Eukaryotic Cell Biology and Eukaryotic Microorganisms

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• CHAPTER 14
• Eukaryotic Cell Biology and Eukaryotic
  Microorganisms

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Eukaryotic Cell Structure/Function
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• A typical eukaryotic cell is shown in Figure
  14.1. Eukaryotes contain a membrane-enclosed
  nucleus and several other organelles, the
  complement of which depends on the organism.

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• The nucleus contains the genome of the
  eukaryotic cell (Figure 14.2).

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Respiratory and Fermentative Organelles The
Mitochondrion and the Hydrogenosome
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• The mitochondrion (Figure 14.3) and the
  hydrogenosome (Figure 14.4) are energy-generating
  organelles of eukaryotic cells.

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Inner structure of mitochondrion
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TEM of mitochondrion
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TEM of hydrogenosomes (Trichmonas and ciliated
protozoa in rumen of animals) lack electron
transport chain and Citric acid cycle.
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Key enzymes of hydrogenosome - Pyruvateferredoxin
oxidoreductase and Hydrogenase Endosymbiotic
methanogens are present in the cytoplasm of
hydrogenosome-containing eukaryotes
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• Mitochondria are involved in aerobic
  respiration. Mitochondria possess a series of
  folded internal membranes called cristae. These
  membranes, formed by invagination of the inner
  membrane, are the sites of enzymes involved in
  respiration and ATP production.

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• The hydrogenosome, found only in certain
  obligately anaerobic eukaryotes, ferments
  pyruvate to yield H2 plus CO2, acetate, and ATP.

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Photosynthetic Organelle The Chloroplast

• The chloroplast is the site of photosynthetic
  energy production and CO2 fixation in eukaryotic
  phototrophs (algae). Like mitochondria,
  chloroplasts have a permeable outermost membrane,
  a much less permeable inner membrane, and an
  intermembrane space.

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• The inner membrane surrounds the lumen of the
  chloroplast, but it is not folded into cristae
  like the inner membrane of the mitochondrion
  (Cristae).
• Instead, chlorophyll and all other components
  needed for photosynthesis are located in a series
  of flattened membrane discs called thylakoids.

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Endosymbiosis Relationships of Mitochondria and
Chloroplasts to Bacteria
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• Key metabolic organelles of eukaryotes are the
  chloroplast, involved in photosynthesis, and the
  mitochondrion or hydrogenosome, involved in
  respiration or fermentation.
• These organelles were originally Bacteria that
  established permanent residence inside other
  cells (endosymbiosis).

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• Several lines of molecular evidence support the
  endosymbiotic theory
• Mitochondria and chloroplasts contain DNA.
• The eukaryotic nucleus contains bacterially
  derived genes.
• Mitochondria and chloroplasts contain their own
  ribosomes.

• Several antibiotics kill or inhibit Bacteria
  specifically by interfering with 70S ribosome
  function. These same antibiotics also inhibit
  protein synthesis in mitochondria and
  chloroplasts.

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• Phylogenetic studies using comparative ribosomal
  RNA sequencing methods and organellar genome
  studies have shown convincingly that the
  chloroplast and mitochondrion originated from the
  Bacteria.

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Other Organelles and Eukaryotic Cell Structures
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• Besides the major organelles of eukaryotes,
  several other structures with defined functions
  are present in the cytoplasm.

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• These include the
• endoplasmic reticulum, the site of ribosomes and
  cellular lipid syntheses
• the Golgi apparatus, involved in protein
  modification and secretion
• lysosomes, which play a role in macromolecular
  digestion
• and the peroxisome, an organelle involved in H2O2
  production.

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• In addition, proteinaceous tubes called
  microfilaments and microtubules are present,
  forming the cell's cytoskeleton.
• Flagella and cilia (Figure 14.10) are organelles
  of motility that have extensive microtubular
  structure.

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Whip-like motion Vs. Propeller on a motor boat
(bacteria)
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Cross section of flagellum
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Essentials of Eukaryotic Genetics and Molecular
Biology  Replication of Linear DNA
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• The ends of linear genetic elements present a
  problem to the replication machinery that
  circular genetic elements do not.
• Some prokaryotic and viral linear elements solve
  this problem by using a protein primer (Figure
  14.11).

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Protein Primer
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• Eukaryotes solve the problem by using a special
  enzyme called telomerase to extend one strand of
  the DNA (Figure 14.12).

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Overview of Eukaryotic Genetics

• Eukaryotic microorganisms can mate and exchange
  DNA during sexual reproduction. Mitosis ensures
  appropriate segregation of the chromosomes during
  asexual cell division.
• Haploid cells formed by meiosis can fuse to form
  a diploid zygote.

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• There are two mating types in yeast, and yeast
  cells can convert from one type to the other
  (Figures 14.14, 14.15).

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Switching of yeast mating types inserted
cassette determines the mating type a and alpha
factors binds to opposite mating type and bring
about changes
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RNA Processing and Ribozymes

• RNA processing, the processing of eukaryotic
  pre-mRNAs, is unique and involves three distinct
  steps splicing, capping, and tailing (Figure
  14.18).

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• Splicing is done by a complex of several
  ribonucleoproteins (enzymes that contain both RNA
  and protein), called the spliceosome.

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• Introns in some other transcripts are
  self-splicing, and the RNA itself catalyzes the
  reaction (Figure 14.19).
• RNA molecules with catalytic activity are called
  ribozymes and play an important role in the cell.

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Self-splicing ribozymal introns of the prozoan
Tetrahymena 413-NT intron
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Eukaryotic Microbial Diversity 
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• As determined by ribosomal RNA sequencing,
  eukaryotic cells form their own major line of
  evolutionary descent (the Eukarya) (Figure
  14.20a).

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• Some microbial eukaryotes, such as Giardia and
  Trichomonas, are early-branching species, and the
  eukaryotic "crown" of the tree contains the
  multicellular plants and animals.

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• Trees based on the comparative sequencing of
  other genes and proteins yield a different
  evolutionary picture (Figure 14.20b).

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Tree based on eukaryotic genes and proteins
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Protozoa

• Protozoa are unicellular microbial Eukarya that
  typically lack cell walls and are usually motile
  by various means. Table 14.1 lists
  characteristics of the major groups of protozoa.

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• Many protozoa are pathogenic to humans and other
  animals.

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• Most protozoa feed by ingesting particulate
  matter, usually other cells, by phagocytosis. In
  phagocytosis, the cell uses a portion of its
  flexible cell membrane to surround a food
  particle and bring it into the cell.

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• Flagellates are all motile by the activity of
  flagella.

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• The sarcodines include Amoebawhich are naked in
  the vegetative phaseand foraminiferaamoebae
  that secrete a shell during vegetative growth.

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• A variety of naked amoebae are parasites of
  humans and other vertebrates, and their usual
  habitat is the oral cavity or the intestinal
  tract. They move in these habitats by cytoplasmic
  streaming, called amoeboid movement.

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• Ciliates are protozoa that, in some stage of
  their life cycle, possess cilia, structures that
  function in motility.

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• Ciliates are also unique among protozoa in
  having two kinds of nuclei the micronucleus,
  which is involved only with inheritance and
  sexual reproduction, and the macronucleus, which
  is involved only in the production of RNA
  (transcription) or various aspects of cell growth
  and function.

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• Sporozoa are a large group of obligately
  parasitic protozoa. These parasites can cause
  severe diseases, such as malaria.

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Slime Molds

• Acellular slime molds are masses of motile
  protoplasm.

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• Cellular slime molds are masses of individual
  cells that aggregate to form fruiting bodies that
  release spores (Figure 14.29).

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Fungi

• Fungi include the molds and yeasts. Table 14.2
  gives the classification and major properties of
  fungi.

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• Fungi differ from protozoa in their rigid cell
  walls, production of spores, lack of motility,
  and phylogenetic position.

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• Fungal cell walls resemble plant cell walls
  architecturally but not chemically. Although the
  plant cell wall polysaccharide cellulose is
  present in the walls of certain fungi, most fungi
  contain chitin, a polymer of the glucose
  derivative N-acetylglucosamine, in their cell
  walls.

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• From the fungal mycelium, other hyphal branches
  may reach up into the air above the surface, and
  spores called conidia are formed on these aerial
  branches (Figure 14.30).

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A typical mold
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• Mushrooms are large, often edible fungi that
  produce fruiting bodies containing basidiospores
  (Figure 14.32).

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Algae

• Algae are phototrophic Eukarya that contain
  chlorophyll and carotenoid pigments within a
  chloroplast. The chloroplast itself has its roots
  in the Bacteria.