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Index >> Genes >> Genes as a Unit of Recombination (Recon)

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Genes All Cases of Genes are DNA Genes of Some Virus are RNA Genome and Plasmon Genes as a Unit of Recombination (Recon) Gene as a Unit of Mutation (Muton) Gene as a Unit of Function (Cistron) Numer of Genes Number of Nucleotides in Average Gene Duplication of Gene

Genes as a Unit of Recombination (Recon)

	Genes as a Unit of Recombination (Recon)- On the basis of work on Drosophila the gene was considered to be the shortest segment of a chromosome which could be separated from its adjacent segments during crossing over. It was thought that genes were those parts of the chromosomes between which crossing over could take place. Crossing over was, however, not supposed to take place within the genes. The gene was thought to be an indivisible unit of a chromosome controlling some phenotypic character.
However, on the basis of tests for recombinations it has been shown that in viral DNA strands,crossing over not only takes place between the genes, but also within genes. One of the subunits of the gene has been called the recon. It is the smallest unit capable of recombining genetically. Recombination studies on microbes indicate that structurally the recon consists of one or two pairs of nucleotides, possibly only one pair
That crossing over takes place within the gene was demonstrated by Benzer (1955, etc.) in the T4 bacteriophage. Before this it was thought that crossing over could take place only between genes. When bacteriophage cultures are grown on agar plates they form colonies or plaques on the plate. Normal wild type bacteriophages from plagues with smooth edges. Mutant bacteriophages called rII, form large rough edged plaques. Benzer found that two adjacent genes, rIIA and rIIB were responsible for the rough edged plaque trait.
Each gene forms a polypeptide, and the two polypeptides from the protein. A change (mutation) in either polypeptide chain would result in phenotypic expression
	Genetic mapping has shown that in the rIIA gene there are atleast 500 mutational sites where crossing over is possible. If crossing over occurs within the gene, then by mating two rII mutants it should be possible to form the normal wild type. Benzer found that this was indeed the case. He calculated that the lowest frequency of crossover was 0.02% and found that crossing over occurs between two adjacent nucleotides of DNA. Thus, the unit of recombination (recon) is much smaller than the unit of function (cistron).

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Gene Regulation Loci and Alleles Split Genes Hexon Gene Ovalbumin Gene β-Globin Gene Genes Within Genes (Overlapping Genes)

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