Vectors in Genetic Engineering

• Vectors is a DNA fragment and serving as an intermediate gene carrier
• It is used for the transport of desirable gene to host cell, for recombination
• Some vector act as cloning vectors producing identical copies, inside host cell

Features of vectors

• Small sized- for easy isolation and handling .

• Presence of replication ‘Ori’ (origin)

• Presence of restriction site outside ori

• Presence of marker genes, for easy identification.

• Absence of Transfer genes

Examples for vector

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• Plasmid
• Bacteriophage
• Cosmid
• Artificial chromosomes

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Plasmid

• They are the extra chromosomal, self replicating and double stranded, circular DNA molecules
• It is observed in bacterial cells.
• Naturally occurring plasmids can be modified by in vitro techniques.
• Plasmid can accommodate an insert size upto 10kb DNA fragment

Egs- PBR 322, PUC 8.

Bacteriophage

• The viruses that infect bacteria are called bacteriophage.
• Bacteriophages have a very high significant mechanism for delivering its genome into bacterial cell. Hence it can be used as a cloning vector to deliver larger DNA segments.
• Most of the bacteriophage genome is non-essential and can be replaced with foreign DNA.
• Using bacteriophage as a vector, a DNA fragment of size up to 20 kb can be transformed.

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Cosmid

• They are hybrid cloning vectors, especially used for long fragments of eukaryotic DNA.
• As the name indicates, cosmid is a plasmid with an integrated “cos site” (cohesive end site) of the lamda phage.
• The cloning capacity of cosmid is almost two times greater than the best lamda phage vectors.
• This high cloning efficiency enables them to serve as an ideal system for making gene libraries

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Artificial chromosome

• Some eukaryotic genes are too large to be cloned.
• So artificial chromosomes, capable of accommodating long DNA are generated by genetic engineering.
• They are chromosomes with a centromere, several ori, large cloning site with telomeres fused.
• Endogenously, enzyme cut telomeres, so circular chromosomes become linear

BAC (Bacterial Artificial Chromosome)

• BACs are bacterial expression vectors
• It is Simple plasmids with 100,000 – 300,000 bp length
• It have selective markers for antibiotic resistance
• BAC are usually used in sequencing the genome of organisms in genome project

YAC (Yeast artificial chromosome)

• Genetically engineered mini chromosome / linear vector, derived from yeast.
• . Since generated from plasmid, initially they are circular. Inside yeast, it cut the telomeres and become linear.
• It has a large cloning site and has two selective markers- TRP 1 & URA 3.
• Initially, it was used in human genome project, but abandoned by the invention of BAC