What is a double-strand break in DNA?
Abstract. The DNA double-strand break (DSB) is the principle cytotoxic lesion for ionizing radiation and radio-mimetic chemicals but can also be caused by mechanical stress on chromosomes or when a replicative DNA polymerase encounters a DNA single-strand break or other type of DNA lesion.
How are double stranded breaks in DNA repair?
DNA double-strand breaks are repaired by means of two main mechanisms: nonhomologous end joining and homologous recombination (see Figure 1). Both mechanisms operate in all eukaryotic cells that have been examined but the relative contribution of each mechanism varies.
What is the importance of double-strand breaks in initiation of recombination?
During meiosis, cells deliberately form numerous DNA double-strand breaks (DSBs) in order to initiate homologous recombination, which exchanges genetic information between homologous chromosomes and promotes accurate chromosome segregation.
What is gene conversion How does it occur in the double-strand break model of homologous recombination?
Gene conversion is explained by the repair of mismatches present in heteroduplex DNA. We propose a new mechanism for meiotic recombination, in which events are initiated by double-strand breaks that are enlarged to double-strand gaps.
How does double strand break occur?
Double-strand breaks (DSBs) in DNA form as a result of exposure to exogenous agents such as radiation and certain chemicals, as well as through endogenous processes, including DNA replication and repair.
What is the role of DNA strand breaks in genetic recombination?
In meiotic cells, double-stranded DNA breaks are programmed, and the ensuing recombination serves to link homologous chromosomes via crossovers to ensure their proper segregation during the first meiotic division.
How does double-strand break occur?
Why is DNA double stranded?
The common theme of all the excision mechanisms is that DNA must be double stranded to provide a template for the repair. Separate from these, there are proteins involved in the direct reversal of damage (e.g., photoreactivation, O6 methylguanine DNA methyl transferase).
How common are double-strand breaks?
Another study estimates that, in normal human cells, ∼1% of single-strand lesions are converted to ∼50 DSBs per cell per cell cycle, that is, about one DSB per 108 bp (Vilenchik and Knudson 2003).
What is the importance of DNA conjugate in reproduction?
3.07. Conjugation is an important process for genetic exchange between bacteria. The process needs mating of donor cell and recipient cell, and involves a cis-acting nick site (oriT) and the trans-acting functions given by a transfer protein.
What is the double-strand-break model?
It is called the double-strand-break model. New features in this model (contrasting with the Holliday model) are initiation at double-strand breaks, nuclease digestion of the aggressor duplex, new synthesis and gap repair.
How are DNA double-strand breaks repaired?
DNA double-strand breaks (DSBs) are one of the most toxic of these lesions and must be repaired to preserve chromosomal integrity. Eukaryotes are equipped with several different, but related, repair mechanisms involving homologous recombination, including single-strand annealing, gene conversion, and break-induced replication.
Can a double-strand gap in a duplex be used to initiate recombination?
Using plasmids transformed into yeast, it was shown that a double-strand gap in the “aggressor” duplex could be used to initiate recombination, and the gap was repaired during the recombination (this experiment is explored in problem 8.___). In this case, the gap in one duplex was filled by DNA donated from the other substrate.
What are the steps of recombination?
This includes homology search, DNA strand invasion, repair DNA synthesis, and restoration of intact chromosomes. Aspects of DNA topology affecting individual steps are highlighted. Overall, recombination is a dynamic pathway with multiple metastable and reversible intermediates designed to achieve DNA repair with high fidelity.