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DNA Topoisomerases

DNA Topoisomerases

Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes regulate positive and negative DNA supercoiling without changing the nucleotide sequence. DNA overwinding in a clockwise direction results in positively supercoiled DNA, whereas underwinding in a counterclockwise direction produces negatively supercoiled DNA.
Types and Mechanism of action
Topoisomerases are divided into two main types. Type I...

Homologous Recombination

Homologous Recombination

The basic reaction of homologous recombination (HR) involves two chromatids that contain DNA sequences sharing a significant stretch of identity. One of these sequences uses a strand from another as a template to synthesize DNA in an enzyme-catalyzed reaction. The final product is a novel amalgamation of the two substrates. To ensure an accurate recombination of sequences, HR is restricted to the S and G2 phases of the cell cycle. At these stages, the DNA has been replicated already and the...

Overview of Transposition and Recombination

Overview of Transposition and Recombination

Transposons make up a significant part of genomes of various organisms. Therefore, it is believed that transposition played a major evolutionary role in speciation by changing genome sizes and modifying gene expression patterns. For example, in bacteria, transposition can lead to conferring antibiotic resistance. Movement of transposable elements within the genetic pool of pathogenic bacteria can aid in transfer of antibiotic-resistant genetic elements. In eukaryotes, transposons can carry out...

Conservative Site-specific Recombination and Phase Variation

Conservative Site-specific Recombination and Phase Variation

Because the DNA segments are cut and reorganized in a direction-specific manner, site-specific recombination has emerged as an efficient genetic engineering technique. Flippase and Cyclization recombinases or Flp and Cre, respectively, are two members of the tyrosine recombinase family derived from bacteriophages, that are used to mediate site-specific DNA insertions, deletions, and targeted expression of proteins in mammalian cell lines.
The recognition sites for Cre recombinase called LoxP...

Homologous Recombination

Homologous Recombination

The basic reaction of homologous recombination (HR) involves two chromatids that contain DNA sequences sharing a significant stretch of identity. One of these sequences uses a strand from another as a template to synthesize DNA in an enzyme-catalyzed reaction. The final product is a novel amalgamation of the two substrates. To ensure an accurate recombination of sequences, HR is restricted to the S and G2 phases of the cell cycle. At these stages, the DNA has been replicated already and the...

Crossing Over

Crossing Over

Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I, duplicated...

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Related Experiment Video

Updated: May 11, 2026

Recombineering Homologous Recombination Constructs in Drosophila

Recombineering Homologous Recombination Constructs in Drosophila

Published on: July 13, 2013

Topological selectivity in Xer site-specific recombination

S D Colloms¹, J Bath, D J Sherratt

¹Department of Biochemistry, University of Oxford, United Kingdom.

|March 21, 1997

Summary

Xer-mediated recombination in plasmids results in specific DNA topologies. This implies fixed mechanisms for synaptic complex formation and strand exchange, guiding recombination efficiency.

Area of Science:

Molecular Biology
Genetics
Biochemistry

Background:

Xer-mediated site-specific recombination is crucial for genetic processes.
Understanding the topological products of this recombination is key to elucidating its mechanism.

Purpose of the Study:

To determine the product topology of Xer-mediated site-specific recombination at plasmid sites.
To infer the topological constraints of the synaptic complex and strand exchange mechanism.

Main Methods:

Analysis of recombination products from pSC101 psi and ColE1 cer plasmids.
Topological analysis of DNA catenanes formed during recombination.

Main Results:

The deletion product at pSC101 psi is a right-handed antiparallel 4-noded catenane.

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Determination of the Optimal Chromosomal Location(s) for a DNA Element in Escherichia coli Using a Novel Transposon-mediated Approach

Determination of the Optimal Chromosomal Location(s) for a DNA Element in Escherichia coli Using a Novel Transposon-mediated Approach

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Related Experiment Videos

Last Updated: May 11, 2026

Recombineering Homologous Recombination Constructs in Drosophila

Recombineering Homologous Recombination Constructs in Drosophila

Published on: July 13, 2013

Genetic Manipulation in Δku80 Strains for Functional Genomic Analysis of Toxoplasma gondii

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Published on: July 12, 2013

Determination of the Optimal Chromosomal Location(s) for a DNA Element in Escherichia coli Using a Novel Transposon-mediated Approach

Determination of the Optimal Chromosomal Location(s) for a DNA Element in Escherichia coli Using a Novel Transposon-mediated Approach

Published on: September 11, 2017

The ColE1 cer deletion product exhibits identical topology with one pair of strands exchanged.

Synapsis traps three negative supercoils, and strand exchange adds a negative node.

Conclusions:

The specific product topologies indicate fixed structures for the synaptic complex and strand exchange.
A model is proposed where interwrapped recombination sites and accessory proteins (ArgR, PepA) ensure efficient recombination between directly repeated sites on the same DNA molecule.