Tobacco’s New Leaf ■ 181specifies only one amino acid. It is also redun-
dant: since there are a total of 64 codons but
only 20 amino acids, several different codons call
for the same amino acid, as already mentioned.
Finally, the genetic code is virtually univer-
sal: nearly every organism on Earth uses the
same code, from agrobacteria to tobacco cells
to human cells—a feature that illustrates the
common descent of all organisms.
Making a protein from an mRNA strand
requires two additional types of RNA: The
first is ribosomal RNA (rR NA), an important
component of ribosomes. The second is transfer
RNA (tRNA), which is the caddy for the process,
delivering specific amino acids to the ribosomes
as the codons are read off the mRNA “list.”
Each tRNA specializes in binding to a specific
amino acid and recognizes and pairs with a
specific codon in the mRNA, like a puzzle piece
that fits one amino acid on one end and one codon
on the other. At one end of a tRNA molecule, a
special sequence of three nitrogenous bases,
called an anticodon, binds the correct codon on
the mRNA. At the other end, the specific amino
acid attaches (see Figure 10.8).
Let’s recap. For translation to occur, an
mRNA molecule must first bind to a ribo-
some. The ribosomal machinery then “scans”
through the mRNA until it finds a start codon
(AUG). Next the ribosome recruits the appro-
priate tRNAs one by one, as determined by
the codons read in the mRNA sequence. A
special site on the ribosome facilitates the link-
ing of one amino acid to another, like beads
on a string. Finally, the ribosome reaches a
stop codon. The amino acid chain cannot be
extended further, because none of the tRNAs
will recognize and pair with any of the three
stop codons. At this point the mRNA molecule
and the completed amino acid chain separate
from the ribosome. The new protein then folds
into its compact, specific three-dimensional
shape and is ready to go to work in the cell.
But this process does not always go as
planned. In Chapter 9 we learned that a muta-
tion is a change in the sequence of DNA bases.
Mutations affect an organism by disrupting or
preventing the healthy formation of a protein.
For example, a mutation can cause a DNA
sequence not to be translated or transcribed,
prompt the amino acid chain to end prema-
turely, or make the final protein fold incorrectly,the protein, the actual product that will be
extracted from the tobacco leaves. First the
mRNA is transported from the nucleus, where
it was made, to the sites of protein synthesis:
the cell structures called ribosomes in the
cytoplasm. To escape the nucleus, the long
strand of mRNA passes through a nuclear
pore, like a noodle slipping through the hole of
a colander. Once the mRNA molecule arrives
in the cytoplasm, the information it contains
must be translated, with the help of ribo-
somes, from the language of mRNA (nitroge-
nous bases) to the language of proteins (amino
acids). Translation is the process by which
ribosomes convert the information in mRNA
into proteins.
During translation, ribosomes “read” the
mRNA code like a grocery list (bread, milk,
etc.), and collect the corresponding amino
acids, linking them in the precise sequence
dictated by mRNA (Figure 10.8). Ribosomes
read the mRNA information in sets of three
bases at a time, and each unique sequence
of three mRNA bases is called a codon. The
hemagglutinin gene has about 1,770 bases, of
which 1,695 code for the protein. That makes
565 codons (1,695 divided by 3), and therefore
the hemagglutinin protein is composed of 565
amino acids.
The four bases of mR NA (A, C, G, U) can be
arranged to create a three-base sequence in
64 different ways (because 4^3 = 64). Therefore,
there are 64 possible codons (Figure 10.9).
Most of the 64 codons specif y a particular
amino acid. A couple of amino acids are spec-
if ied by only one codon, while other amino
acids are specif ied by any where from two
to six different codons. Some codons do not
code for any amino acid and instead act as
signposts that communicate to the ribosomes
where they should start or stop reading the
mRNA. The start codon (AUG) is the ribo-
some’s starting point on the mR NA strand,
and there are three possible stop codons
(UA A, UAG, and UGA). By beginning and
ending at f ixed points, the cell ensures that
the mR NA message is read in precisely the
same way every time.
The information specified by all 64 possible
codons is the genetic code (see Figure 10.9).
The genetic code has several significant charac-
teristics. First, it is unambiguous: each codon