Biology 12

264 MHR • Unit 3 Molecular Genetics

Two parts of the tRNA structure are particularly

important to its role in translation. At the end of

one lobe, an anticodoncontains a nucleotide triplet

with a sequence that is complementary to the codon

of the mRNA molecule. At the 3 ′end of the strand

across from this lobe is an amino acid attachment

site. This site binds the particular amino acid

specified by the mRNA codon. A tRNA molecule

bound to its particular amino acid is called an

amino-acyl tRNA, or aa-tRNA.

As you saw in section 8.1, there are 64 possible

combinations of nucleotide triplets that could form

anticodon sequences. Of these combinations, 61 code

for amino acids and three are stop signals. Therefore,

you might expect to see 61 different kinds of tRNA

molecules in each cell. However, a typical cell

contains only about 30 to 45 different kinds of

tRNA molecules, a result possible because of the

wobble in the genetic code. In other words, the

anticodons of some tRNAs can pair with more than

one codon because the strict base-pairing rules are

relaxed somewhat for the nucleotide in the third

position of the anticodon triplet. For example, in

many cases a base U in the third position of an

anticodon can pair with either A or G in the third

position of a codon. This versatility of the third

nucleotide position helps to explain the pattern of

redundancy you saw in Table 8.1, page 254. As a

result, the cell can save energy by reducing the

number of different tRNAs it must synthesize in

order to manufacture a full range of proteins.

Activating Enzymes

Activating enzymesare the genetic code-breakers

or chemical translators of the cell. These enzymes

are responsible for attaching the correct amino acid

to the tRNA molecule with the correct anticodon.

Each activating enzyme has two binding sites. One

recognizes the anticodon on the tRNA molecule,

while the other recognizes the amino acid

corresponding to that anticodon. When both the

tRNA and the amino acid are bound to the enzyme,

the enzyme catalyzes a reaction that attaches the

amino acid to the hydroxyl group at the 3 ′end of

the tRNA molecule. The aa-tRNA molecule is then

released. In the reaction illustrated in Figure 8.16,

the enzyme that recognizes and binds the sequence

UAC in its anticodon binding site will only bind

methionine in its amino acid binding site. This

means that the UAC tRNA will be attached only

to methionine, and never to any other amino acid.

Figure 8.16Activating enzymes link the correct amino acid

to each tRNA molecule. The 3 ′amino acid binding sites

do not differ among different tRNA molecules. Instead,

the match between a tRNA with a particular anticodon

sequence and its one corresponding amino acid comes

from the very specific nature of the binding sites on the

activating enzyme.

Ribosomes

Along with a translator, the translation process

also requires a structure that can bring together

the mRNA strand, the aa-tRNAs, and the enzymes

involved in building polypeptides. Structures called

ribosomesperform this function and provide the

site for protein synthesis within the cytoplasm. A

ribosome is a complex that contains a cluster of

different kinds of proteins together with a third

type of RNA known as ribosomal RNAor rRNA.

Ribosomal RNA is a linear strand of RNA that always

stays bound to proteins in the ribosome assembly.

Each ribosome has two sub-units, one large and

one small. The two sub-units fit together to produce

an active ribosome, as shown in Figure 8.17. In a

prokaryotic cell, the small unit contains one rRNA

strand and about 20 different proteins, while the

large sub-unit contains two rRNA strands interwoven

with about 30 different proteins. Eukaryotic

ribosomes tend to be larger and contain more

rRNA and proteins than prokaryotic ribosomes.

amino

acid

aminoacyl-tRNA

synthetase

(enzyme)

aminoacyl-tRNA

tRNA

C The aa-tRNA is then released.

When both the correct

amino acid and the correct

tRNA molecule are in the

enzyme’s active sites, the

enzyme catalyzes a

reaction that binds the

molecules together.

B

The activating enzyme has

two binding sites. One is

specific to a particular

amino acid (in this case,

methionine). The other is

specific to a particular

anticodon sequence (in

this case, UAC, which is

complementary to the

codon AUG).

A