Biology Now, 2e

162 ■ CHAPTER 09 What Genes Are

GENETICS

polymerase—a key enzyme in the replication

of DNA—builds the two new strands of DNA,

starting from primers that pair comple-

mentary nucleotides at specific sites near the

origins of replication.

3. When construction is completed, there are
   two identical copies of the original DNA
   molecule. Each copy is composed of a
   template strand of DNA (from the original
   DNA molecule) and a newly synthesized
   strand of DNA.

This mode of replication is known as semicon-

servative replication because one “old” strand

(the template strand) is retained, or “conserved,”

in each new double helix (Figure 9.8).

The mechanics of copying DNA are far from

simple. More than a dozen enzymes and proteins

are needed to unwind the DNA, to stabilize the

separated strands, to start the replication process,

to attach nucleotides that are complementary

genome much more recently, and that viral DNA

is still active.

All this viral DNA remains in these genomes

because it is passed from generation to genera-

tion via DNA replication, the duplication of a

DNA molecule. DNA replication is ongoing in

our bodies: It occurs right before a cell enters

mitosis, so that there is a copy of the DNA to pass

along to the new cell. It occurs feverishly when a

new embryo is being formed. And it occurs when

viruses hijack our cell machinery and copy their

own DNA.

Cells replicate DNA in three steps:

1. The DNA molecule unwinds through special
   proteins that bind the DNA at sequences
   known as origins of replication, which then
   break the hydrogen bonds connecting the two
   strands of DNA.


2. Each strand is then used as a template for the
   construction of a new strand of DNA. DNA

C A

G T

G T

C A

C

G

A

T

G

C

T

A

C

G

A

T

G

C

T

A

C

G

A

T

G

C

T

A

C

G

A

T

G

C

T

C A G T

A

G T C A

As DNA replication

begins, the

hydrogen bonds

connecting the

two strands are

broken, causing

the strands to

separate.

Each old strand

of DNA serves

as a template for

the construction

of a new

complementary

strand.

When DNA

replication is

completed, there are

two DNA molecules,

each with one

template strand and

one new strand.

Template strand

Template

strand

New strand

New strands

New strands

forming

New strand

1

2 3

Figure 9.8

DNA replication is semiconservative

In this overview of DNA replication, the template

DNA strands are blue, and the newly synthesized

strands are magenta. M

Q1: Which of the DNA strands here are

the template strands? Why are they called

“template” strands?

Q2: What must be broken before

replication can begin?

Q3: In your own words, explain

why replication is described as

“semiconservative.”