Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 69

second tRNA with its dipeptide thereby moves down one position

in the ribosome. A third tRNA, bearing its specific amino acid,

then attaches by its anticodon to the third codon of the mRNA.

The previously formed dipeptide is now moved to the amino acid

carried by the third tRNA as the mRNA again moves a distance

of one codon within the ribosome. This is followed by the release

of the second tRNA (minus its dipeptide), as the third tRNA,

which now carries a tripeptide, moves up a distance of a codon in

the ribosome. A polypeptide chain, bound to one tRNA, thereby

grows as new amino acids are added to its growing tip ( fig. 3.21 ).

This process continues until the ribosome reaches a “stop” codon

in the mRNA, at which point genetic translation is terminated and

the fully formed polypeptide is released from the last tRNA.

As the polypeptide chain grows in length, interactions

between its amino acids cause the chain to twist into a helix

(secondary structure) and to fold and bend upon itself (tertiary

structure). At the end of this process, the new protein detaches

from the tRNA as the last amino acid is added. Although,

under ideal conditions, the newly formed polypeptide chain

could fold correctly to produce its proper tertiary structure, this

may not happen in the cell. For example, one region of the

newly forming polypeptide chain may improperly interact with

specific for each codon and amino acid. Each synthetase
enzyme recognizes its amino acid and joins it to the tRNA that
bears a specific anticodon. The cytoplasm of a cell thus con-
tains tRNA molecules that are each bonded to a specific amino
acid, and each of these tRNA molecules is capable of bonding
with a specific codon in mRNA via its anticodon base triplet.

Formation of a Polypeptide

The anticodons of tRNA bind to the codons of mRNA as the mRNA
moves through the ribosome. Because each tRNA molecule carries
a specific amino acid, the joining together of these amino acids by
peptide bonds creates a polypeptide whose amino acid sequence
has been determined by the sequence of codons in mRNA.
Two tRNA molecules containing anticodons specific to the
first and second mRNA codons enter a ribosome, each carrying its
own specific amino acid. After anticodon-codon binding between
the tRNA and mRNA, the first amino acid detaches from its tRNA
and bonds to the second amino acid, forming a dipeptide attached
to the second tRNA. While this occurs, the mRNA moves down
a distance of one codon within the ribosome, allowing the first
tRNA (now minus its amino acid) to detach from the mRNA. The

Figure 3.19 Transcription and translation. The genetic code is first transcribed into base triplets (codons) in mRNA and
then translated into a specific sequence of amino acids in a polypeptide.
See the Test Your Quantitative Ability section of the Review Activities at the end of this chapter.

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Transcription

DNA

double

helix

Translation

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DNA coding strand

Messenger RNA

Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 Codon 7

Protein

Methionine Glycine Serine Isoleucine Glycine Alanine Alanine