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Chapter 16 Retroviruses And retroposons
16.1 Introduction 16.2 The retrovirus life cycle involves transposition-like events16.3 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
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.
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
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.
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
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.
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
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.
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
16.4 Yeast Ty elements resemble retroviruses Figure 16.14 Ty elements generate virus-like particles. Photograph kindly provided by Alan Kingsman.
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
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.
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).
