Biology Now, 2e

158 ■ CHAPTER 09 What Genes Are

GENETICS

information can be stored in a DNA sequence

and in a genome. The genome of the domestic

pig, for example, has about 3 billion base pairs,

the human genome has about 3.2 billion base

pairs, a tomato has only about 900 million base

pairs, and the bacterium Escherichia coli has a

measly 4.6 million. The sequence of nucleotides

in DNA differs among species and among indi-

viduals within a species, and these differences

in genotype can result in different phenotypes

(Figure 9.5).

In the mid-1990s, scientists became very

excited about the idea of using pig organs in

humans, but testing stalled because of the

fear that humans would become infected with

PERVs. Just breeding pigs in sterile conditions

can’t get rid of the virus; it’s integrated right

there in the double helix. The Harvard team

believed CRISPR might be able to solve that

Nucleotide bases:

Adenine

Sugar-phosphate Base Nucleotide

+ =

Cytosine

Thymine

A pairs only with T. C pairs only with G.

The nucleotides

in one strand

are paired with

the nucleotides

in the

complementary

strand.

The two strands

of DNA are held

together by

hydrogen

bonds (dotted

lines) between

the bases.

Nucleotides

are linked

together by

covalent

bonds to form

one strand of

DNA.

Guanine

A

A A

C

C C

C

T

T T

T

T

P P

G

G G

G

Phosphate

Sugar (deoxyribose)

A

C

T

G

Figure 9.3

The DNA double helix and its building blocks

A molecule of DNA consists of two complementary strands of nucleotides that are

twisted into a spiral around an imaginary axis, rather like the winding of a spiral staircase. M

Q1: Name two base pairs.

Q2: Why is the DNA structure referred to as a “ladder”? What part of the DNA

represents the rungs of the ladder? What part represents the sides?

Q3: Is the hydrogen bond that holds the base pairs together a strong or weak chemical

bond? (Hint: Refer to Chapter 3 to review chemical bonds, if necessary.)

Figure 9.4

What DNA actually looks like

In November 2012, Italian researchers used an

electron microscope to directly visualize DNA for

the first time. This is the single thread of double-

stranded DNA that they saw.