Biology Now, 2e

Pigs to the Rescue ■ 159

part of a defense system used by bacteria. Bacte-

ria are constantly under bombardment from

viruses that try to sneak into and take over their

genomes, so bacteria evolved a set of defensive

measures, including a tool to recognize and cut

foreign, interloping DNA. Pioneered by micro-

biologists Jennifer Doudna of UC Berkeley and

Emmanuelle Charpentier at Umeå University in

Sweden, the CRISPR-Cas9 system is composed

problem by destroying the PERV DNA in pig

cells once and for all.

Precise Cuts

The DNA sequences called CRISPR, a bless-

edly easy acronym for clustered regularly inter-

spaced short palindromic repeats, are actually

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Chicken

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Figure 9.5

The sequence of bases in DNA differs among species and among individuals

within a species

The sequence of bases in a hypothetical gene is compared for two humans (A and B) and a chicken.

Base pairs highlighted in blue are variant; that is, they differ between the genes of persons A and B,

and between the same genes in humans and chickens.

Q1: If all genes are composed of just four nucleotides, how can different genes carry different

types of information?

Q2: Would you expect to see more variation in the sequence of DNA bases between two

members of the same species (such as humans) or between two individuals of different species

(for example, humans and chickens)? Explain your reasoning.

Q3: Do different alleles of a gene have the same DNA sequence or different DNA sequences?