页数：34页文件大小：939.0 KB来源： 分子生物学基因Geneppt课件
本课件为《分子生物学》第16章逆转录病毒与逆转录转座子（Retroviruses and retroposons），依据 Gene VII 编排，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 16 Retroviruses and retroposons
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.
Figure 16.1 The reproductive cycles of retroviruses and retroposons involve altern at ion of reverse transcription from RNA to DNA with transcription from DNA to RNA. Only retroviruses can generate infectious particles. Retroposons are confined to an intr a cellular cycle.
LTR is an abbrevi at ion for long-terminal repeat.
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.
Figure 16.3 The genes of the retrovirus are expressed as poly proteins 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.
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.
Figure 16.6 Minus strand DNA is generated by switching templates during reverse transcription.
Figure 16.7 Synthesis of plus strand DNA requires a second jump.
Figure 16.9 Integrase is the only viral protein required for the integr at ion 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.
Helper virus provides functions absent from a defective virus, enabling the latter to complete the infective cycle during a mixed infection.
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.
Figure 16.11 Replication-defective viruses may be generated through integr at ion 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.
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.
Figure 16.3 The genes of the retrovirus are expressed as poly proteins 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.
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.
Figure 16.6 Minus strand DNA is generated by switching templates during reverse transcription.
Figure 16.14 Ty elements generate virus-like particles. Photograph kindly provided by Alan Kingsm an.
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.
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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 pseudogeneis 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.
Figure 16.16 Retroposons can be divided into the viral or nonviral superfamilies.
Figure 16.17 Retroposons of the viral family have terminal repeats and include open reading frames.
Figure 16.6 Minus strand DNA is generated by switching templates during reverse transcription.
Figure 16.18 Retrotranspositi on of non-LTR elements occurs by nicking the target to provide a primer for cDNA synthesis on an RNA template.
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.
16.7 Summary•Reverse transcription is the unifying mechanism for reproduction of retroviruses and perpetu at ion 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 integr at ion 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).
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