This document discusses bacteria and their cellular structure. It begins by introducing bacteria and noting that while some cause disease, most are harmless or beneficial. It then describes the key differences between prokaryotic and eukaryotic cells, noting that prokaryotes like bacteria lack a nucleus and membrane-bound organelles. The document proceeds to describe the external structures of bacterial cells like the cell wall and plasma membrane. It concludes by covering bacterial reproduction through binary fission and how genetic transfer can occur between bacteria through transformation and transduction.

1. BACTERIA
Presented By : Talha Chishti

2. Introduction
 Bacteria often get a bad rap: they’re described as unsafe
“bugs” that cause disease. Although some types of bacteria
are pathogenic (disease-causing), most kinds are actually
harmless, or even beneficial.
 Prokaryotes, organisms that include bacteria and another
group called archaea, dominate the Earth. They live nearly
everywhere – on every surface, on land and in water, and
even inside of our bodies. In fact, the prokaryotic cells in your
digestive tract outnumber the human cells that make up your
tissues! That may sound unappealing, but our prokaryotic
inhabitants are actually essential for maintaining our health.
 In this article, we'll first discuss what prokaryotes are and how
they differ from eukaryotes. Then, we'll walk through some of
the unique structural features of prokaryotic cells.
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3. What are prokaryotes?
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 Prokaryotes are microscopic organisms
belonging to the domains Bacteria and
Archaea, which are two out of the three major
domains of life.
 Fossils show that prokaryotes were already
present on Earth 5 billion years ago, and
prokaryotic ancestors are thought to have
given rise to all present-day organisms.
 Most bacteria and archaea are unicellular, with
a few multicellular exceptions. In contrast,
most eukaryotes are multicellular.

4. Prokaryotes vs. eukaryotes
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 Prokaryotes and eukaryotes have many similarities (a reflection of
their shared evolutionary ancestry). For instance, both types of
organisms decode genes into proteins through the processes of
transcription and translation.
 Both prokaryotes and eukaryotes pass genetic information to their
offspring in the form of DNA and have mechanisms for increasing
genetic variation within populations.
 However, in other ways, prokaryotes and eukaryotes are quite
different. Perhaps the most basic difference between prokaryotes
and eukaryotes is the way their cells are set up.
 The defining difference between prokaryotes and eukaryotes is
whether their cells have a membrane-bound nucleus. Eukaryotes
have a nucleus, while prokaryotes don't.
 Eukaryotic cells also typically have other membrane-bound
organelles, which prokaryotes lack.
 Prokaryotic cells are tiny, typically ranging from0.5-5 μm in
diameter. By comparison, eukaryotic cells typically range from 10-
100 μm.

5. Many prokaryotic cells have sphere, rod, or spiral shapes (as shown
below). In the following sections, we’ll walk through the structure of a
prokaryotic cell, starting on the outside and moving towards the inside
of the cell.
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Cocci Bacilli Spiral

6. External structures
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 Capsule or slime layer
 Many prokaryotes have an organized outermost layer called
the capsule, which is typically composed of polysaccharides.
If the outer layer is unorganized, it is instead referred to as
the slime layer.
 The cell wall
 Underneath the capsule or slime layer, all prokaryotic cells
have a stiff cell wall. This structure maintains the cell’s
shape, protects the cell interior, and prevents the cell from
bursting when it takes up water. The cell wall of most bacteria
contains peptidoglycan, a polymer of linked sugars and
polypeptides. Archaea, on the other hand, lack peptidoglycan,
but have cell walls made of proteins or glycoproteins
(proteins with attached carbohydrate groups).

7. External structures
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Some of the antibiotics used to treat bacterial infections in humans and other animals act
by targeting the bacterial cell wall. Penicillin and ampicillin, for instance, disrupt the
production of peptidoglycan in prokaryotic cell walls but do not harm the cells of animal
tissues (since these cells do not produce peptidoglycan to begin with).

8. The plasma membrane
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 Underneath the cell wall lies the plasma membrane.
The basic building block of the plasma membrane is the
phospholipid, a lipid composed of a glycerol molecule
attached a hydrophilic (water-attracting) phosphate head
and to two hydrophobic (water-repelling) fatty acid tails.
The phospholipids of a eukaryotic or bacterial
membrane are organized into two layers, forming a
structure called a phospholipid bilayer.

9. Key points:
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 Prokaryotes are single-celled organisms belonging to the
domains Bacteria and Archaea.
 Prokaryotic cells are much smaller than eukaryotic cells, have
no nucleus, and lack organelles.
 All prokaryotic cells are encased by a cell wall. Many also
have a capsule or slime layer made of polysaccharide.
 Prokaryotes often have appendages (protrusions) on their
surface. Flagella and some pili are used for
locomotion, fimbriae help the cell stick to a surface, and sex
pili are used for DNA exchange.
 Most prokaryotic cells have a single circular chromosome.
They may also have smaller pieces of circular DNA
called plasmids.

10. Prokaryotes reproduce through a cell division process called binary fission. Like mitosis in eukaryotes,
this process involves copying the chromosome and separating one cell into two. Binary fission is
an asexual form of reproduction, meaning that it does not involve production of gametes (eggs or sperm),
and it typically produces daughter cells – new prokaryote individuals – that are genetically identical to the
mother cell.
How do prokaryotes reproduce?10
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11. Binary Fission
 Before binary fission can begin, the prokaryotic cell must
copy its DNA. Prokaryotes usually have a single, circular
chromosome that forms their genome, although they may
also have smaller circular DNA molecules
called plasmids (not pictured). Copying of the chromosome
begins at the origin of replication, a region of DNA
recognized by special initiator proteins.
 The initiator proteins recruit other proteins to separate the
DNA strands of the chromosome, forming a “replication
bubble.” Then, replication proteins copy the DNA, moving in
both directions around the circle. The origin is the first part
of the DNA to be copied, and as replication continues, the
two origins move towards opposite ends of the cell. The cell
also begins to elongate, adding to the separation of the
copied chromosomes.
 Replication continues until the entire chromosome is copied
and the replication complexes meet at a site called
the terminus of replication. Once replication is complete,
each new chromosome forms a complete circle of DNA, but
the two chromosomes may still be tangled together (like a
pair of linked rings) and must be disentangled by enzymes.
 Once the chromosomes have moved to opposite cell ends,
clearing the center of the cell, division of the cytoplasm can
take place. In this process, the membrane pinches inward
and a septum, or new cell wall, forms down the middle of
the cell, partitioning it into two compartments. Finally, the
septum itself splits down the middle, and the two cells are
released to continue their lives as individual prokaryotes.
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12. Steps Of Transformation
 In a typical transformation
experiment, the target gene (blue
DNA above) is first inserted into a
plasmid. In addition to the target
gene, the plasmid also contains a
gene that provides resistance to a
particular antibiotic (red DNA above).
If the goal is to use the bacteria to
synthesize protein from the gene, the
plasmid will also contain apromoter,
or control sequence, that allows the
target gene to be expressed in
bacteria (green DNA above).
 When copies of the plasmid are
mixed with E. coli cells and the cells
are heat-shocked (exposed briefly to
high temperature), a small fraction of
them will take up the plasmid. All of
the E. coli are then spread on a
nutrient plate containing the
antibiotic. The purpose of the
antibiotic is to only let bacteria with
the plasmid survive and grow.
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E. coli lacking the plasmid will be killed by the antibiotic. E.
coli that contain the plasmid, however, can survive and
reproduce (thanks to the antibiotic resistance gene in the
plasmid). Each resistant cell will form a colony of genetically
identical bacteria, which appears on the agar plate as small
dot. An antibiotic-resistant colony can be analyzed (checked
by other methods to confirm it contains the correct plasmid),
then grown up to make a large culture of identical, plasmid-
bearing bacteria.

13. Transformation
 Transformation is when a
prokaryotic cell takes in foreign DNA
directly from its environment. This
DNA may come from other bacterial
cells that have died, or, in a
laboratory, may be introduced by
scientists.
 If the DNA is in the form of a plasmid,
or circular DNA that can replicate
independently in a bacterium, it will
be copied in the receiving cell and
passed on to its descendants.
 Alternatively, a fragment of DNA may
be incorporated into the cell’s
chromosome through homologous
recombination. This process
involves the swapping of similar
nucleotide sequences between two
double-stranded DNA molecules. In
some cases, homologous
recombination may replace a
harmless allele of a bacterial gene
with a disease-causing allele, making
the bacterium pathogenic (disease-
causing).
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14. Transduction
 Transduction is when a virus transfers
bacterial DNA from one prokaryote to
another. Viruses that infect bacteria,
called bacteriophages, or phages for
short, reproduce by injecting their DNA
(or RNA) into host bacterial cells. The
DNA programs the cell to become a
phage factory, making the nucleic acid
and protein parts to build more viral
particles. In many cases, the phage
also causes the bacteria chromosome
to be broken into fragments.
 During the assembly of the particles,
viral DNA or RNA is packed into a
protein shell called the capsid.
However, fragments of the host cell's
DNA are sometimes accidentally put
into the capsid instead. When a viral
particle containing a bacterial DNA
fragment infects another cell, the DNA
is transferred to the new cell and may
be incorporated into the chromosome
by homologous recombination.
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15. Conjugation
 Conjugation is a process in
which one prokaryotic cell
transfers DNA to another
through physical contact.
 In conjugation, a rod-like
structure called a sex
pilusextends from the donor
cell, attaches to the recipient
cell, and pulls the two cells
together. DNA is then copied
and transferred from the
donor cell to the recipient cell
through a channel called
the mating bridge. In most
cases, the DNA that is
transferred between cells is a
plasmid. The transferred
plasmid can replicate in the
receiving cell and will be
passed on to its descendants.
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16. Conjugation
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 In the best-understood conjugation system, donor
cells act as donors because they carry a segment of
DNA called the fertility factor (or F factor). This
segment of DNA encodes the proteins of the sex pilus,
along with other molecular components needed for
conjugation. It also contains the initiation site where
DNA transfer during conjugation begins.
 Cells with F factor are said to be F​+. The F factor may
be carried on a plasmid, or it may be integrated into
the donor cell’s chromosome. Recipient cells
(denoted F​−) lack F factor and thus cannot transfer
their DNA. However, if a recipient cell gets the F factor
through conjugation, it will become a donor.
Conjugation cannot occur between two F​+ cells.

17. Bacteria
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18. Thanks For Watching
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