Interaction Between F Factor And Chromosomes

Interaction Between F Factor And Chromosomes

	Interaction Between F Factor And Chromosomes - Donor cells are of three types, F+ donors, Hfr donors and F prime (F') donors.The F+ donor transfers the F factor as described previously. The Hfr donor is formed by the integration of the F factor into the chromosome. The probability of the generation of an Hfr cell from an F+ population is about 10-5 per cell, per generation. A break occurs in both stands of plasmid as well as chromosomal DNA in regions having similarity of nucleotide sequences. The plasmid DNA is inserted into the chromosome to form an Hfr donor cell. This insertion is analogous to that of prophage formation in bacteriophages
The bacterial chromosome has only a limited n-umber of sites where integration of the sex factor can take place. Hfr strains thus appear to arise by nonrandom integration of the F factor at specific sites Recombination takes place between homologous sites on the F factor and the chromosome. During conjugation of an Hfr cell with an F - cell, one strand of the F plasmid breaks, and a part of the plasmid strand is transferred to the recipient cell along with the chromosome The F prime (F') factor is formed by the excision of the F factor from the chromosome along with a segment of the chromosome. This factor was isolated by Adelberg and Burns (1960). The release of the F factor occurs by breakage and reunion at a point different from the point of plasmid insertion
This results in an interchange of plasmid and chromosomal DNA segments. The F -prime factor consists of the plasmid along with the segment picked up from the chromosome. It carries certain chromosomal genes but lacks a part of the F factor. The bacterial chromosome has lost certain genes, but has gained a segment of the F factor. Bacterial strains containing F-prime factors are called F-prime donors. The characteristics of the F-prime donor strain are intermediate between those of the F+ and the Hfr strains
The F-prime episome is transferred infectiously during conjugation of the F-prime cell with an F - cell. Since the F-prime factor also carries bacterial genes, it is able to pair preferentially with a corresponding region of the bacterial chromosome. The frequency of transfer of the F-prime factor is, therefore, greater than that shown by the F+ strain (10-1 per division cycle for F-prime as compared to 10-5 per cell per generation for F+). It is, however, less than that of the Hfr strain
	Because the F-prime factor contains genes of the bacterial chromosome, the F- recipient cells acquire the characteristics coded by these genes. Thus if an F-prime factor containing the lactose operon (lac +) is transferred to an F- lac- cell the latter become Flac+ (The F-prime factor carrying the lactose operon IS designated as Flac+). If the recipient chromosome already carries a lac + operon, it now bas two lac operons, and is diploid for the bacterial genes carried by the episome. The whole transfer sequence IS, therefore, as follows: Flac+ x F-lac- -+ Flac+ + lac+

Transformation
Competence
Binding
Penetration
Synapsis
Integration
Conjugation
Unidirectional Genetic Transfer
Properties of Sexual Mating Types
F Factor
Interaction Between F Factor and Chromosome
Chromosome Transfer
Integration of Donor DNA
Transduction
Generalised Transduction
Role of Defective Phage
Complete and Abortive Transduction
Specialized - Restricted Genetic Transduction
Low Frequency Transduction
High Frequency Transduction

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