Transcript Modules10-17to10

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 10
Molecular Biology of the Gene
Modules 10.17 – 10.22
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
VIRUSES: GENES IN PACKAGES
The Molecular Genetics of Viruses
Viral DNA may become part of the host chromosome
Phage
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA
Many cell
divisions
Occasionally a prophage
may leave the bacterial
chromosome
LYTIC CYCLE
Phages
assemble
LYSOGENIC CYCLE
Phage DNA
circularizes
Prophage
Lysogenic bacterium
reproduces normally,
replicating the prophage
at each cell division
OR
New phage DNA and
proteins are synthesized
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Phage DNA inserts into the bacterial
chromosome by recombination
Translation of viral genes in Prokaryotes
• Prophage genes inserted in the DNA of
essentially harmless bacteria cause botulism,
diptheria, and scarlet fever. This occurs when
prophage genes become active in the host
genome. Also, an environmental signal such as
radiation or a certain chemical triggers a
switchover from the lysogenic cycle to the lytic
cycle.
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10.18 Connection: Many viruses cause disease in
animals
• Many viruses have
RNA, rather than DNA,
as their genetic material
Membranous
envelope
• RNA viruses include:
flu, cold, measles,
mumps as well as AIDS
and polio.
RNA
• DNA viruses include:
hepatitis, chicken pox,
herpes
• Not all viruses
reproduce in the
cytoplasm, some like
herpes reproduce in the
nucleus.
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Protein
coat
Glycoprotein spike
Figure 10.18A
Viral Classification
20 - 300 nm, origin
not clear because
ther is no fossil
record. The smallest
genomes code for
only four proteins
and weigh about
106 daltons, while
the largest weigh
about 108 daltons
and code for over
one hundred
proteins.
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• Baltimore Classification
• IdsDNA viruses
• IIssDNA viruses
• IIIdsRNA viruses
• IV(+)ssRNA viruses
• V(-)ssRNA viruses
• VIssRNA-RT viruses
• VIIdsDNA-RT viruses
• ss: single-stranded
•
ds: double stranded
RT: reverse transcribing
In taxonomy, the classification of viruses is rather difficult due to the
lack of a fossil record and the dispute over whether they are living or
non-living. They do not fit easily into any of the domains of
biological classification and therefore classification begins at the
family rank. However, the domain name of Acytota (without cells)
has been suggested. This would place viruses on a par with the other
domains of Eubacteria, Archaea, and Eukarya. Not all families are
currently classified into orders, nor all genera classified into families.
.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
As an example of viral classification, the chicken pox virus
belongs to family Herpesviridae, subfamily
Alphaherpesvirinae and genus Varicellovirus. It remains
unranked in terms of order. The general structure is as follows:
Order (-virales)
Family (-viridae)
Subfamily (-virinae)
Genus (-virus)
Species (-virus)
The International Committee on Taxonomy of Viruses (ICTV)
developed the current classification system.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Some animal
viruses steal a bit of
the host cell’s
membrane (Herpes
virus)
• Some remain latent
unless cell is
stressed
• How quickly we
fight them off with
our immune system
and how quickly our
cells go through
mitosis is a factor in
how and IF we
recover 100% from
a viral infection.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Glycoprotein spike
VIRUS
Protein coat
Viral RNA
(genome)
Plasma
membrane
of host
cell
Envelope
1
Entry
2
Uncoating
3
RNA synthesis
by viral enzyme
Viral RNA
(genome)
4
Protein
synthesis
5 RNA synthesis
(other strand)
Template
mRNA
New
viral proteins
protein
6
New viral
genome
Assembly
Exit
7
Figure 10.18B
10.19 Connection: Plant viruses are serious
agricultural pests
• Most plant viruses have RNA
– Example: tobacco mosaic disease
– These viruses enter damaged plants and spread through
the plasmodesmata
Protein
RNA
Figure 10.19
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10.20 Connection: Emerging viruses threaten
human health
• The deadly Ebola virus causes
hemorrhagic fever (1976, Africa)
– Each virus is an enveloped
thread of protein-coated RNA
• Hantavirus is another enveloped
RNA virus caused by a virus
infecting rodents (1993)
• Viral Encephalitis-inflammation of
the brain
• West Nile (1999-48 states by 2004)
• SARS (severe acute respiratory
syndrome) (China 2003-3months
8,450 infected)
Figure 10.20A, B
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10.21 The AIDS virus makes DNA on an RNA
template
• HIV is a retrovirus using reverse transcriptase
to make a DNA copy of its genome.
Envelope
Glycoprotein
Protein
coat
RNA
(two identical
strands)
Reverse
transcriptase
Figure 10.21A
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• Inside a cell,
HIV uses its
RNA as a
template for
making DNA
to insert into
the host
chromosome
Viral RNA
CYTOPLASM
1
NUCLEUS
DNA
strand
Chromosomal
DNA
2
3
Doublestranded
DNA
Provirus
DNA
4
5
RNA
Viral
RNA
and
proteins
6
Figure 10.21B
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Spread of Lethal viruses
• Viral disease can go from a small population to
an epidemic or even pandemic
• Lethal viruses are more threatening due to
biological and technological factors:
international travel, blood transfusions, sexual
promiscuity, IV drug abuse, jumping the
“species barrier”
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The Molecular Genetics of the
Simplest Living Organisms
• Bacteria replicate through binary fission
• Bacteria can transfer DNA in three ways
– Bacteria can transfer genes from cell to cell by one of
three processes
• Transformation, transduction, or conjugation
DNA enters
cell
Fragment of DNA
from another
bacterial cell
Bacterial
chromosome
(DNA)
Mating bridge
Phage
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
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Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
– Once new DNA gets into a bacterial cell
• Part of it may then integrate into the recipient’s
chromosome
Donated DNA
Figure 10.22D
Recipient cell’s
chromosome
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Crossovers
Degraded DNA
Recombinant
chromosome
The ability of
E.coli to carry
out conjugation
• 10.23 Bacterial F factors and plasmids can
serve as carriers for gene transfer
• F Factor (fertility) is integrated into the host
genome
• Plasmids are not integrated
• Are small circular DNA molecules separate from the
bacterial chromosome
• Confers antibiotic resistence
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– Plasmids can serve as carriers
• For the transfer of genes
F factor (plasmid)
F factor
(integrated)
Male (donor) cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
Male (donor) cell
Bacterial chromosome
F factor starts replication
and transfer
Only part of the chromosome
transfers
Plasmid completes transfer
and circularizes
Plasmids
Recombination
can occur. Cell still
female
Figure 10.23A–C
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Cell now male
Colorized TEM 2,000
Recipient cell
10.22 Virus research and molecular genetics are
intertwined
• Virus studies help
establish molecular
genetics
• Molecular genetics helps
us understand viruses
– such as HIV, seen here
attacking a white blood
cell
Figure 10.22
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings