Biology 12

Chapter 8 Protein Synthesis • MHR 255

mechanism in the cell that can reset the translation
process back to the correct frame.
Redundancy There are a total of 64 possible
codons but only 20 amino acids. As you can see
from Table 8.1, only three RNA codons do not code
for any amino acid. These three codons make protein
synthesis stop. This means that each amino acid is
associated with, on average, about three possible
codons. This pattern offers a significant biological
advantage. If there were a one-to-one correlation
between codons and amino acids, 44 codons would
code for nothing and therefore be read as “stop”
signals to end protein synthesis. This would mean
that a random mutation would be more than twice
as likely to terminate protein synthesis than to code
for an altered protein. Termination of protein
synthesis too early is likely to result in a more severe
mutation than simply building in a wrong amino
acid. So, having a limited number of “stop” signals
offers some protection against serious mutations.
Furthermore, the redundancy is not random — it
follows a particular pattern. For example, Table 8.1
shows that four different codons code for the amino
acid proline, and they all have the nucleotide
sequence CC. Similarly, of the six codons for
arginine, four have the sequence CG. The third
position in a codon is often referred to as the
“wobble” position, since in many cases it can
accommodate a number of different nucleotides
without changing the resulting amino acid. The
wobble feature serves as an additional guard
against harmful mutations. It also contributes to the
efficiency of protein synthesis, as you will see later
in this chapter. Redundancy and continuity are
compared in Figure 8.4.
It is important not to confuse redundancy with
ambiguity. The code is redundant in that several
different codons can code for the same amino acid,
but it is not ambiguous. No codon ever has more
than one amino acid counterpart. While you cannot
always read backwards from an amino acid to
determine the precise nucleotide sequence of the
codon, you can always read from the codon to its
one correct amino acid.
Universality The genetic code shown in Table 8.1
is the same in almost all living organisms, from
bacteria to mammals. That is, the same RNA codons
correspond to the same amino acids in almost all
organisms that have genetic material. The only
known exceptions are found in a few types of
unicellular eukaryotes and in the mitochondria and
chloroplasts in eukaryotic cells. The universality of
the genetic code provides evidence that all these

organisms share a common ancestor, and that the

code was established very near the outset of life on

Earth (see Figure 8.5).

Figure 8.4Two characteristics of the genetic code are

continuity and redundancy. Here, a portion of a DNA molecule

serves as a template for the synthesis of a strand of RNA.

The genetic code is made up of three-letter codons along

the RNA strand. Each codon correlates with one amino acid.

Note that the codons are read as an unbroken series of non-

overlapping words (continuity). Note also that two different

codons can code for the same amino acid (redundancy).

Figure 8.5Mitochondria contain their own DNA in the form

of a closed, circular molecule like the one found in bacteria.

This DNA has a slightly different genetic code than the DNA

found in most living cells, including the cells in which the

mitochondria are found. This distinct code suggests that

mitochondria might once have been separate organisms

that became incorporated into eukaryotic cells early in the

evolution of life forms. The fact that mitochondria have other

similarities with bacteria lends support to this idea.

In addition to providing some hints about the

origins of life on Earth, the universality of the

genetic code also has important implications for

society today. Since the same code is used by all

cells, a gene that is taken from one kind of

organism and inserted into another can still

3 ′ TACTTACTCGTCTTG 5 ′

5 ′ AUGAAUGAGCUGAAC 3 ′

DNA template

RNA strand

transcription

translation

polypeptide

key: met = methionine; asn = asparagine;

glu = glutamate; leu = leucine

met asn glu leu asn