The document discusses several topics related to prokaryotic cells: 1) Osmosis is the spontaneous movement of water through a selectively permeable membrane towards a region of higher solute concentration. 2) Bacterial genetic material consists of double-stranded DNA that is not attached to the cell membrane. 3) Polysomes/polyribosomes are multiple ribosomes translating a single mRNA strand simultaneously to produce multiple proteins. They are found in both prokaryotes and eukaryotes.

1. 1. What is Osmosis?
RECAP…
 Spontaneous net movement of solvent molecules through a selectively
permeable membrane from a region of high water potential (region of
lower solute concentration) to a region of low water potential (region of higher
solute concentration), in the direction that tends to equalize the solute
concentrations on the two sides.
 Water potential- is the potential energy of water per unit volume relative to pure
water in reference conditions.
 Osmotic pressure- is the external pressure required to be applied so that there is
no net movement of solvent across the membrane.
2. What is prokaryotic genetic material?
 DNA or RNA?
 Single stranded or Double stranded?

2. 3. Is bacterial chromosome attached to the cell membrane?
 No definitive answer- however there are various models for explanation.
- single site of replication in the bacterial chromosome: Replication begins at the
origin and proceeds linearly until the entire chromosome has been duplicated.
- model proposes that the chromosome is attached to the bacterial membrane.
-model suggests that the DNA-synthesizing complex is fixed to the bacterial
membrane and that the DNA moves through this complex.
-the membrane is thought to provide a mechanism for the segregation of the
daughter replicons by growth of the cell surface between their sites of
attachment.
-The bacterial membrane would thereby perform the function of the mitotic
apparatus of higher organisms, as well as the site of DNA synthesis.
 However, there are contradictory models also, the overall evidence suggests
potential scope for future work.

3. 4. What are Polysome/Polyribosome?
 In prokaryotes, during the transcription, the mRNA formed are polycistronic in
nature while the mRNA formed during the eukaryotic transcription is
monocistronic in nature.
 Monocistronic mRNA (eukaryotic mRNA) consists of a single cistron (can produce
a single protein) & Polycistronic mRNA (prokaryotic mRNA) consisting of two or
more cistrons (can produce multiple proteins).
 In prokaryotes, functionally-related genes assemble in groups in such a way that
all proteins can be transcribed at once when needed. E.g., Lac operon is one such
famous operon in E. coli (encodes β-galactosidase LacZ)
 A polysome/polyribosome is simply a whole bunch of ribosomes (> two) that
actively and simultaneously translate one single mRNA strand forming
polypeptide chains during active translation.
 Therefore, more than one ribosomes translate an mRNA at one time, making it
possible to produce many polypeptides simultaneously from a single mRNA.

4. -Eukaryotic mRNA is monocistronic
-Ribosomes read mRNA strands like ‘ticker tape
readers’, i.e, once one ribosome reads through
the start of an mRNA strand, there is nothing
stopping another ribosome from binding and
beginning translation on the same mRNA even
before the first ribosome is finished translating.
-So for a time, more than one ribosome will be
attached to the same mRNA.
-Each ribosome will finish coding the entire
protein from the mRNA strand, but since they are
all sequentially bound to the same mRNA they
form a polyribosome during the process.
5. Are Polyribosome found in eukaryotes?

5. 2. CYTOPLASM & CYTOPLASMIC INCLUSIONS
 These are extra-chromosomal hereditary determinants- either independent
of bacterial chromosome or integrated with them.
[D]. PLASMIDS
 Small, circular molecules of double-stranded DNA physically separated
from the bacterial chromosomal DNA (with size 20-100 kbp)
 Plasmids often carry genes that benefit
the survival of the organism e.g, antibiotic
resistance, virulence and metabolic
capacities (each carrying upto as many as
100 genes).
 Plasmids are usually very small compared
to the bacterial chromosomes and contain
only additional genes that may be useful to
the organism under certain situations.

6.  All plasmids possess an origin of replication for DNA polymerase to bind and
replicate plasmid DNA, and can only multiply inside a host cell.
 They may be found as single or multiple copies with few to several hundred
genes (COPY NUMBER)- number of copies of the plasmid in each bacterial cell.
 The number of copies influences the strength of plasmid-borne
characteristics, especially antibiotic resistance. The more copies of the plasmid
per cell, the more copies will be of the antibiotic resistance genes, and
therefore, the higher the resulting level of antibiotic resistance.
 The size of plasmids varies enormously. The F-plasmid of E. coli is fairly
average in this respect and is about 1% the size of the E. coli chromosome.
 Most multicopy plasmids are much smaller (ColE plasmids are about 10% the
size of the F-plasmid). Very large plasmids, up to 10% of the size of a
chromosome are sometimes found (megaplasmid), but they are difficult to work
with and only few have been properly characterized.
 Certain plasmids can move themselves from one bacterial cell to another
(plasmid transferability), e.g, the F-type and P-type plasmids.

7.  Plasmids carry non-essential genes and has no role in viability and growth of
bacteria- “dispensable autonomous elements”- provide genetic advantages.
 Independent genetically determined system of replication control- hence
their rate of replication may be different from chromosome replication.
 e.g., “F – factor”- which determines the
maleness in bacteria; separate from the
bacterial chromosome & transmitted from
cell to cell by contact.
 A number of bacterial properties are now
known that are determined by plasmid
carried genes.
 e.g., Plasmids can provide bacteria with the ability to fix nitrogen, while some
plasmids, have no observable effect on the phenotype of the host cell or their
benefit to the host cells have not been determined yet- “Cryptic plasmids”.

8. Classification and Types of PLASMIDS
Conjugative plasmids: Contain a set of transfer or tra genes which promote
sexual conjugation between different cells. In this process of conjugation
plasmid may be transferred from one bacterium to another via
sex pili encoded by some of the tra genes.
Non-conjugative plasmids: Incapable of initiating conjugation, hence they
can be transferred only with the assistance of conjugative plasmids.
Compatible and In-compatible plasmids: A microbe can harbor different
types of plasmids, however, different plasmids can only exist in a single
bacterial cell if they are compatible.
 If two plasmids are not compatible, one will be rapidly lost from the cell.
 Different plasmids may therefore be assigned to different incompatibility
groups depending on whether they can coexist together.
 Incompatible plasmids normally share the same replication mechanisms
and thus cannot be kept together in a single cell.

9. Classification of PLASMIDS based on function (5 main classes)
Fertility F-plasmids: Contain tra genes. They are capable of conjugation and
result in the expression of sex pili.
Resistance (R) plasmids: Contain genes that provide resistance
against antibiotics or poisons. Historically known as R-factors.
Col plasmids: Contain genes that code for bacteriocins- proteins that can kill
other bacteria.
Degradative plasmids: Enable the digestion of unusual substances,
e.g. toluene and salicylic acid in Pseudomonas.
Virulence plasmids: Turn the bacterium into a pathogen.
 Many others e.g., mercury-resistant plasmids, tumor inducing plasmids of
Agrobacterium tumifaciens, penicillinase plasmids of Staphylococcus aureus,
cryptic plasmids, etc.

10. [E]. MESOSOMES
 Complex localized infoldings of cytoplasmic membrane in most of the
prokaryotic cell- mainly in the form of vesicles, tubules, and lamellae- initially
referred as “peripheral bodies”.
 Number may be 2-4 in each cell & believed to be higher in bacteria showing
high respiratory activity e.g., nitrifying bacteria.
 The major function of mesosomes was believed to increase the surface area
of the plasma membrane to carry out cellular respiration more efficiently.
 Believed to play a role in several cellular processes- such as, cell wall
formation, DNA replication and cell division, excretion of exoenzymes,
photosynthesis, electron transport, and cell compartmentalization.
 Enzymes like, ATPase, dehydrogenase and cytochrome are “Absent”
During 1970’s majority of these beliefs/hypothesis were questioned,
tested and proven false, and these were primarily concluded not to be
the sites of respiration.

11.  Therefore, it was stated that ‘mesosomes are artifacts’ formed through
damage to the membrane during the process of chemical fixation, and do not
occur in cells that have not been chemically fixed.
 Late in 1980s, with advances in cryofixation and freeze substitution methods
for electron microscopy, it was generally concluded that mesosomes do not
exist in living cells.
 Recently, similar folds have been observed in bacteria that have been exposed
to some classes of antibiotics, and antibacterial peptides.
 The appearance of these mesosome like structures may be the result of these
chemicals damaging the plasma membrane and/or cell wall.
“Mesosomes or chondrioids” are
folded invaginations in the plasma
membrane of bacteria that are produced by the
chemical fixation techniques used to prepare samples
for electron microscopy.