BIOL 201 Lecture Notes - Lecture 9: Signal Recognition Particle, Start Codon, Eukaryotic Translation
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Eukaryotic mRNA that has been processed and transported to the cytoplasm (i.e., mature
mRNA) can then be translated by the ribosome. Translation may occur at ribosomes free-floating
in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle.
Therefore, unlike in prokaryotes, eukaryotic translation is not directly coupled to transcription.
The 5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using
a 5'-5'-triphosphate linkage. This modification is critical for recognition and proper attachment of
mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for
other essential processes, such as splicing and transport.
Coding regions are composed of codons, which are decoded and translated (in eukaryotes usually
into one and in prokaryotes usually into several) into proteins by the ribosome. Coding regions
begin with the start codon and end with a stop codon. In general, the start codon is an AUG
triplet and the stop codon is UAA, UAG, or UGA. The coding regions tend to be stabilised by
internal base pairs, this impedes degradation.in addition to being protein-coding, portions of
coding regions may serve as regulatory sequences in the pre-mRNA as exonic splicing
enhancers orexonic splicing silencers.
Untranslated regions (UTRs) are sections of the mRNA before the start codon and after the stop
codon that are not translated, termed the five prime untranslated region (5' UTR) and three prime
untranslated region (3' UTR), respectively. These regions are transcribed with the coding region
and thus are exonic as they are present in the mature mRNA. Several roles in gene expression
have been attributed to the untranslated regions, including mRNA stability, mRNA localization,
and translational efficiency. The ability of a UTR to perform these futions depends on the
sequence of the UTR and can differ between mRNAs.
The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity
for RNA degrading enzymes called ribonucleases and for ancillary proteins that can promote or
inhibit RNA degradation.
Translational efficiency, including sometimes the complete inhibition of translation, can be
controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by
influencing the ribosome's ability to bind to the mRNA. MicroRNAs bound to the 3' UTR also
may affect translational efficiency or mRNA stability.
Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are
needed in a particular region of the cell can also be translated there; in such a case, the 3' UTR
may contain sequences that allow the transcript to be localized to this region for translation.
Some of the elements contained in untranslated regions form a characteristic secondary
structure when transcribed into RNA. These structural mRNA elements are involved in
regulating the mRNA. Some, such as the SECIS element, are targets for proteins to bind. One
class of mRNA element, the riboswitches, directly bind small molecules, changing their fold to
modify levels of transcription or translation. In these cases, the mRNA regulates itself.
The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to
the 3' end of the pre-mRNA. This tail promotes export from the nucleus and translation, and
protects the mRNA from degradation.