Nature上报道了一个新奇的技术叫做DNA复印。原始的PCR只是扩增两条引物之间的序列，但你有没有想过扩增整个基因组。科学家已经这样想了，这种技术称为WGA（Whole-genome amplification 全基因组扩增）。
在过去的12年内已经有了基于PCR的WGA技术，但是此技术在扩增整个基因组的时候会“漏掉”10％到15的区域不能被扩增。新的方法已经克服了这个缺点。在商业上已经有相关的试剂盒出售，如GE Healthcare公司的GenomiPhi DNA amplification kit，利用短的引物随机与整个基因组结合，使用来自phi29噬菌体的高扩增能力的单链置换DNA聚合酶（single-strand displacing DNA polymerase ）扩增总基因组，此聚合酶会切除掉“挡在”它前面的引物并继续合成DNA。
来自Sigma-Aldrich 公司的GenomePlex WGA kit 先把总DNA打断为约400碱基对的片段，然后接上特定的接头，利用与接头匹配的引物，通过PCR的方法扩增出整个基因组的序列。
参考文献：
Photocopiers for DNA.Nature 435, 237 (12 May 2005)
http://www.nature.com/nature/journal/v435/n7039/full/435237a.html
http://www.nature.com/nature/journal/v435/n7039/pdf/435237a.pdf
Nature 435, 237 (12 May 2005)
Photocopiers for DNA
Rather than amplifying just the DNA between two primers, many researchers want to amplify entire genomes, to make archive copies of the total DNA from a unique biopsy sample, for example. "If you have a piece of paper with valuable information on it, obviously you don‘t want to lose the paper, but instead photocopy it many times, store some copies appropriately and use others," says Andy Betera vice-president of product management of GE Healthcare, based in Little Chalfont, UK. Whole-genome amplification (WGA) systems aim to act as photocopiers for DNA. "From the point of view of the molecular epidemiologist or molecular geneticist, the overwhelming potential advantage of the newer methods of WGA is the possibility of producing additional genomic DNA, or amplified WGA DNA, for genotyping, sequencing or other types of genetic analysis like loss of heterozygosity (LOH)," says Andrew Bergen, staff scientist at the National Cancer Institute in Bethesda, Maryland.
GE HEALTHCARE
GenomiPhi-amplified salmon sperm.
"For the past 12 years there have been PCR-based methods of WGA with about a 10 or 15% locus bias, meaning that 10 or 15% of the genetic loci in the genome would not be amplified," explains Bergen. Newer methods can avoid this loss in the right conditions. But problems really arise when you have only a very small sample from a slide or biopsy to play with. Bergen says colleagues who have almost exhausted the material from a particular sample ask him: "Can this sample be rescued?" And no WGA method is really effective with less than 10 ng of genomic DNA, he says.
On the commercial front, GE Healthcare‘s GenomiPhi DNA amplification kit uses the highly processive single-strand displacing DNA polymerase from phage phi29. Short primers are bound randomly throughout the genome and the enzyme copies the DNA, starting from each primer and displacing the primer ahead of it. The result is a soup of genomic fragments of varying lengths, averaging around 10,000 bases. The phi29 method is able to efficiently amplify a whole genome with no loss of sequence if high molecular weight DNA is used as the starting material.
The GenomePlex WGA kit from Sigma-Aldrich uses technology licensed from Rubicon Genomics of Ann Arbor, Michigan. In this method, the genomic DNA is initially broken up into fragments of around 400 bases long, which are then attached to an identical sequence. The collection is amplified by PCR using a primer that recognizes this sequence. Because short fragments are being copied, this method can cope with starting DNA of high or low molecular weight.