Zinc-Finger double stranded break. The DNA binding domain of

Zinc-Finger Nucleases (ZFNs)
have a high degree of specificity, theu can recognise a sequence between 9-18 basepairs.
ZFNs are made up of two domains, the binding domain and the cleavage domain (Horizon). The cleavage domain is made up of the
non-specific, restriction endonuclease, Fokl, which is responsible for introducing
the double strand break. To target a specific gene, Fokl must dimerise (combine with smaller molecules) with zinc fingers
which make up the binding domain. There is normally between 3 and 6 zinc
fingers which recognise 3 basepairs each, zinc fingers are synthetically
synthesised to recognise larger specific sequences by combining smaller zinc
finger modules of known sequence (Gene Therapy Net, n.d.). This method is
higly complex and costly (Nemudryi, et al., 2014)

TALE proteins were discovered
when studying the bacteria genus Xanthomonas,
they have a central domain capable of DNA binding and and a domain that
activates target gene transcription. The discovery of TALE proteins lead to the
development of the TALENs genome editing technique by the combination of the TALE
proteins binding domain and the FokI cleavage
domain which as in ZFNs introduces the double stranded break. The DNA binding
domain of TALE proteins consist of multiple monomers which bind one nucleotide
in the target sequence, meaning that many different sequences can be targeted
by combining monomers in different orders (Nemudryi,
et al., 2014).
The versatility when creating the binding sequences makes TALENs artificial
nucleases favourable over Zinc-Finger Nucleases because they easier to engineer
(Yeadon,
n.d.)

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CRISPR-Cas9 is a very
versatile RNA guided nuclease it has two core components, the Cas9 nuclease and
a single guided RNA (sgRNA), which are in complex together. The sgRNA has a
20-nucleotide guide sequence which is used to specifically target a gene and
Cas9 is used to create a double stranded break (DBS). The DBS is repaired by
either non-homologous end joining or homologous directed repair. CRISPR-Cas9 is
capable of inducing gene loss of function and gene gain of function (Liu, et
al., 2016).

Another possible application of genome editing is for the
treatment of cancers such as sarcomas, which creates very aggressive tumours that
don’t typically respond very well to chemotherapy. Sarcomas are closely linked
with specific mutations leading to gene over-expression, this makes genome
editing a possible method of treatment. (Liu, et al., 2016) Other cancers, genome editing has been trialled
as a treatment method for, are leukemia …

Genome editing could be used to modify plant DNA to increase
crop yields. An example of this is the modification of Hexaploid bread wheat’s
genome with CRISPR-Cas9 and TALENs to introduce heritable alleles for resistance
to the fungus Blumeria graminis which
causes powdery mildew.