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DNA bases at specific sequences. Another restriction enzyme labels the bacterium’s DNA as self, and so it
distinguishes self from foreign DNA that would enter the cell during a bacteriophage infection” (Glass 2012).
Yeast does not have this system. The bacterial restriction-modification genes are not expressed while the
genome is cloned in yeast. Any bacterial genome that is isolated from a yeast cell will not have the specific
methylation needed to keep the recipient cell restriction modification system from recognizing it as foreign
DNA and cutting it to pieces. Glass and coworkers managed to mutate the
Mycoplasma capricolum
restriction enzyme so that it could no longer cut donor genomes lacking the appropriate methylation.
Obviously there were no differences, other than the restriction-modification systems issues, between
eukaryotic and prokaryotic DNA that would interfere with the genome transplantation process (Glass 2012).
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Yeast
The yeast species which is mainly used as a chassis
in synthetic biology is Saccharomyces cerevisiae.
It is largely
used in molecular biology, particularly in relation to research on the eukaryotic cell, which links directly into
human biology. They appear to be able to accommodate larger sequences of modified DNA than
Escherichia
coli. The eukaryotic microbe has remarkable capacity to combine homologous pieces of DNA, allowing for up
to 200 base pair overlaps between synthetic fragments (Glass 2012).
Cell engineering
This approach pursues the synthesis of minimal but complete genomes and their insertion in cells to redesign
and control metabolic processes (Moya et al. 2009).
According to the kind of modification and the genetic level where the modification takes place Ellis et al.
(2011) categorised three issues:
1.
Parts to genes
2.
Genes
to pathways
3.
Pathways to genomes
Roughly, these steps comprise combining parts to produce genes, linking genes to make pathways
and devices,
and finally arranging these together to create synthetic chromosomes and genomes.
Table 1: Possibilities for DNA assembly in synthetic biology (Ellis et al. 2011)
The DNA assembly process is still a limiting factor for most laboratories. The methods differ in mechanism and
scale, offering the self-assembly of many parts in a single reaction (parallel assembly), giving constructs with a
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pre-defined physical arrangement (ordered assembly), or allowing multiple versions of parts to be used
simultaneously (combinatorial assembly). The challenge for synthetic biology is to develop standardised
assembly methods allowing work at all levels of abstraction – genes, pathways and genomes – and to clearly
understand the context dependencies when parts are physically placed next to other parts (Ellis et al. 2011).
Assembly of a gene from its individual parts functionally requires ordered assembly. Scar-sequences – bases
without function left behind by some assembly methods – are undesirable as they often affect how parts
function together, and individual parts possibly cannot be replaced in another way once assembled (Ellis et al.
2011).
An idealised assembly method should be suitable for combinatorial construction from standardised part
libraries, have no forbidden site requirements, and allow for pre-determined order in the final product. It
should also allow rapid assembly in a parallel
reaction, working at any scale and only leave scars
between parts
that can tolerate them. As currently no single technique is capable to fulfil all these requirements the most
appropriate strategy involves using several techniques in tandem (Ellis et al. 2011).
Genome synthesis
Genome synthesis,
i.e. DNA synthesis of entire genomes, entails chemically synthesising DNA molecules from
the four different DNA nucleotides or bases: Adenine, Guanine, Thymine and Cytidine. These four bases are
chemically synthesised from glucose and are linearly assembled in specific sequences using an instrument
called an oligonucleotide synthesiser (Glass 2012).
Figure 2: Strategy for synthesising the genome of Mycoplasma genitalium (Glass 2012)
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Genome transplantation
Is the process of installing a naked bacterial chromosome into a suitable recipient cell in such a way that the
installed genome commands and reprograms the machinery of the recipient cell. The resulting cell has only
the attributes encoded by the new genome (Glass 2012).
Figure 3: Strategy for making a synthetic bacterial cell (from Glass (2012))
Parts to genes
Current techniques employ standardised restriction enzyme assembly protocols such as BioBricks™, BglBricks
and Golden Gate methods. Alternatively, sequence independent overlap techniques, such as InFusion™, SLIC
and Gibson assembly® (an isothermal assembly) are becoming popular for larger assemblies, and
in vivo DNA
assembly in yeast and bacillus appears adept for chromosome fabrication (Ellis et al. 2011).
BioBrick™: a DNA unit with standardised flanking sequences that enables assembly to be achieved by a cheap,
simple and standardised restriction/ligation method. The major downside of the BioBrick™ approach is that
the same eight bp scar sequence is found at every junction. The presence of this scar
sequence is unacceptable
at certain positions, notably the RBS (Ribosome Binding Site). The scar is also problematic when assembling
fusion proteins as it encodes an in-frame stop codon.
Another standard,
BglBricks has been described that uses different sequences for assembly and leaves a
smaller six bp scar. BglBrick has the advantage of using highly efficient and commonly-used restriction
enzymes whose recognition sequences are not blocked by the most common DNA methylases, Dam and Dcm.
Scarless assembly is possible using other methods, as described in
OE-PCR (overlap extension polymerase
chain reaction). This method uses chimeric PCR primers of more than 40 bases in length to create homologous
ends between different DNA molecules. The homology is then used to prime extension in a second round of
PCR between the initial products, and is the basis of most routine gene fusion techniques. The sequences of
the homologous primers direct which parts follow each other, allowing ordered assembly which can be done
sequentially, or even as a parallel reaction. OE-PCR has the power to assemble not just a gene, but the whole
plasmid (CPEC – circular polymerase extension cloning). Like all PCR methods, assembly is problematic when
sequences contain many repeats or are GC-rich. These methods are also limited in their ability to scale up,
plasmids becoming less efficient at larger sizes and by the error rate of PCR.