Question

Describe the features that distinguish plasmid, phage, lambda, BAC and YAC cloning vectors.

Answer 1

Cloning vector is a DNA molecule used as a vehicle to transfer a foreign DNA fragment into the host cell where it can be cloned easily. The cloning vectors and their features have been mentioned as the following:

1. Plasmid: Plasmid is extra- chromosomal, circular, double stranded DNA present in bacteria, which is an autonomously replicating unit. It can be used as vehicle to transfer a segment of foreign DNA to the host organism hence called vector. Such plasmid vectors with inserted DNA segment are called recombinant plasmid vector. Plasmid can accommodate a DNA insert upto 15kb in size. Plasmid vectros have very high copy number (number of copies of the vector in the host cell), upto 500 per cell. Plasmic vectors contain origin of replication (ori) essential for initiation of replication.

2. Phage lambda: Phage is a virus that infects bacteria, also called bacteriophage. DNA inserts upto 53kb can be inserted into a bacteriophage. The bacteriophaage is used to transfer foreign DNA into the host bacterial cell. Phage vectrs can be used to insert and clone larger fragments of DNA in comparison to plasmid vectors. Example of bacteriophage vectors are phage lambda (phageλ) and M13 cloning vectors. Phageλ and M13 phage are E.coli phages. Phage vectors also possess ori.

Phage lambda vector: It has a genome size of 48.5kb with 12bp single stranded cohesive ends called cos sites at both the terminals. It has higher transformation efficiency in comparison to plasmid vectors. It has an insert capacity of 9-25kb DNA fragment. It can be used as an insert or a replacement vector means either DNA segment can be inserted into the vector or the DNA from the may be replaced with the DNA to be added.

4. BAC: BAC stands for Bacterial Artificial Chromosome. BACs are similar to the bacterial plasmids but are linear and have higher insert capacity of 150-350kb. BAC has been derived from the Fertility plasmid (F plasmid) of bacteria. BAC cloning vectors have very low copy number of 1 or 2 per cell. Similar to plasmid and phage cloning vectors, BAC cloning vector is also used for DNA transfer and cloning in bacterial cells, but is used in mapping and sequencing of eukaryotic genomes. BAC vectors also have ori.

5. YAC: YAC stands for Yeast Artificial Chromosome.  It is a linear plasmid vector. The Dna molecule isolated from Saccharomyces cerevisae and joined together with bacterial plasmid, with yeast telomers at both the ends gives the artificial chromosome called YAC. When used as a vector can clone upto 100-1.4 Mb DNA fragment. It is used for cloning in bacterial as well as in yeast cells, and are utilized for studying eukaryotic gemones. YAC vectors have an Autonomous Replicating Sequence (ARS) instead of ori. Similar to BAC, it has low copy number of 1 or 2 per cell.