Anatomy and Physiology - Introduction to Biomedical Engineering - page 92

Cytoplasm

Nucleus

G
A
U
C

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

mRNA

Ribosome

DNA mRNA

tR NA

amino acid

Polypeptide

FIGURE 3.15 Following transcription from DNA and processing in the nucleus, mRNA moves from the
nucleus to the cytoplasm. In the cytoplasm, the mRNA joins with a ribosome to begin the process of translation.
During translation, tRNA delivers amino acids to the growing polypeptide chain. Which amino acid is delivered
depends on the three-base codon specified by the mRNA. Each codon is complementary to the anticodon of a
specific tRNA. Each tRNA binds to a particular amino acid at a site that is opposite the location of the anticodon.
For example, the codon CUG in mRNA is complementary to the anticodon GAC in the tRNA that carries leucine
and will result in adding the amino acid leucine to the polypeptide chain.

EXAMPLE PROBLEM 3.5
Consider a protein that contains the amino acids asparagine, phenylalanine, histidine, and
serine in sequence. Which nucleotide sequences on DNA (assuming that there were no introns)
would result in this series of amino acids? What would be the anticodons for the tRNAs that
delivered these amino acids to the ribosomes during translation?
Solution
The genetic code (see Table 3.1) provides the sequence for the mRNA codons that specify these
amino acids. The mRNA codons can be used to determine the sequence in the original DNA and
the anticodons of the tRNA, since the mRNA bases must pair with the bases in both DNA
and tRNA. Note that DNA contains thymine (T) but no uracil (U) and that both mRNA and tRNA
contain U and not T. See Figures 3.3 and 3.14 for examples of base pairing.

Asparagine
(Asn)

Phenylalanine
(Phe)

Histidine
(His)

Serine
(Ser)

mRNA codon AAU or AAC UUU or UUC CAU or CAC UC(A, G, U, or C)

DNA TTA or TTG AAA or AAG GTA or GTG AG(T, C, A, or G)

tRNA anticodon UUA or UUG AAA or AAG GUA or GUG AG(U, C, A, or G)