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Depurination of DNA as a possible mutagenic pathway for cells
Abstract:
The possible consequences of depurination for both spontaneous and induced mutagenesis were investigated using in vitro and in vivo assays. Depurination of synthetic polynucleotide templates such as poly [d(A-T)] or poly [d(G-C)] leads to increased misincorporation of noncomplementary nucleotides when these templates are copied by prokaryotic and eukaryotic DNA polymerases. The ability of Escherichia coli DNA polymerase I to copy over apurinic sites was demonstrated using single-stranded circular DNA of bacteriophage 0X174 as a template and starting DNA synthesis at a fixed point. Analysis of the newly synthesized 0X174 restriction fragments on neutral and alkaline sucrose gradients shows that synthesis proceeded past apurinic sites. When using depurinated 0X174 DNA containing the am3 amber mutation as a template for copying by E. coli DNA polymerase I, an increased reversion to wild type is observed after transfection into E. coli spheroplasts. The enhancement in reversion frequency is proportional to the extent of depurination, suggesting that depurination is also mutagenic during copying natural DNA in vitro. When noncopied depurinated 0X174 am3 DNA is transfected in E. coli spheroplasts, no increase in reversion frequency is observed above background level. However, when the spheroplasts are derived from bacteria in which the SOS response had been induced by UV irradiation, a substantial increase is observed for depurinated molecules, whereas no increase is observed for nondepurinated templates, suggesting in vivo mutagenesis at depurinated sites. In each of the different assay systems investigated, the increase in misincorporation or reversion frequency is a linear function of the number of sites and is abolished by treatment of the depurinated templates with alkali, which rapidly induces strand breakage at apurinic sites.
Insights
Depurination, the loss of DNA bases, increases errors during DNA copying by polymerases. This process is mutagenic both in vitro and in vivo, particularly when the SOS DNA repair response is active.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Depurination is a DNA damage event that can arise spontaneously or be induced.
- Understanding the mutagenic potential of depurination is crucial for comprehending DNA repair and mutation processes.
Purpose of the Study:
- To investigate the consequences of depurination on mutagenesis using in vitro and in vivo assays.
- To determine if depurination leads to increased errors during DNA replication and if it causes mutations.
Main Methods:
- In vitro assays using synthetic polynucleotide templates (poly [d(A-T)], poly [d(G-C)]) and DNA polymerases.
- In vivo assays using bacteriophage 0X174 DNA with am3 amber mutation and Escherichia coli spheroplasts.
- Analysis of DNA synthesis past apurinic sites using sucrose gradient centrifugation.
- Induction of the SOS response in E. coli to assess in vivo mutagenesis.
Main Results:
- Depurination of synthetic templates increased misincorporation of nucleotides by DNA polymerases.
- Escherichia coli DNA polymerase I copied over apurinic sites in 0X174 DNA.
- Depurinated 0X174 DNA showed increased reversion frequency to wild type after transfection, proportional to depurination extent.
- In vivo, depurinated DNA induced significant reversion only in UV-irradiated E. coli (SOS response active).
- Alkali treatment abolished the mutagenic effects by inducing strand breaks.
Conclusions:
- Depurination is mutagenic, causing misincorporation and mutations during DNA replication in vitro.
- Depurination can lead to mutagenesis in vivo, especially when the SOS DNA repair pathway is activated.
- The mutagenic effect of depurination is linear with the number of damaged sites and can be prevented by alkali-induced strand breakage.