Biology Now, 2e

Tobacco’s New Leaf ■ 183

among other possibilities. While single-base

substitutions are not always a problem, single-

base insertions and deletions cause a genetic

“frameshift,” shifting all subsequent codons

“dow nstrea m” by one ba se (Figure 10.10).

This shift scrambles the entire downstream

DNA message, in turn scrambling the entire

RNA message and causing the ribosomes to

assemble a very different sequence of amino

acids from the mutation point onward—as if

every letter in this phrase was shifted to the

left one space while the word length and spaces

between words were retained (a si fever ylette

ri nthi sphras ewa sshifte dt oth erigh ton espac

ewhil eth ewor dlengt han dspace sbetwee

nword swer eretaine d).

Tweaking Gene Expression

Inside a tobacco cell, as in most living cells, the

expression of many genes can be turned on or

off, slowed down (down-regulated), or sped up

(up-regulated). This gene regulation enables

organisms to change which genes they express

in response to internal signals (from inside the

body) or external cues in the environment. In this

way, by producing different proteins as needed,

organisms can adapt to their surroundings.

All cells in a multicellular individual have

essentially the same DNA, yet different cells

express different sets of genes, and within a

given cell the pattern of gene expression can

change over time. Single-celled organisms, such

as bacteria, face a more difficult challenge: they

are directly exposed to their environment, and

they have no specialized cells to help them deal

with changes in that environment. One way they

meet this challenge is to express different genes

at different times.

The expression of most genes in prokary-

otes and eukaryotes is regulated by both inter-

nal and external signals. Many genes are also

developmentally regulated, meaning that their

expression can change, sometimes dramati-

cally, as an organism grows and develops. Gene

expression is regulated at many different points

in the cell, including DNA packing (the way

DNA is compressed or unwound in the genome),

transcription, mRNA processing, and several

T

DNA

template

mRNA

Protein

DNA

template

mRNA

Protein

DNA

template

mRNA

Protein

Base substitution

Mutation

Mutation

“Normal” gene

Base insertion

... ...

...

...

... ...

...

... ...

...

... ...

C A T A G G T C G C A A G G C G

G U A U C C A G C G U U C C G C

Val Ser Ser Val Pro

C A T A G G T C G T A A G G C G

G U A U C C A G C A U U C C G C

Val Ser Ser Ile Pro

C A T A G G T C G T A A G G C G

G U G G

A

U A C C A C U A U U C C C

Val Ser Ser Tyr Ser

C T

A

DNA

template

mRNA

Protein

Mutation

Base deletion

... ...

... ...

C A T A G G T C G A A G G C G

G U A U C C A G C U U C C G C

Val Ser Ser Phe Arg

Figure 10.10

Effects of point mutations

Q1: Why is an insertion or a deletion in a gene more likely to alter the

protein product than a substitution, such as A for C, would?

Q2: Which would you expect to have more impact on an organism: a

point mutation as shown here, or the insertion or deletion of a whole

chromosome (discussed in Chapter 8)?

Q3: Which mechanisms in a cell prevent mutations? (Hint: Refer back

to Chapter 6 if needed.)