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CHAPTER 14 Eukaryotic Cell Biology and Eukaryotic Microorganisms. Eukaryotic Cell Structure/Function. 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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CHAPTER 14 Eukaryotic Cell Biology and Eukaryotic Microorganisms
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.
The nucleus contains the genome of the eukaryotic cell (Figure 14.2).
Respiratory and Fermentative Organelles: The Mitochondrion and the Hydrogenosome
The mitochondrion (Figure 14.3) and the hydrogenosome (Figure 14.4) are energy-generating organelles of eukaryotic cells.
TEM of hydrogenosomes (Trichmonas and ciliated protozoa in rumen of animals) – lack electron transport chain and Citric acid cycle.
Key enzymes of hydrogenosome - Pyruvate:ferredoxin oxidoreductase and Hydrogenase Endosymbiotic methanogens are present in the cytoplasm of hydrogenosome-containing eukaryotes
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.
The hydrogenosome, found only in certain obligately anaerobic eukaryotes, ferments pyruvate to yield H2 plus CO2, acetate, and ATP.
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.
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 calledthylakoids.
Endosymbiosis: Relationships of Mitochondria and Chloroplasts to Bacteria
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).
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.
Phylogenetic studies using comparative ribosomal RNA sequencing methods and organellar genome studies have shown convincingly that the chloroplast and mitochondrion originated from the Bacteria.
Besides the major organelles of eukaryotes, several other structures with defined functions are present in the cytoplasm.
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.
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.
Whip-like motion Vs. Propeller on a motor boat (bacteria)
Cross section of flagellum
Essentials of Eukaryotic Genetics and Molecular Biology Replication of Linear DNA
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).
Eukaryotes solve the problem by using a special enzyme called telomerase to extend one strand of the DNA (Figure 14.12).
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.
There are two mating types in yeast, and yeast cells can convert from one type to the other (Figures 14.14, 14.15).
Switching of yeast mating types – inserted cassette determines the mating type – a and alpha factors binds to opposite mating type and bring about changes
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).
Splicing is done by a complex of several ribonucleoproteins (enzymes that contain both RNA and protein), called the spliceosome.
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.
Self-splicing ribozymal introns of the prozoan Tetrahymena 413-NT intron
As determined by ribosomal RNA sequencing, eukaryotic cells form their own major line of evolutionary descent (the Eukarya) (Figure 14.20a).
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.
Trees based on the comparative sequencing of other genes and proteins yield a different evolutionary picture (Figure 14.20b).
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.
