
2. diploid stages
1. dominant alleles vs recessive alleles
2. homozygotes vs heterozygotes
3. natural selection in sexual populations, especially with diploid dominant life-cycles
1. calculating allele frequencies across diploid generations
1. example: no selection
2. example: calculating frequencies when heterozygotes unknown
3. example: effects of natural selection
2. some conclusions
1. Hardy-Weinberg conditions and genetic equilibrium
2. possible results of selection
3. possible effects of small populations

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