Transcript Freeman 1e: How we got there

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 membraneenclosed 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 energygenerating 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 hydrogenosomecontaining 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 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
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).
Protein
Primer
• 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 selfsplicing, 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.