Translation

Translation is protein synthesis at the ribosomes in the cytoplasm,
based on the genetic information in the transcribed mRNA. Another
type of RNA, called transfer RNA (tRNA), is important in
the translation process. It brings the different amino
acids coded for by the mRNA into alignment so that a polypeptide
can be made. Complementary pairing of bases across the molecule

maintains tRNA’s configuration. The presence of some unusual
bases (i.e., other than adenine, thymine, cytosine, guanine, or
uracil) disrupts the normal base pairing and forms loops in the
molecule. The center loop (the “anticodon loop”) has a sequence
of three unpaired bases called the anticodon. During translation,
pairing of the mRNA codon with its complementary anticodon of
tRNA appropriately positions the amino acid that tRNA carries.
Ribosomes, the sites of protein synthesis, consist of large
and small subunits that organize the pairing between the codon
and the anticodon. Several sites on the ribosome are binding sites
for mRNA and tRNA. At the initiation of translation, mRNA
binds to a small, separate ribosomal subunit. Attachment of the
mRNA requires that the initiation codon (AUG) of mRNA be
aligned with the P (peptidyl) site of the ribosome. A tRNA with a
complementary anticodon for methionine binds to the mRNA,
and a large subunit joins, forming a complete ribosome.
Polypeptide formation can now begin. Another site, the A
(aminoacyl) site, is next to the P site. A second tRNA, whose anticodon
is complementary to the codon in the A site, is positioned.
Two tRNA molecules with their attached amino acids are now
side-by-side in the P and A sites. This step requires enzyme
aid and energy, in the form of guanine triphosphate (GTP).
An enzyme (peptidyl transferase), which is actually a part of the
larger ribosomal subunit, breaks the bond between the amino acid
and tRNA in the P site, and catalyzes the formation of a peptide
bond between that amino acid and the amino acid in the A site.
The mRNA strand then moves along the ribosome a distance
of one codon. The tRNA with two amino acids attached to
it that was in the A site is now in the P site. A third tRNA can
now enter the exposed A site. This process continues until the entire
mRNA has been translated, and a polypeptide chain has been
synthesized. Translation ends when a termination codon (e.g.,
UAA) is encountered.
Protein synthesis often occurs on ribosomes on the surface
of the rough endoplasmic reticulum. The positioning of ribosomes
on the ER allows proteins to move into the ER as the protein is
being synthesized. The protein can then be moved to the Golgi
apparatus for packaging into a secretory vesicle or a lysosome.