UT Arlington BIOL BIOL 3427 - ch19 (21 pages)

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ch19



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ch19

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21
School:
University of Texas at Arlington
Course:
Biol Biol 3427 - Plant Science
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ALGAE AND THE ORIGIN OF EUKARYOTIC CELLS Chapter 19 Life began about 3 5 billion years ago in the oceans with the appearance of prokaryotes The oldest reliable date for the appearance of the eukaryotes is about 1 9 billion years ago when the first members of a group of unicellular organisms called acritarchs appear in the fossil record in China Acritarchs Are probably the remains of a group of ancient eukaryotes Were plankton Some resemble dinoflagellates while others resemble green algae Their relationship among living organisms is uncertain http www ucl ac uk GeolSci micropal acritarch html http www geo arizona edu palynology ppacrtrc html Eukaryotic cells came into existence probably by a process called endosymbiosis Mitochondria arose first as an early eukaryotic cell engulfed but did not digest a bacterium capable of aerobic respiration The two organisms lived together one inside the other and both benefited Fungi plants and animals are all probably derived from protists Fungi and animals are eukaryotes organisms that lack plastids Another line of evolution one that had mitochondria entered another endosymbiosis with a photosynthetic cyanobacterium which later evolved into a chloroplast This line gave rise to algae including green algae which in turn produced true plants the embryophytes Several clades exist that still have some extant members whose plastids have numerous prokaryotic characters Chloroplasts of red algae especially resemble cyanobacteria The kingdom Protista contains eukaryotes that cannot be assigned with certainty to other kingdoms The kingdom Protista is an artificial grouping and classification does not represent evolutionary relationships This kingdom is also known as Protoctista Protists covered in this course are those photosynthetic organisms that function like plants in ecosystems They are the grass of the ocean Protists to be studied include 1 Algae photosynthetic organisms studied by phycologists 2 Slime molds and oomycetes heterotrophic organisms that are traditionally studied by mycologists although these organisms are not fungi Another group of protists not included in this course are the ciliates flagellates and other heterotrophs The phylogenetic relationship among the different groups of protists is controversial e g the relationship between the green and brown algae ORIGIN OF EUKARYOTIC CELLS DNA Structure In prokaryotes proteins do not surround the DNA Its numerous negative charges are neutralized by calcium ions In eukaryotes the DNA is packaged with histones forming nucleosomes The DNA condenses into chromosomes The genome is a short circle of DNA containing about 3 000 genes and lack introns In eukaryotes the DNA molecule carries thousands of genes The chromosomes of eukaryotes have a homologous and never occur as a single chromosome in normal circumstances Eukaryotic genes have introns which do not code for any type of RNA Nuclear structure and division Prokaryotic cells lack nucleus The DNA circle is attached to the plasma membrane As the cell grows and the plasma membrane expands the two daughter DNA molecules are separated The nuclei of plants animals and fungi are very similar in structure metabolism mitosis and meiosis Apparently these three clades diverged after the nucleus had achieved a high level of complexity In eukaryotes most of the DNA is found in the nucleus The nucleus is surround by two double layered membranes with nuclear pores A nucleolus is present The nuclei are typically haploid or diploid Mitosis assures that each daughter cell receives one of each type of chromosome to maintain the species number of chromosomes Meiosis usually occurs as part of sexual reproduction The pairing of paternal and maternal homologous chromosomes followed by crossing over and genetic recombination assures genetic diversity Some groups of organisms have a unique mitotic process that may represent an earlier divergence in the history of eukaryotes Organelles Prokaryotes lack membrane bound organelles They have ribosomes and storage granules which are notmembrane bound organelles Photosynthetic prokaryotes have folded plasma membrane that projects into the cytoplasm Eukaryotes have membrane bound organelles that compartmentalize the cell and perform different functions simultaneously Ribosomes of prokaryotes are 70S being smaller and denser than the 80S ribosomes of eukaryotes Flagella and cilia are uniform in eukaryotes having a 9 2 arrangement of microtubules A few prokaryotes have flagella and never have the 9 2 arrangement They are not composed of microtubules or tubulin Endosymbiotic Theory This hypothesis attempts to explain the origin of eukaryotic organelles mitochondria and chloroplasts In 1905 K C Mereschkowsky had speculated that plastids were prokaryotes living inside eukaryotic cells In the 1960s plastids and mitochondria were discovered to have their own DNA and ribosomes both with prokaryotic features Plastids and mitochondria divide similarly to prokaryotes They lack microtubules Their DNA is small and circular contains a small number of genes and is organized like prokaryotic DNA Their ribosomes are sensitive to the same antibiotics that interfere with prokaryotic ribosomes 1 Chloroplasts and mitochondria could have originated from bacteria that were phagocytized by a large heterotrophic prokaryote Mitochondria could have derived from an aerobic prokaryote that was ingested but not digested Chloroplasts could have been derived from a photosynthetic prokaryote probably a cyanobacterium Chloroplasts originated several times An endosymbionts is an organism that lives within another dissimilar organism 2 These bacteria were then adopted as endosymbionts rather than being digested 3 With time these endosymbionts became simplified and specialized to perform only photosynthesis or respiration 4 The DNA of the endosymbionts and many or its functions were transferred to the nuclear DNA The nuclear membrane could have originated from an infolding of the plasma membrane of a prokaryote Prokaryotes have their single circular chromosome attached to the plasma membrane Infolding of other portions of the plasma membrane may have given origin to the ER and Golgi complex Primary endosymbiosis gave rise to a clade containing red algae green algae and a small group called glaucophytes Glaucophyte chloroplasts still produce a thin film of cyanobacterial wall between themselves and the cell Red algal chloroplasts have chlorophyll a but not b


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