Molecular Biology: General theory
Molecular Biology:
General Theory
Author: Dr Darshana Morar
Licensed under a Creative Commons Attribution license.
TRANSLATION
Translation is the process of converting the mRNA sequence into an amino acid sequence. It occurs in
the cytoplasm where the ribosomes are located. Ribosomes are made of a small and a large subunit
which surround the mRNA. In translation, an mRNA sequence is used by the ribosome as a template to
guide the synthesis of a chain of amino acids.
DNA transfers information to mRNA in the form of a code defined by the sequence of nucleotide bases.
Since DNA and RNA are constructed from four types of nucleotides, there are 64 possible triplet
sequences or codons (4x4x4); many more than the 20 needed to specify the common amino acids
present in nature. Three of the possible codons specify the termination of the polypeptide chain. They are
called "stop codons". That leaves 61 codons to specify only 20 different amino acids. Therefore, most of
the amino acids are represented by more than one codon. The genetic code is therefore said to be
degenerate.
The vast majority of genes are encoded with exactly the same code, known as the canonical genetic
code, or simply the genetic code. In fact there are many variant codes; so it should be noted that the
canonical genetic code is not universal. For example, in humans, protein synthesis in mitochondria relies
on a genetic code that varies from the canonical code.
During protein synthesis, a ribosome moves along an mRNA molecule from the 5' end to the 3' end and
"reads" its sequence three nucleotides at a time (codon). Each amino acid is specified by the mRNA's
codon, which pairs with a sequence of three complementary nucleotides (anticodon) carried by a
particular transfer RNA (tRNA) molecule. The other end of the tRNA has the amino acid attached to the
3'-OH group via an ester linkage. A tRNA molecule with an attached amino acid is said to be "charged".
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Molecular Biology: General theory
Figure 10 Structure of the charged transfer RNA (tRNA) molecule
When a small subunit of a ribosome charged with a tRNA + methionine (initiator tRNA) encounters an
mRNA, it attaches and starts to scan for a start signal or start codon (AUG). When it finds the start
sequence AUG, the codon for methionine, the large subunit joins the small one to form a complete
ribosome and protein synthesis is initiated. A new charged tRNA (tRNA + amino acid) enters the
ribosome, at the next codon downstream of the AUG codon. If its anticodon matches the mRNA codon it
binds and the ribosome can link the two amino acids together (Note: if a tRNA with the wrong anticodon
(and therefore the wrong amino acid) enters the ribosome, it cannot bind with the mRNA and is rejected).
The ribosome then moves one triplet forward and a new charged tRNA can enter the ribosome and the
procedure is repeated. When the ribosome reaches one of three stop codons, UAG, UAA or UGA, there
are no corresponding tRNAs to that sequence. Instead termination proteins bind to the ribosome and
stimulate the release of the polypeptide chain (the protein), and the ribosome dissociates from the mRNA.
Figure 11 Process of DNA translation
(Adapted from http://www.duke.edu/web/MAT/jennifer_sohn/unit/translation_oh.htm)
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