Download 2. Movement in bacteria.

Benha University
Faculty of Education
Botany Dept.
1st year students
General Biology
Jan. 2011
1st semester
Prof. Dr. Mohammed Reda Metwali
Question I: Answer three only of the following?
1. Plant cell structure;
Plant cells are eukaryotic cells that differ in several key respects from the cells of
other eukaryotic organisms. Their distinctive features include:
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A large central vacuole, a water-filled volume maintains the cell's turgor,
controls movement of molecules between the cytosol and sap, stores useful
material and digests waste proteins and organelles.
A cell wall composed of cellulose and hemicelluloses, pectin and in many
cases lignin, and secreted by the protoplast on the outside of the cell
membrane. This contrasts with the cell walls of fungi (which are made of
chitin), and of bacteria, which are made of peptidoglycan.
Specialized cell-cell communication pathways known as plasmodesmata,
pores in the primary cell wall through which the plasmalemma and
endoplasmic reticulum of adjacent cells are continuous.
Plastids, notably the chloroplasts which contain chlorophyll and the
biochemical systems for light harvesting and photosynthesis, but also
amyloplasts specialized for starch storage, elaioplasts specialized for fat
storage and chromoplasts specialized for synthesis and storage of pigments.
Cell division by construction of a phragmoplast as a template for building a
cell plate late in cytokinesis is characteristic of land plants and a few groups of
algae, notably the Charophytes and the Order Trentepohliales.
2. Movement in bacteria.
Many but not all bacteria exhibit motility, i.e. self-propelled motion,
under appropriate circumstances. Motion can be achieved by one of three
mechanisms:
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Most motile bacteria move by the use of flagella (singular,
flagellum), rigid structures 20 nm in diameter and 15-20 µm long
which protrude from the cell surface (e.g. Chromatium).
Spirochaetes are helical bacteria which have a specialized internal
structure known as the axial filament which is responsible for
rotation of the cell in a spiral fashion and consequent locomotion
(e.g. Rhodospirillum).
Gliding bacteria all secrete copious slime, but the mechanism
which propels the cells is not known
In some bacteria, there is only a single flagellum - such cells are called
monotrichous. In these circumstances, the flagellum is usually located at
one end of the cell (polar). Some bacteria have a single flagellum at both
ends - amphitrichous. However, many bacteria have numerous flagella; if
these are located as a tuft at one end of the cell, this is described as
lophotrichous (e.g. Chromatium), if they are distributed all over the cell,
as peritrichous.
Flagella consist of a hollow, rigid cylinder composed of a protein called
flagellin, which forms a filament anchored to the cell by a curved
structure called the hook, which is attached to the basal body. Flagellae
are, in effect, rotary motors comprising a number of proteinaceous rings
embedded in the cell wall. These molecular motors are powered by the
phosporylation cascade responsible for generating energy within the cell.
In action, the filament rotates at speeds from 200 to more than 1,000
revolutions per second, driving the rotation of the flagellum. The
organization of these structures is quite different from that of eukaryotic
flagella. The direction of rotation determines the movement of the cell.
Anticlockwise rotation of monotrichious polar flagella thrusts the cell
forward with the flagellum trailing behind. Peritrichous cells operate in
the same way.
Periodically the direction of rotation is briefly reversed, causing what is
known as a "tumble", and results in reorientation of the cell. When
anticlockwise rotation is resumed, the cell moves off in a new direction.
This ability is important, since it allows bacteria to change direction.
Bacteria can sense nutrient molecules such as sugars or amino acids and
move towards them - a process is known as chemotaxis. Additionally,
they can also move away from harmful substances such as waste products
and in response to temperature, light, gravity, etc. This apparently
intelligent behaviour is achieved by changes in the frequency of tumbles.
When moving towards a favourable stimulus or away from an
unfavourable one, the frequency of tumbles is low, thus the cells moves
towards or away from the stimulus as appropriate. However, when
swimming towards an unfavourable or away from a favourable stimulus,
the frequency of tumbles increases, allowing the cell to reorient itself and
move to a more suitable growth.
Gliding motility is the movement of cells over surfaces without the aid
of flagella, a trait common to many bacteria, yet the mechanism of
gliding motility is unknown. The gliding motility apparatus which propels
the cells involves a complex of proteins, yet the actual nature of the
"motor" and how the components interact is not understood
3. General life cycle of Ascomycetes.
Reproduction in the Ascomycetes:
In this group of fungi there are no specialized organs of hyphal fusion,
different mating type mycelia merely fuse with each other to form
transient dikaryons, mycelia with two mating type nuclei within it. The
dikaryotic mycelium can differentiate to from varying amounts of sterile
mycelium around what is to become the fertile tissue of the fruit body. In
yeasts, a single, diploid yeast will undergo meiosis, producing four
haploid progeny cells, but in more complex fungi there are a sequence of
cellular and nucleic events that ensure an organized fertile layer. The
events are illustrated below in figure.
Spores are delineated around these nuclei in a process called free cell
formation, and as most of the cytoplasm is contained around the nucleus
and within the spore wall, all that is left outside is cell sap. These
modified hyphae are termed Asci, and the spores that are held within
them are termed ascospores. The asci are often found packed tightly with
other asci, and between a dense layer of supporting sterile tissue. Often
the structure is large enough to be seen with the naked eye.
The asci can be aggregated together in various sorts of fruit body which
we will see in the practical, including the, cup fungi (Discomycetes,
apothecial), the flask fungi, (Pyrenomycetes, perithecial), the mildews
(Plectomycetes cleistothecial) and the fungi with black, crusty stromata
(Loculoascomycetes, pseudothecial fungi). There are also the yeasts,
Hemiascomycetes,. Their ascospores are normally formed in loose asci
and are not actively discharged. We have not looked at these. When they
form ascospores in fruit pulps or liquids they are usually liberated by the
disintegration of the ascus wall.
4. Lytic cycle in case of viruses.
The lytic cycle is one of the two cycles of viral reproduction, the other
being the lysogenic cycle. The lytic cycle is typically considered the main
method of viral replication, since it results in the destruction of the
infected cell.
Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a sixstage cycle. In the first stage, called "penetration," the virus injects its
own nucleic acids into a host cell. Then the viral acids form a circle in the
center of the cell. The cell then mistakenly copies the viral acids instead of
its own nucleic acids. Then the viral DNA organize themselves as viruses
inside the cell. When the number of viruses inside becomes too much for
the cell to hold, the membrane splits and the viruses are free to infect
other cells.
Penetration
To infect a cell, a virus must first enter the cell through the plasma
membrane and (if present) the cell wall. Viruses do so by either attaching
to a receptor on the cell's surface or by simple mechanical force. The virus
then releases its genetic material (either single- or double-stranded RNA
or DNA) into the cell. In doing this, the cell is infected and can also be
targeted by the immune system.
Biosynthesis
The virus' nucleic acid uses the host cell’s machinery to make large
amounts of viral components. In the case of DNA viruses, the DNA
transcribes itself into messenger RNA (mRNA) molecules that are then
used to direct the cell's ribosomes. One of the first polypeptides to be
translated destroys the host's DNA. In retroviruses (which inject an RNA
strand), a unique enzyme called reverse transcriptase transcribes the viral
RNA into DNA, which is then transcribed again into RNA.
The biosynthesis is (e.g. T4) regulated in three phases of mRNA
production followed by a phase of protein production.[1]
*Early phase
Enzymes involved to modify the hosts DNA replication by RNA
polymerase. Amongst other modifications, virus T4 changes the
sigma factor of the host by producing an anti-sigma factor so that
the host promotors are not recognized any more but now recognize
T4 middle proteins.
*Middle phase
Virus nucleic acid (DNA or RNA depending on virus type).
*Late phase
Structural proteins including those for the head and the tail.
Maturation and lysis
After many copies of viral components are made, they are assembled into
complete viruses. The phage then directs production of an enzyme that
breaks down the bacteria cell wall and allows fluid to enter. The cell
eventually becomes filled with viruses (typically 100-200) and liquid, and
bursts, or lyses; thus giving the lytic cycle its name. The new viruses are
then free to infect other cells.
Lytic cycle without lysis
Some viruses escape the host cell without bursting the cell membrane, but
rather bud off from it by taking a portion of the membrane with them.
Because it otherwise is characteristic of the lytic cycle in other steps, it
still belongs to this category, although it is sometimes named the
Productive Cycle. HIV, influenza and other viruses that infect eukaryotic
organisms generally use this method.
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