Biology Now, 2e

Pigs to the Rescue ■ 157

general, people tend to prefer the idea of trans-

plants from pigs over transplants from mammals

more closely related to us, such as baboons. If we

were able to transplant organs from animals into

humans, a process called xenotransplantation,

healthy organs could be available on demand

and in essentially limitless supply.

Ye t t her e h a s b e en a ba r r ier t o h a r ve s t-

ing pig organs for humans: the pig genome

is dotted with DNA from a family of viruses

called porcine endogenous retroviruses, or

PERVs. DNA is built from two parallel strands

of repeating units called nucleotides. Each

nucleotide is composed of the sugar deoxy-

ribose, a phosphate group, and one of four

bases: adenine, cytosine, guanine, or thymine.

We identify nucleotides by their bases, using

“adenine nucleotide” as shorthand for “nucleo-

tide with an adenine base.”

The nucleotides of a single strand are

connected by covalent bonds between the phos-

phate group of one nucleotide and the sugar of

the next nucleotide. The two DNA strands are

connected by hydrogen bonds linking the bases

on one strand to the bases on the other, like the

rungs that connect the two sides of a ladder

(Figure 9.3). The term base pair (or nucleotide

pair) refers to two nucleotides held together by

one of these bonds between their bases; that

is, a base pair corresponds to one rung of the

DNA ladder. The ladder twists into a spiral

called a double helix (Figure 9.4). Within the

long, winding double helix of the pig genome,

short sections of DNA from PERVs are scat-

tered about—sections made up of the same four

nucleotides but encoding information for viral

proteins rather than pig proteins.

Nucleotides do not form base pairs willy-

nilly. The adenine (A) nucleotide on one strand

can pair only with thymine (T) on the other

strand (see Figure 9.3); similarly, cytosine

(C) on one strand can pair only with guanine

(G) on t he ot her s t ra nd. The se base-pairing

rules, which provide complementary base-

pairing between two nucleic acid strands, have

an important consequence: when the sequence

of nucleotides on one strand of the DNA mole-

cule is known, the sequence of nucleotides on

the other, complementary strand of the mole-

cule is automatically known as well. The fact

that A can pair only with T and that C can

pair only with G allows the original strands

Figure 09.02

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Kidney Pancreas Liver Heart Lung

Figure 9.2

Pig and human organs are remarkably similar in size

Q1: Name one reason why, for a potential transplant, matching the

size of organs shown would be important.

Q2: Name a tissue transplant for which size matching would not be

as important.

Q3: Name a tissue transplant for which size matching would not be

important at all.

to serve as “template strands” on which new

strands can be built through complementary

base-pairing. (We will see more in Chapter 10

about building new DNA strands, including

how RNA can pair with DNA, which CRISPR

takes advantage of.)

Still, the four nucleotides can be arranged

in any order along a single strand of DNA, and

each DNA strand is composed of millions of

these nucleotides, so a tremendous amount of