Yes and no. Most nascent (newly made)
mRNAs are delivered into the cytoplasm of
the cell (where protein synthesis occurs) and are
translated when docked with the ribosome, but many
are not free-floating. This is especially true in
eggs and early embryos, where many mRNAs are
specifically localized to certain areas by binding
proteins or by tethering to the cytoskeleton.
In bacteria (no nuclear compartment), mRNA is
often translated (loaded onto ribosomes) as it is
being transcribed from the DNA, and thus is never
freely diffusing. And even in bacteria, certain
mRNAs may be localized to specific regions of the
cell by these targeting/tethering mechanisms. In
this way, the cell can regulate when and where the
protein gets made "post-transcriptionally."
In regard to tRNAs, the aminoacyl tRNA is
literally delivered to the ribosome by an
elongation factor called Tu (EF-Tu) in bacteria
and EF1alpha in eukaryotes, which takes that
aa-tRNA right to the ribosomal "A site" as soon as
it is made by the synthetase. Why? The
simplest explanation is efficiency. It turns
out that the activated ester bond of aa-tRNA is
quite labile -- if left to diffuse, it would be
hydrolyzed by water before it reached the
ribosome. EF-Tu protects this bond from
hydrolysis and helps "target" the aa-tRNA to the
proper site on the ribosome.
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