Presentation on theme: "CHAPTER 14 Eukaryotic Cell Biology and Eukaryotic Microorganisms."— Presentation transcript:
CHAPTER 14 Eukaryotic Cell Biology and Eukaryotic Microorganisms
Eukaryotic Cell Structure/Function
A typical eukaryotic cell is shown in Figure 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.
Inner structure of mitochondrion
TEM of mitochondrion
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 H 2 plus CO 2, acetate, and ATP.
Photosynthetic Organelle: The Chloroplast The chloroplast is the site of photosynthetic energy production and CO 2 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 called thylakoids.
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: 1.Mitochondria and chloroplasts contain DNA. 2.The eukaryotic nucleus contains bacterially derived genes. 3.Mitochondria and chloroplasts contain their own ribosomes. 4.Several antibiotics kill or inhibit Bacteria specifically by interfering with 70S ribosome function. These same antibiotics also inhibit protein synthesis in mitochondria and chloroplasts.
5.Phylogenetic studies using comparative ribosomal RNA sequencing methods and organellar genome studies have shown convincingly that the chloroplast and mitochondrion originated from the Bacteria.
Other Organelles and Eukaryotic Cell Structures
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 H 2 O 2 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
Eukaryotic Microbial Diversity
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).
Tree based on eukaryotic genes and proteins
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.
Many protozoa are pathogenic to humans and other animals.
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.
Flagellates are all motile by the activity of flagella.
The sarcodines include Amoeba—which are naked in the vegetative phase—and foraminifera—amoebae that secrete a shell during vegetative growth.
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.
Ciliates are protozoa that, in some stage of their life cycle, possess cilia, structures that function in motility.
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.
Sporozoa are a large group of obligately parasitic protozoa. These parasites can cause severe diseases, such as malaria.
Slime Molds Acellular slime molds are masses of motile protoplasm.
Cellular slime molds are masses of individual cells that aggregate to form fruiting bodies that release spores (Figure 14.29).
Fungi Fungi include the molds and yeasts. Table 14.2 gives the classification and major properties of fungi.
Fungi differ from protozoa in their rigid cell walls, production of spores, lack of motility, and phylogenetic position.
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.
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).
A typical mold
Mushrooms are large, often edible fungi that produce fruiting bodies containing basidiospores (Figure 14.32).
Algae Algae are phototrophic Eukarya that contain chlorophyll and carotenoid pigments within a chloroplast. The chloroplast itself has its roots in the Bacteria.