Viruses and Bacteria.pptx

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Dec 14, 2012 (4 years and 8 months ago)

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Viruses and Bacteria

Ch 19 & 27

Campbells

Structure of Viruses


Viruses are not cells


Viruses are very small infectious particles consisting
of nucleic acid enclosed in a protein coat and, in
some cases, a membranous envelope


Capsids and Envelopes


A
capsid
is the protein shell that encloses the viral
genome


Capsids are built from protein subunits called
capsomeres


A capsid can have various structures


Fig. 19
-
3

RNA

Capsomere

Capsomere

of capsid

DNA

Glycoprotein

18

250 nm

70

90 nm (diameter)

Glycoproteins

80

200 nm (diameter)

80

225 nm

Membranous

envelope

RNA

Capsid

Head

DNA

Tail

sheath

Tail

fiber

50 nm

50 nm

50 nm

20 nm

(a) Tobacco mosaic



virus

(b) Adenoviruses

(c) Influenza viruses

(d) Bacteriophage T4


Some viruses have membranous envelopes that
help them infect hosts


These
viral envelopes
surround the capsids of
influenza viruses and many other viruses found in
animals


Viral envelopes, which are derived from the host
cell’s membrane, contain a combination of viral and
host cell molecules


Bacteriophages
, also called
phages
, are viruses that
infect bacteria


They have the most complex capsids found among
viruses


Phages have an elongated capsid head that
encloses their DNA


A protein tail piece attaches the phage to the host
and injects the phage DNA inside


General Features of Viral Reproductive
Cycles


Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins


The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules


Viral nucleic acid molecules and capsomeres
spontaneously self
-
assemble into new viruses

Animation: Simplified Viral Reproductive Cycle


Transcription

and manufacture

of capsid proteins


Self
-
assembly of


new virus particles


and their exit from


the cell


Entry and

uncoating

Fig. 19
-
4

VIRUS

1

2

3

DNA

Capsid

4

Replication

HOST CELL

Viral DNA

mRNA

Capsid

proteins

Viral DNA

The Lytic Cycle


The
lytic cycle
is a phage reproductive cycle that
culminates in the death of the host cell


The lytic cycle produces new phages and digests
the host’s cell wall, releasing the progeny viruses


A phage that reproduces only by the lytic cycle is
called a
virulent phage


Bacteria have defenses against phages, including
restriction enzymes
that recognize and cut up
certain phage DNA

Animation: Phage T4 Lytic Cycle

Fig. 19
-
5
-
5

Phage assembly

Head

Tail

Tail fibers

Assembly

Release

Synthesis of viral

genomes and

proteins

Entry of phage

DNA and

degradation of

host DNA

Attachment

1

2

4

5

3

The Lysogenic Cycle


The
lysogenic cycle
replicates the phage genome
without destroying the host


The viral DNA molecule is incorporated into the
host cell’s chromosome


This integrated viral DNA is known as a
prophage


Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells

Animation: Phage Lambda Lysogenic and Lytic Cycles

Fig. 19
-
6

Phage

DNA

Phage

The phage injects its DNA.

Bacterial

chromosome


Phage DNA

circularizes.

Daughter cell

with prophage

Occasionally, a prophage

exits the bacterial

chromosome,

initiating a lytic cycle.

Cell divisions

produce

population of

bacteria infected

with the prophage.

The cell lyses, releasing phages.

Lytic cycle

Lytic cycle

is induced

or

Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,

copying the prophage and

transmitting it to daughter cells.

Phage DNA integrates into

the bacterial chromosome,

becoming a prophage.

New phage DNA and proteins

are synthesized and

assembled into phages.

Table 19
-
1a

Table 19
-
1b

RNA as Viral Genetic Material


The broadest variety of RNA genomes is found in
viruses that infect animals


Retroviruses

use
reverse transcriptase
to copy
their RNA genome into DNA


HIV (human immunodeficiency virus)
is the
retrovirus that causes
AIDS (acquired
immunodeficiency syndrome)

Fig. 19
-
8a

Glycoprotein

Reverse

transcriptase

HIV

RNA (two

identical

strands)

Capsid

Viral envelope

HOST CELL

Reverse

transcriptase

Viral RNA

RNA
-
DNA

hybrid

DNA

NUCLEUS

Provirus

Chromosomal

DNA

RNA genome

for the

next viral

generation

mRNA

New virus


Vaccines
are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the actual pathogen


Vaccines can prevent certain viral illnesses


Viral infections cannot be treated by antibiotics


Antiviral drugs can help to treat, though not cure,
viral infections

Fig. 19
-
9c

(c) Vaccinating ducks

Viral Diseases in Plants


More than 2,000 types of viral diseases of plants
are known and cause spots on leaves and fruits,
stunted growth, and damaged flowers or roots


Most plant viruses have an RNA genome



Viroids and Prions: The Simplest
Infectious Agents


Viroids
are circular RNA molecules that infect
plants and disrupt their growth


Prions
are slow
-
acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals


Prions propagate by converting normal proteins
into the prion version


Scrapie in sheep, mad cow disease, and Creutzfeldt
-
Jakob disease in humans are all caused by prions

Fig. 27
-
2

(a) Spherical


(cocci)

1

µm

(b) Rod
-
shaped


(bacilli)

2

µm

(c) Spiral

5

µm

Cell
-
Surface Structures


An important feature of nearly all prokaryotic
cells is their cell wall, which maintains cell
shape, provides physical protection, and
prevents the cell from bursting in a hypotonic
environment


Eukaryote cell walls are made of cellulose or
chitin


Bacterial cell walls contain
peptidoglycan
, a
network of sugar polymers cross
-
linked by
polypeptides


Archaea contain polysaccharides and proteins
but lack peptidoglycan


Using the
Gram stain
, scientists classify many
bacterial species into
Gram
-
positive
and
Gram
-
negative
groups based on cell wall composition


Gram
-
negative bacteria have less peptidoglycan
and an outer membrane that can be toxic, and
they are more likely to be antibiotic resistant

Fig. 27
-
3

Cell

wall

Peptidoglycan

layer

Plasma membrane

Protein

Gram
-

positive

bacteria

(a) Gram
-
positive: peptidoglycan traps


crystal violet.

Gram
-

negative

bacteria

(b) Gram
-
negative: crystal violet is easily rinsed away,


revealing red dye.

20 µm

Cell

wall

Plasma membrane

Protein

Carbohydrate portion

of lipopolysaccharide

Outer

membrane

Peptidoglycan

layer

Fig. 27
-
3c

Gram
-

positive

bacteria

Gram
-

negative

bacteria

20 µm

Fig. 27
-
7

(a) Aerobic prokaryote

(b) Photosynthetic prokaryote

Thylakoid

membranes

Respiratory

membrane

0.2 µm

1 µm

Fig. 27
-
8

Chromosome

Plasmids

1 µm


The typical prokaryotic genome is a ring of DNA
that is not surrounded by a membrane and that
is located in a
nucleoid region

Reproduction and Adaptation


Prokaryotes reproduce quickly by binary fission
and can divide every 1

3 hours


Many prokaryotes form metabolically inactive
endospores
, which can remain viable in harsh
conditions for centuries

Fig. 27
-
9

Endospore

0.3 µm

Transformation and Transduction


A prokaryotic cell can take up and incorporate
foreign DNA from the surrounding environment
in a process called
transformation


Transduction

is the movement of genes
between bacteria by bacteriophages (viruses
that infect bacteria)

Fig. 27
-
11
-
4

Recombinant cell

Recipient

cell

A
+

B


B


A
+

A


Recombination

A
+

Donor

cell

A
+

B
+

A
+

B
+

Phage DNA

Conjugation and Plasmids


Conjugation
is the process where genetic
material is transferred between bacterial cells


Sex pili allow cells to connect and pull together
for DNA transfer


A piece of DNA called the
F factor
is required for
the production of sex pili


The F factor can exist as a separate plasmid or
as DNA within the bacterial chromosome

Fig. 27
-
12

Sex pilus

1 µm

Concept 27.3: Diverse nutritional and
metabolic adaptations have evolved in
prokaryotes


Phototrophs

obtain energy from light


Chemotrophs

obtain energy from chemicals


Autotrophs

require CO
2

as a carbon source


Heterotrophs

require an organic nutrient to
make organic compounds


These factors can be combined to give the four
major modes of nutrition: photoautotrophy,
chemoautotrophy, photoheterotrophy, and
chemoheterotrophy

Table 27
-
1

The Role of Oxygen in Metabolism


Prokaryotic metabolism varies with respect to
O
2
:


Obligate aerobes
require O
2

for cellular respiration


Obligate anaerobes
are poisoned by O
2

and use
fermentation or
anaerobic respiration


Facultative anaerobes
can survive with or without
O
2

Nitrogen Metabolism


Prokaryotes can metabolize nitrogen in a variety
of ways


In
nitrogen fixation
, some prokaryotes convert
atmospheric nitrogen (N
2
) to ammonia (NH
3
)

Fig. 27
-
16

UNIVERSAL

ANCESTOR

Eukaryotes

Korarcheotes

Euryarchaeotes

Crenarchaeotes

Nanoarchaeotes

Proteobacteria

Chlamydias

Spirochetes

Cyanobacteria

Gram
-
positive

bacteria

Domain

Eukarya

Domain Archaea

Domain Bacteria

Table 27
-
2


Some archaea live in extreme environments and
are called
extremophiles


Extreme halophiles
live in highly saline
environments


Extreme thermophiles
thrive in very hot
environments

Fig. 27
-
17


Methanogens
live in swamps and marshes and
produce methane as a waste product


Methanogens are strict anaerobes and are
poisoned by O
2


In recent years, genetic prospecting has
revealed many new groups of archaea


Some of these may offer clues to the early
evolution of life on Earth


Example:
Rhizobium
,

which forms root nodules
in legumes and fixes atmospheric N
2


Example:
Agrobacterium
,

which produces
tumors in plants and is used in genetic
engineering

Fig. 27
-
18c

Rhizobium

(arrows) inside a root

cell of a legume (TEM)

2.5 µm

Chlamydias


These bacteria are parasites that live within
animal cells


Chlamydia trachomatis
causes blindness and
nongonococcal urethritis by sexual transmission

Chemical Cycling


Prokaryotes play a major role in the recycling of
chemical elements between the living and
nonliving components of ecosystems


Chemoheterotrophic prokaryotes function as
decomposers
, breaking down corpses, dead
vegetation, and waste products


Nitrogen
-
fixing prokaryotes add usable nitrogen
to the environment


In
mutualism
, both symbiotic organisms benefit


In
commensalism
, one organism benefits while
neither harming nor helping the other in any
significant way


In
parasitism
, an organism called a
parasite

harms but does not kill its host


Parasites that cause disease are called
pathogens


Pathogenic prokaryotes typically cause disease
by releasing exotoxins or endotoxins


Exotoxins
cause disease even if the prokaryotes
that produce them are not present


Endotoxins
are released only when bacteria die
and their cell walls break down


Many pathogenic bacteria are potential
weapons of bioterrorism