Answer:
they can insert the codon that encodes phenylalanine by CRISPR-Cas mediated homology-directed repair
Step-by-step explanation:
The CRISPR (clustered regularly interspaced short palindromic repeats,)-Cas (CRISPR-associated protein) is a versatile technology for genome editing which has been developed by adapting a natural defense mechanism found in prokaryotic organisms. The CRISPR-Cas genome editing tool can be used to edit DNA sequences in eukaryotic genomes (for example, in human genomes) and thus restore gene function (in this case, the CFTR gene function). The system consists of a Cas protein that cleaves the DNA at specific genomic sites and a single guide RNA (sgRNA) that guides the Cas protein at specific genomic locations. CRISPR-Cas mediated homology-directed repair refers to the process where Cas cuts the DNA to triggers the homologous recombination (HR) DNA repair pathway, by which it is possible to insert new genetic sequences at specific genomic sites (in this case, the codon that encodes phenylalanine within the specific CFTR gene).