Biology Now, 2e

178 ■ CHAPTER 10 How Genes Work

GENETICS

Once bound to the promoter, the RNA

polymerase unzips the DNA double helix at

the beginning of the gene, separating a short

portion of the two strands. Only one of the two

DNA strands is used as a template, and thus

it is called the template strand. The RNA

polymerase begins to move down the DNA

template strand, constructing a messenger

RNA (mRNA) molecule, a strand of nucleo-

tides complementary to the DNA, from free

nucleotides f loating around in the nucleus

(Figure 10.6). RNA does not have all the same

bases as DNA: its four bases are adenine (A),

cytosine (C), guanine (G), and uracil (U). Those

bases pair with the four DNA bases according

to the following rules: RNA’s A with DNA’s T,

C with G, G with C, and U with A.

Part of the reason that tobacco cells produce so

much hemagglutinin so quickly is that the hemag-

glutinin gene inserted into the cells contains

a special DNA sequence that triggers multiple

RNA polymerases to transcribe a hemaggluti-

nin gene at a single time. As for any gene, as an

RNA polymerase moves away from the promoter

and travels down the template strand, another

RNA polymerase can bind at the promoter and

start synthesizing a second mRNA on the heels

of the first. At any given time, therefore, many

RNA polymerases can be traveling down a DNA

template simultaneously, each synthesizing an

mRNA.

Transcr iption stops when the R NA poly-

merase reads through a special sequence of

bases called a terminator. In eukaryotic

cells, the mRNA then undergoes an elaborate

sequence of modifications that prepare it to

leave the nucleus. These steps include chemi-

cal modification of both ends of the mRNA, as

well as a process called RNA splicing.

Most eukaryotic genes (and many viral genes)

are embedded with stretches of sequences that

don’t code for anything, called introns. The

stretches of DNA in a gene that carry instruc-

tions for building the protein are called exons.

Because of this patchwork construction, with

genes made of introns and exons interspersed,

newly transcribed mRNA (pre-mRNA) is

also a patchwork of coding sequences inter-

mixed within noncoding sequences. During

RNA splicing, the introns are snipped out

of a pre-mRNA and the remaining pieces of

mRNA—the exons—are joined to generate the

Transcription begins when RNA

polymerase binds to the promoter.

1

An mRNA molecule

is produced as RNA

polymerase moves

down the template

strand of DNA.

2

Transcription ends

when RNA

polymerase reads

through the

terminator.

3

RNA polymerase

RNA polymerase

Promoter

(in pink)

Terminator

(in pink)

Direction of

transcripton

Hemagglutinin gene

Tobacco cell

New RNA

strand

Template

strand of

DNA

RNA

nucleotides

Direction of

transcripton

Figure 10.6

Plants making proteins, I: Transcription

RNA polymerase transcribes the hemagglutinin gene into a molecule of RNA. M

Q1: Why is only one strand of DNA used as a template?

Q2: If a mutation occurred within the promoter or terminator region,

do you think it would affect the mRNA transcribed? Why or why not?

Q3: The template strand of part of a gene has the base sequence

TGAGAAGACCAGGGTTGT. What is the sequence of RNA transcribed

from this DNA, assuming that RNA polymerase travels from left to

right on this strand?