Chapter 16
Retroviruses
And
retroposons
16.1 Introduction
16.2 The retrovirus life cycle involves transposition-like events
16.3 Retroviral genes codes for polyproteins
16.4 Viral DNA is generated by reverse transcription
16.5 Viral DNA integrates into the chromosome
16.6 Retroviruses may transduce cellular sequences
16.7 Yeast Ty elements resemble retroviruses
16.8 Many transposable elements reside in D,melanogaster
16.9 Retroposons fall into two classes
16.10 The Alu family has many widely dispersed members
Retroposon is a transposon that mobilizes via
an RNA form; the DNA element is
transcribed into RNA,and then reverse-
transcribed into DNA,which is inserted at a
new site in the genome.
16.1 Introduction
Figure 16.1 The reproductive
cycles of retroviruses and
retroposons involve
alternation of reverse
transcription from RNA to
DNA with transcription from
DNA to RNA,Only
retroviruses can generate
infectious particles,
Retroposons are confined to
an intracellular cycle.
9.1 Introduction
LTR is an abbreviation
for long-terminal repeat.
16.2 The retrovirus life cycle
involves transposition like events
Figure 16.2 The retroviral
life cycle proceeds by
reverse transcribing the
RNA genome into duplex
DNA,which is inserted
into the host genome,in
order to be transcribed
into RNA.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.3 The genes of the retrovirus are expressed as
polyproteins that are processed into individual products.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.4
Retroviruses (HIV)
bud from the plasma
membrane of an
infected cell,
Photograph kindly
provided by
Matthew Gonda.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.5
Retroviral RNA
ends in direct
repeats (R),the
free linear DNA
ends in LTRs,
and the provirus
ends in LTRs
that are
shortened by
two bases each.
16.2 The retrovirus life cycle involves transposition like events
Figure 16.6 Minus strand
DNA is generated by
switching templates during
reverse transcription.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.7 Synthesis of
plus strand DNA requires a
second jump.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.9 Integrase
is the only viral protein
required for the
integration reaction,in
which each LTR loses
2 bp and is inserted
between 4 bp repeats
of target DNA.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.8 Copy choice
recombination occurs when
reverse transcriptase releases its
template and resumes DNA
synthesis using a new template,
Transfer between template strands
is probably occurs directly,but is
shown here in separate steps to
illustrate the process.
16.2 The retrovirus life
cycle involves
transposition like events
Helper virus provides functions absent
from a defective virus,enabling the
latter to complete the infective cycle
during a mixed infection.
16.3 Retroviruses may
transduce cellular sequences
Figure 16.10 Replication-defective transforming viruses have
a cellular sequence substituted for part of the viral sequence,
The defective virus may replicate with the assistance of a
helper virus that carries the wild-type functions.
16.3 Retroviruses may transduce cellular sequences
Figure 16.11 Replication-
defective viruses may be
generated through
integration and deletion of
a viral genome to generate
a fused viral-cellular
transcript that is packaged
with a normal RNA
genome,Nonhomologous
recombination is
necessary to generate the
replication-defective
transforming genome.
16.3 Retroviruses may transduce cellular sequences
Figure 16.12 Ty
elements terminate in
short direct repeats and
are transcribed into
two overlapping RNAs,
They have two reading
frames,with sequences
related to the retroviral
gag and pol genes.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.13 A unique Ty
element,engineered to
contain an intron,transposes
to give copies that lack the
intron,The copies possess
identical terminal repeats,
generated from one of the
termini of the original Ty
element.
16.4 Yeast Ty elements
resemble retroviruses
Figure 16.3 The genes of the retrovirus are expressed as
polyproteins that are processed into individual products.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.4
Retroviruses (HIV)
bud from the
plasma membrane
of an infected cell,
Photograph kindly
provided by
Matthew Gonda.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.5 Retroviral
RNA ends in direct
repeats (R),the free
linear DNA ends in
LTRs,and the
provirus ends in LTRs
that are shortened by
two bases each.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.6 Minus strand
DNA is generated by
switching templates during
reverse transcription.
16.4 Yeast Ty elements
resemble retroviruses
Figure 16.14 Ty
elements generate
virus-like particles,
Photograph kindly
provided by Alan
Kingsman.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.15 Three types of
transposable element in D,
melanogaster have different
structures.
16.5 Many
transposable elements
reside in D,
melanogaster
Figure 16.15 Three types of
transposable element in D,
melanogaster have different
structures.
16.5 Many
transposable elements
reside in D,
melanogaster
Alu family is a set of dispersed,related sequences,each
~300 bp long,in the human genome,The individual
members have Alu cleavage sites at each end (hence the
name).
Processed pseudogene is an inactive gene copy that lacks
introns,contrasted with the interrupted structure of the
active gene,Such genes presumably originate by reverse
transcription of mRNA and insertion of a duplex copy
into the genome.
16.6 Retroposons fall into two classes
Figure 16.16 Retroposons can be divided
into the viral or nonviral superfamilies.
16.6 Retroposons fall into two classes
Figure 16.17
Retroposons of
the viral family
have terminal
repeats and
include open
reading frames.
16.6 Retroposons fall into two classes
Figure 16.6 Minus
strand DNA is
generated by
switching templates
during reverse
transcription.
16.6
Retroposons fall
into two classes
Figure 16.18
Retrotransposition of non-
LTR elements occurs by
nicking the target to provide a
primer for cDNA synthesis on
an RNA template.
16.6
Retroposons fall
into two classes
Figure 16.19 Pseudogenes could arise by reverse
transcription of RNA to give duplex DNAs that
become integrated into the genome.
16.6 Retroposons fall into two classes
Figure 23.11 An
intron codes for an
endonuclease that
makes a double-
strand break in DNA,
The sequence of the
intron is duplicated
and then inserted at
the break.
16.6 Retroposons fall into two classes
16.7 Summary
?Reverse transcription is the unifying mechanism
for reproduction of retroviruses and perpetuation of
retroposons,
?Retroviruses have genomes of single-stranded
RNA that are replicated through a double-stranded
DNA intermediate,
?Reverse transcriptase is the major component of
pol,and is responsible for synthesizing a DNA
(minus strand) copy of the viral (plus strand) RNA,
16.7 Summary
?Switches in template during nucleic acid
synthesis allow recombination to occur by
copy choice,
?The integration event generates direct target
repeats (like transposons that mobilize via
DNA),
?Another class of retroposons have the
hallmarks of transposition via RNA,but
have no coding sequences (or at least none
resembling retroviral functions).
Retroviruses
And
retroposons
16.1 Introduction
16.2 The retrovirus life cycle involves transposition-like events
16.3 Retroviral genes codes for polyproteins
16.4 Viral DNA is generated by reverse transcription
16.5 Viral DNA integrates into the chromosome
16.6 Retroviruses may transduce cellular sequences
16.7 Yeast Ty elements resemble retroviruses
16.8 Many transposable elements reside in D,melanogaster
16.9 Retroposons fall into two classes
16.10 The Alu family has many widely dispersed members
Retroposon is a transposon that mobilizes via
an RNA form; the DNA element is
transcribed into RNA,and then reverse-
transcribed into DNA,which is inserted at a
new site in the genome.
16.1 Introduction
Figure 16.1 The reproductive
cycles of retroviruses and
retroposons involve
alternation of reverse
transcription from RNA to
DNA with transcription from
DNA to RNA,Only
retroviruses can generate
infectious particles,
Retroposons are confined to
an intracellular cycle.
9.1 Introduction
LTR is an abbreviation
for long-terminal repeat.
16.2 The retrovirus life cycle
involves transposition like events
Figure 16.2 The retroviral
life cycle proceeds by
reverse transcribing the
RNA genome into duplex
DNA,which is inserted
into the host genome,in
order to be transcribed
into RNA.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.3 The genes of the retrovirus are expressed as
polyproteins that are processed into individual products.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.4
Retroviruses (HIV)
bud from the plasma
membrane of an
infected cell,
Photograph kindly
provided by
Matthew Gonda.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.5
Retroviral RNA
ends in direct
repeats (R),the
free linear DNA
ends in LTRs,
and the provirus
ends in LTRs
that are
shortened by
two bases each.
16.2 The retrovirus life cycle involves transposition like events
Figure 16.6 Minus strand
DNA is generated by
switching templates during
reverse transcription.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.7 Synthesis of
plus strand DNA requires a
second jump.
16.2 The retrovirus
life cycle involves
transposition like
events
Figure 16.9 Integrase
is the only viral protein
required for the
integration reaction,in
which each LTR loses
2 bp and is inserted
between 4 bp repeats
of target DNA.
16.2 The retrovirus life cycle involves
transposition like events
Figure 16.8 Copy choice
recombination occurs when
reverse transcriptase releases its
template and resumes DNA
synthesis using a new template,
Transfer between template strands
is probably occurs directly,but is
shown here in separate steps to
illustrate the process.
16.2 The retrovirus life
cycle involves
transposition like events
Helper virus provides functions absent
from a defective virus,enabling the
latter to complete the infective cycle
during a mixed infection.
16.3 Retroviruses may
transduce cellular sequences
Figure 16.10 Replication-defective transforming viruses have
a cellular sequence substituted for part of the viral sequence,
The defective virus may replicate with the assistance of a
helper virus that carries the wild-type functions.
16.3 Retroviruses may transduce cellular sequences
Figure 16.11 Replication-
defective viruses may be
generated through
integration and deletion of
a viral genome to generate
a fused viral-cellular
transcript that is packaged
with a normal RNA
genome,Nonhomologous
recombination is
necessary to generate the
replication-defective
transforming genome.
16.3 Retroviruses may transduce cellular sequences
Figure 16.12 Ty
elements terminate in
short direct repeats and
are transcribed into
two overlapping RNAs,
They have two reading
frames,with sequences
related to the retroviral
gag and pol genes.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.13 A unique Ty
element,engineered to
contain an intron,transposes
to give copies that lack the
intron,The copies possess
identical terminal repeats,
generated from one of the
termini of the original Ty
element.
16.4 Yeast Ty elements
resemble retroviruses
Figure 16.3 The genes of the retrovirus are expressed as
polyproteins that are processed into individual products.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.4
Retroviruses (HIV)
bud from the
plasma membrane
of an infected cell,
Photograph kindly
provided by
Matthew Gonda.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.5 Retroviral
RNA ends in direct
repeats (R),the free
linear DNA ends in
LTRs,and the
provirus ends in LTRs
that are shortened by
two bases each.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.6 Minus strand
DNA is generated by
switching templates during
reverse transcription.
16.4 Yeast Ty elements
resemble retroviruses
Figure 16.14 Ty
elements generate
virus-like particles,
Photograph kindly
provided by Alan
Kingsman.
16.4 Yeast Ty elements resemble retroviruses
Figure 16.15 Three types of
transposable element in D,
melanogaster have different
structures.
16.5 Many
transposable elements
reside in D,
melanogaster
Figure 16.15 Three types of
transposable element in D,
melanogaster have different
structures.
16.5 Many
transposable elements
reside in D,
melanogaster
Alu family is a set of dispersed,related sequences,each
~300 bp long,in the human genome,The individual
members have Alu cleavage sites at each end (hence the
name).
Processed pseudogene is an inactive gene copy that lacks
introns,contrasted with the interrupted structure of the
active gene,Such genes presumably originate by reverse
transcription of mRNA and insertion of a duplex copy
into the genome.
16.6 Retroposons fall into two classes
Figure 16.16 Retroposons can be divided
into the viral or nonviral superfamilies.
16.6 Retroposons fall into two classes
Figure 16.17
Retroposons of
the viral family
have terminal
repeats and
include open
reading frames.
16.6 Retroposons fall into two classes
Figure 16.6 Minus
strand DNA is
generated by
switching templates
during reverse
transcription.
16.6
Retroposons fall
into two classes
Figure 16.18
Retrotransposition of non-
LTR elements occurs by
nicking the target to provide a
primer for cDNA synthesis on
an RNA template.
16.6
Retroposons fall
into two classes
Figure 16.19 Pseudogenes could arise by reverse
transcription of RNA to give duplex DNAs that
become integrated into the genome.
16.6 Retroposons fall into two classes
Figure 23.11 An
intron codes for an
endonuclease that
makes a double-
strand break in DNA,
The sequence of the
intron is duplicated
and then inserted at
the break.
16.6 Retroposons fall into two classes
16.7 Summary
?Reverse transcription is the unifying mechanism
for reproduction of retroviruses and perpetuation of
retroposons,
?Retroviruses have genomes of single-stranded
RNA that are replicated through a double-stranded
DNA intermediate,
?Reverse transcriptase is the major component of
pol,and is responsible for synthesizing a DNA
(minus strand) copy of the viral (plus strand) RNA,
16.7 Summary
?Switches in template during nucleic acid
synthesis allow recombination to occur by
copy choice,
?The integration event generates direct target
repeats (like transposons that mobilize via
DNA),
?Another class of retroposons have the
hallmarks of transposition via RNA,but
have no coding sequences (or at least none
resembling retroviral functions).
