ColeochaetalesCharalesUnique Features Of CellsComplicated Life HistoriesThe ThallusPigmentsKelpsLife CycleThe Walls Of Diatoms Consist Of Two HalvesReproduction In Diatoms Is Mainly AsexualDivision DinophytaSome Terrestrial Oomycetes Are Important Plant PathogensChapter 19 ALGAE AND THE ORIGIN OF EUKARYOTIC CELLSLife 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 uncertainhttp://www.ucl.ac.uk/GeolSci/micropal/acritarch.htmlhttp://www.geo.arizona.edu/palynology/ppacrtrc.htmlEukaryotic 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 kingdomsThe 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 CELLSDNA StructureIn 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 neveroccur as a single chromosome in normal circumstances. Eukaryotic genes have introns, which do not codefor any type of RNA.Nuclear structure and divisionProkaryotic cells lack nucleus. The DNA circle is attached to the plasma membrane. As the cell grows andthe 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.OrganellesProkaryotes lack membrane bound organelles. They have ribosomes and storage granules, which are not-membrane 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 withprokaryotic 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