Download 3rd lecture Cell Biology The ultrastructures of prokaryotic cells The

3rd lecture
Cell Biology
 The ultrastructures of prokaryotic cells
1) The prokaryotic cell is simpler than the eukaryotic cell at every level,
with one exception: The cell envelope is more complex.
2) The ultrastructures of prokaryotic cells include:
a) The Nucleoid
b) The cytoplasmic structures, which include (ribosomes, plasmids
and episomes, inclusion bodies and storage granules and
endospores).
c) The cell envelope, which include (cytoplasmic membrane, the
periplasmic space, the cell wall and the outer membrane).
d) The exterior components of the cell wall, which include
Extracellular polymeric substance (EPS), flagella, pili and
fimbriae).
 The Nucleoid
1) Prokaryotes have no true nuclei; instead, they package their DNA in a
structure known as the nucleoid.
2) The negatively charged DNA is at least partially neutralized by small
polyamines and magnesium ions.
3) Histone-like proteins exist in bacteria and presumably play a role
similar to that of histones in eukaryotic chromatin.
4) Electron micrographs of a typical prokaryotic cell reveal the absence
of a nuclear membrane and a mitotic apparatus.
5) The exception to this rule is the planctomycetes, a divergent group of
aquatic bacteria, which have a nucleoid surrounded by a nuclear
envelope consisting of two membranes.
6) The distinction between prokaryotes and eukaryotes that still holds is
that prokaryotes have no eukaryotic-type mitotic apparatus.
7) The nuclear region (Figure 1) is filled with DNA fibrils.
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Dr. Mohanad J.K. Al-Dawah
3rd lecture
Cell Biology
8) The nucleoid of most bacterial cells consists of a single continuous
circular molecule ranging in size from 0.58 to almost 10 million base
pairs.
9) In bacteria, the numbers of nucleoids, and therefore the number of
chromosomes, depend on the growth conditions.
10) Rapidly growing bacteria have more nucleoids per cell than slowly
growing ones; however, when multiple copies are present, they are all
the same (prokaryotic cells are haploid).
Figure 1: nuclear region of E. coli chromosome
 The cytoplasmic structures (matrix)
1) The cytoplasm is the aqueous solution containing all cytoplasmic
components and constitutes the region within the cytoplasmic
membrane, excluding the nucleoid.
2) The cytoplasmic matrix is about 70% water and lacks a cytoskeleton.
3) Despite the apparent homogeneity, the matrix is highly organized with
respect to protein location.
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Dr. Mohanad J.K. Al-Dawah
3rd lecture

Cell Biology
Ribosomes
1) The ribosome is the platform and active site of protein synthesis.
2) Prokaryotic ribosomes constitute up to 10% of the dry cell mass, have
a sedimentation coefficient of 70S, and are composed of a large (50S)
and small (30S) ribosomal subunit.
3) Their small subunit has a 16S rRNA subunit (consisting of 1540
nucleotides) bound to 21 proteins.
4) The large subunit is composed of a 5S RNA subunit (120 nucleotides)
and a 23S RNA subunit (2900 nucleotides) and 31 proteins.
5) The rRNAs form stable three-dimensional structures, which serve as a
structural scaffold for ribosomal protein attachment, as depicted in
Figure 2.
6) In addition to their structural role, the 16S rRNA has a region
complementary to the Shine-Delgarno region of mRNA, which
facilitates mRNA binding to the ribosome for translation.
 Plasmids and episomes:
1) Plasmids are autonomous extrachromosomal replicons, which are
widespread
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Dr. Mohanad J.K. Al-Dawah
3rd lecture
Cell Biology
physiological flexibility of the organism’s response to environmental
changes and stresses.
2) With rare exceptions, plasmids exist within the cell as highly
supercoiled circular dsDNA molecules of a few kilobases, such as
ColVK30 which is 2 kbp in length, to over a hundred kilobases in
length, for example the CAM plasmid of Pseudomonas.
3) Plasmids are present in host cells as either single copies or multiple
copies in excess of 40 per cell.
4) Plasmids may encode one or more phenotypic features, which may in
turn be expressed by the host.
5) These features include antibiotic resistance, toxin production,
adherence antigens, bacteriocin production and sex pilus and
conjugation
6) A representation of R100 is included as a typical plasmid in Figure 3.
7) Plasmids are commonly classified according to their molecular
characteristics, gene functions (particularly antibiotic resistance
patterns that they confer), incompatibility groups, host range, and
bacteriophage susceptibility of hosts.
8) A few types of plasmids are also capable of inserting into the host
chromosome, and these integrative plasmids are sometimes referred to
as episomes in prokaryotes.
Figure 3: Atypically plasmid R100
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Dr. Mohanad J.K. Al-Dawah
3rd lecture
Cell Biology
 Inclusion bodies and storage granules
1) Bacteria often store reserve materials in the form of insoluble
granules, which appear as refractile bodies in the cytoplasm when
viewed by phase contrast microscopy. These so-called inclusion
bodies
2) Usually function in the storage of energy or as a reservoir of structural
building blocks.
3) Most cellular inclusions are bounded by a thin nonunit membrane
consisting of lipid, which serves to separate the inclusion from the
cytoplasm proper.
4) Poly-a-hydroxybutyric acid (PHB) is one of the most common
inclusion bodies produced when the source of nitrogen, sulfur, or
phosphorous is limited and there is excess carbon in the medium.
5) Another storage product formed by prokaryotes when carbon is in
excess is glycogen, which is a polymer of glucose.
6) PHB and glycogen are used as carbon sources when protein and
nucleic acid synthesis are resumed.
 Endospores:
1) Members of several bacterial genera are capable of forming
endospores.
2) The two most common are gram-positive rods: the obligatory aerobic
genus Bacillus and the obligatory anaerobic genus Clostridium.
3) These organisms undergo a cycle of differentiation in response to
environmental conditions:
4) The process, sporulation, is triggered by near depletion of any of
several nutrients (carbon, nitrogen, or phosphorous).
5) Each cell forms a single internal spore that is liberated when the
mother cell undergoes autolysis.
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Dr. Mohanad J.K. Al-Dawah
3rd lecture
Cell Biology
6) The spore is a resting cell, highly resistant to desiccation, heat, and
chemical agents; when returned to favorable nutritional conditions and
activated, the spore germinates to produce a single vegetative cell.
 The cell envelope:
1) Prokaryotic cells are surrounded by complex envelope layers that
differ in composition among the major groups.
2) These structures protect the organisms from hostile environments,
such as extreme osmolarity, harsh chemicals, and even antibiotics.
 Cell membrane
 Structure:
1) The bacterial cell membrane, also called the cytoplasmic membrane, is
visible in electron micrographs of thin sections (figure 4).
2) It is a typical “unit membrane” composed of phospholipids and
upward of 200 different kinds of proteins.
3) Proteins account for approximately 70% of the mass of the membrane,
which is a considerably higher proportion than that of mammalian cell
membranes.
4) At least 50% of the cytoplasmic membrane must be in the semifluid
state for cell growth to occur.
5) The membranes of prokaryotes are distinguished from those of
eukaryotic cells by the absence of sterols.
6) The only exception being mycoplasmas that incorporate sterols, such
as cholesterol, into their membranes when growing in sterolcontaining media.
 Differences between bacterial and Archaeal cell membrane
a) Some Archaeal cell membranes contain unique lipids, isoprenoids,
while bacterial cell membrane contain fatty acids
b) Isoprenoids linked to glycerol by ether, while fatty acid by an ester
linkage.
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Dr. Mohanad J.K. Al-Dawah
3rd lecture
Cell Biology
c) Some of these lipids have no phosphate groups, and therefore, they
are not phospholipids.
d) In other species, the cell membrane is made up of a lipid
monolayer consisting of long lipids with glycerol ethers at both
ends.
e) These unusual lipids contribute to the ability of many Archaea to
grow under environmental conditions such as high salt, low pH, or
very high temperature.
Figure 4: electron micrograph of Bacillus megaterium cell membrane.
poly-β-hydroxybutyric acid inclusion body, PHB; cell wall, CW;
nucleoid, N; plasma membrane, PM; “mesosome,” M; and ribosomes, R.
(Reproduced with permission).
 Function of cell membrane
a) Selective permeability and transport of solutes.
b) Electron transport and oxidative phosphorylation in aerobic
species.
c) Excretion of hydrolytic exoenzymes.
d) Bearing the enzymes and carrier molecules that function in the
biosynthesis of DNA, cell wall polymers, and membrane lipids.
e) Bearing the receptors and other proteins of the chemotactic and
other sensory transduction systems.
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Dr. Mohanad J.K. Al-Dawah