Recombination:
An Old and New tool for plant breeding
7 EU

Keywords

 

 

 

 

 

Meiotic Recombination (MR)

 

 

 

Meiosis

Meiosis is the specialized cell division used by sexually reproducing organisms to halve the chromosome complement in the gametes. During meiosis, a single round of DNA replication is followed by two successive divisions: the first (reductional) division segregates homologous chromosomes, and the second (equational) division separates sister chromatids from each other, resulting in four nuclei with a single complement of chromosomes. For a recent review see: Osman et al. "Pathways to meiotic recombination in Arabidopsis thaliana" New Phytologist (2011) 190: 523-544.

 

 

Homologous recombination (HR)

Homologous Recombination relates to the exchange of DNA between sequences that share substantial homology. It is a central cellular process involved in many aspects of genome maintenance such as DNA repair, replication, telomere maintenance and meiotic chromosomal segregation. HR is highly conserved in pro- and eukaryotes, contributing to genome stability as well as to the generation of genetic diversity. In principle, HR may occur via a variety of mechanisms. Experimental data support DNA double-strand break (DSB)-induced HR as a prominent recombination pathway. Several HR mechanisms enable the precise repair of a DSB caused by external environmental sources (e.g., genotoxic agents, and ionizing radiation), or naturally occurring, during endogenous processes such as unwinding stalled replication forks and the initiation of meiosis. HR events occurring during meiotic prophase I ensure the proper segregation of homologous chromosomes at the first meiotic division. These events are initiated by induction of programmed double-strand breaks by the SPO11 protein and repair of such breaks by homologous recombination which requires many proteins, including the strand exchange activity provided by the RAD51-like protein(s). Homologous recombination can result in either Crossover or Gene conversion (Figure).

 

Homologous recombination

 

 

 

Gene Targeting (GT)

Gene targeting (GT, also referred to in literature as targeted gene replacement), corresponds to the homologous recombination-mediated integration of an extrachromosomal DNA segment into a chromosomal target sequence, enables the precise modification or replacement of any gene. Despite its value as a molecular genetic tool, GT remains a hardly applicable technology in most species. GT is inefficient in most higher eukaryotes, and especially in plants, because the frequency of random DNA integration exceeds that of homologous integration by three to four orders of magnitude. An exception in the plant kingdom is the moss Physcomitrella patens, in which HR is the major pathway of exogenous DNA integration and thus GT rates are as high as in yeast and can reach up to 100%.

 


Arabidopsis thaliana

Arabidopsis thalianaArabidopsis thaliana is a small flowering plant that is widely used as a model organism in plant biology. Arabidopsis is a member of the mustard (Brassicaceae) family, which includes cultivated species such as cabbage and radish. Arabidopsis is not of major agronomic significance, but it offers important advantages for basic research in genetics and molecular biology. A Wikipedia entry is also available.

(see  the TAIR website: http://www.arabidopsis.org/portals/education/aboutarabidopsis.jsp)

 

Some useful statistics:

  • Small genome (114.5 Mb/125 Mb total) has been sequenced in the year 2000.
  • Extensive genetic and physical maps of all 5 chromosomes .
  • A rapid life cycle (about 6 weeks from germination to mature seed).
  • Prolific seed production and easy cultivation in restricted space.
  • Efficient transformation methods utilizing Agrobacterium tumefaciens.
  • A large number of mutant lines and genomic resources many of which are available from Stock Centers.
  • Multinational research community of academic, government and industry laboratories.

Such advantages have made Arabidopsis a model organism for studies of the cellular and molecular biology of flowering plants. TAIR collects and makes available the information arising from these efforts.