The way in which the phage λ DNA is replicated is of particular interest in the development of larger insert cloning vectors termed cosmids ( Figure 1). These are especially useful for the analysis of highly complex genomes and are an important part of various genome mapping projects. The upper limit of the insert capacity of phage λ is approximately 21 kb. This is because of the requirement for essential genes and the fact that the maximum length between the cos sites is 52 kb. Consequently, cosmid vectors have been constructed that incorporate the cos sites from phage λ and also the essential features of a plasmid, such as the plasmid origin of replication, a gene for drug resistance, and several unique restriction sites for insertion of the DNA to be cloned. When a cosmid preparation is linearised by restriction digestion, and ligated to DNA for cloning, the products will include concatemers of alternating cosmid vector and insert. Thus, the only requirement for the length of DNA to be packaged into viral heads is that it should contain cos sites spaced apart at the correct distance; in practice, this spacing can range between 37 and 52 kb. Such DNA can be packaged in vitro if phage head precursors, tails and packaging proteins are provided. Since the cosmid is very small, inserts of about 40 kb in length will be most readily pack aged. Once inside the cell, the DNA recircularises through its cos sites, and from then onwards behaves exactly like a plasmid.

Fig1. Scheme for cloning foreign DNA fragments in cosmid vectors.

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مواضيع ذات صلة

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Cloning Vectors : Virus-Based Vectors

Cloning Vectors : Plasmids

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Chromosome Painting and DNA Sequencing Reveal the Evolution of Chromosomes

During Metaphase, Chromosomes Can Be Distinguished by Banding Patterns and Chromosome Painting

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Origins and Types of Mutations

Regulation of Gene Expression

The Human Genome

The Polymerase Chain Reaction (PCR)

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