Molecular Biology

How Yeast Artificial Chromosome Yac Libraries are Made



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Artificial chromosomes are one of the major determinants of the successes made in the field of cloning and yeast artificial chromosomes (YAC) are one of its many forms. Among the other forms of artificial chromosomes, bacterial artificial chromosomes, phage artificial chromosomes and p1-derived artificial chromosomes have proven their scientific might as effective cloning vectors.

Difference between YAC and other artificial chromosomes:

However, YAC differ in its production as well as in its capacity to hold external DNA during the cloning process, which makes it a unique and a useful vector in cloning large DNA sequences such as the ‘human genome’. In general, YAC will be able to accommodate 20 times more genetic material as a vector when compared to other vectors in DNA cloning, such as the bacterial artificial chromosomes. Numerically, YAC has the capacity to handle about 1000 base pairs of DNA material, which will be able to accommodate most of the genes in higher order species such as humans.

Origins of DNA material for YAC:

In the process of engineering a yeast artificial chromosome, the DNA sequence used as the vector originates from the chromosomes of the yeast cell, which most likely to be belonging to the species, ‘Saccharomomyces cerevesiae’.

Splicing and creating the YAC vector:

Following splicing the yeast chromosomes into DNA fragments using special techniques, each fragment will contain specific ‘end DNA sequences’ and ‘beginning DNA sequences’. In the next step, DNA from a target cell such as from a bacterial cell or a human cell, will be added into the separated chromosome segments of the yeast, which will recognize the ‘foreign DNA’ as part of its own DNA sequence. This makes it to replicate all DNA segments including the ‘foreign DNA’ into a single chromosome, which is the YAC vector. Such vectors with incorporated genetic material are sometimes referred to as, chromosomal libraries.

Cloning process:

When it is time to clone, chemically induced procedure will enable the yeast cell to take up the engineered YAC vector as part of its own chromosome. Thus, following its incorporation, each time the yeast cell multiply, the incorporated genetic material will also multiply proportionately. Therefore, the ultimate result will be a colony of yeasts with genetically engineered chromosomes, which contains the targeted DNA sequence in plenty.

Necessity for large number of cloned DNA material:

The necessity to have large number of copies of the same DNA sequence is because the automated sequencing machines cannot work on single DNA strands. Thus, having enough copies will enable a rapid and accurate sequencing, which made sequencing the human genome, a reality.

Usefulness in the ‘human genome project’:

When sequencing the human genome, different parts of the human genome was replicated in different YACs and following colonizing enough of each YAC, the sequencing machines were able to determine the entire DNA sequence at a rapid rate.

Conclusion:

However, use of YAC has its own limitations and the scientists prefer to use bacterial artificial chromosomes as much as possible due to some of the inherent weaknesses of yeasts in becoming a cloning vector.

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More about this author: Dr Pandula Siribaddana

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