Question

In actuality, the DNA sequence of the human genome is NOT random. Some sequences, including some very large sequences, are repeated many times throughout the human genome. Write two ideas you have for how this fact complicates the use of CRISPR gene-editing technology in humans.

Answer 1

repeated DNA sequences decrease the target specificity of the CRISPR-Cas system, and may lead to off-target effects

Step-by-step explanation:

The CRISPR-Cas system is a versatile and powerful tool for gene editing. This gene-editing tool consists of a single guide RNA (sgRNA) that guides the Cas endonuclease protein to the appropriate genomic locus, where Cas generate a double-strand break in the DNA and thus induces DNA repair either by Non-homologous end joining (NHEJ) or Homologous Recombination (HR) repair pathways. The sgRNA consists of a 17-20 nucleotide sequence which is complementary to the target DNA, this sequence is called crispr RNA (crRNA). The existence of repeated DNA sequences hampers the specificity of the CRISPR-Cas tool because a particular sgRNA is able to guide Cas9 to different genomic loci that have the same (repeated) nucleotide sequence complementary to the crRNA. In consequence, repeated DNA sequences may lead to undesired off-target effects, i.e., unintended cleavage (and therefore unintended mutations) at untargeted genomic sites.