Biology 12

Chapter 8 Protein Synthesis • MHR 253

genetic code codons are made up of nucleotide
triplets, is known as the triplet hypothesis.
Crick tested his hypothesis by inserting different
numbers of nucleotides into bacterial DNA and
then observing the effects on the resulting bacterial
colonies. If his triplet hypothesis was correct,
the insertion of a new nucleotide triplet would
produce only a minor disruption in the coding
sequence. In contrast, the insertion of a single
nucleotide or a pair of nucleotides would cause a
major change in the triplet sequence following the
insertion point, as shown in Figure 8.2. The results
of Crick’s experiments, which were reported in
1961, supported the triplet hypothesis.

Figure 8.2A schematic of Crick’s experiments to test the
triplet hypothesis

The Transfer of

Genetic Information

Before they discovered the structure of DNA,
Watson and Crick had started to devise a theory
that genetic information is somehow transmitted
from DNA to RNA and then to proteins. Part of the
supporting evidence for this sequence of events lay
in experiments that demonstrated that DNA never
leaves the nucleus of a eukaryotic cell. These
experiments also showed that most of the structures
and processes involved in protein synthesis are found
only in the cytoplasm of the cell. RNA, however, is
found in both the nucleus and the cytoplasm.

Not long after Watson and Crick had established

their model for the structure of DNA and its

process of replication, Francis Crick coined the

phrase “central dogma” to describe the two-step

process by which he believed genes were expressed

and proteins built. According to this dogma, a term

he used to suggest a powerful theory that had

little experimental support, a strand of DNA first

serves as the template for the construction of a

complementary strand of RNA. This strand of RNA

then moves from the nucleus to the cytoplasm.

Here, it guides the synthesis of a polypeptide

chain. Over the next few years, experimental data

confirmed that genetic information is indeed

passed from DNA to RNA to protein.

Figure 8.3 summarizes the two main steps

involved in the process of gene expression. The

synthesis of an RNA molecule from a strand of DNA

is called transcription. To transcribe means to make

a copy of or otherwise transfer information from

one medium to another in a way that preserves the

original language of the information. During genetic

transcription, the nucleotide sequence (the language)

of the DNA strand is copied into the nucleotide

sequence of the synthesized RNA molecule. In

contrast, during the second step, called translation,

the language preserved in the RNA must be

converted to the amino acid sequence of a

polypeptide in order for a protein to be synthesized.

You will learn more about each of these steps later

in this chapter.

Figure 8.3Crick’s “central dogma” proposes a two-step

process of gene expression. In this process, genetic

information is first transcribed from DNA to RNA, and

then translated from RNA to protein.

Why does DNA not code directly for a

polypeptide product? At first glance, the two-step

process seems like an unnecessary waste of cellular

energy. However, the additional energy involved in

producing an RNA copy of the DNA information is

DNA

protein

RNA

transcription translation

TAG TAG TAG TAG TAG TAG

CTA GTA GTA GTA GTA GTA

CCT

or

AGT AGT AGT AGT AGT

TAG CCC TAG TAG TAG TAG

TAG

or

normal

bacterial

colonies

no

colonies

mutant

TCC CAG TAG TAG TAG colonies

This is a normal bacterial coding sequence for a

hypothetical polypeptide product. Normal bacteria

reproduce in viable colonies.

A

The insertion of a single nucleotide or a pair of

nucleotides alters all subsequent nucleotide triplets. No

viable colony is produced, indicating that the insertion

has caused a mutation so severe that the bacteria are

unable to manufacture a functioning polypeptide.

B

The insertion of a nucleotide triplet alters at most two

“words” of the code, after which the normal coding

sequence resumes. The result is viable but mutant

colonies that manufacture a modified version of the

polypeptide.

C