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Hello, my name is Dr. Jeffrey Kieft.
For the next portion of the series, understanding ribosomes,
I'm going to lecture on factor independent initiation of protein synthesis by IRES RNAs.
During the course of this lecture,
I'm going to talk about a mode of initiating
translation in eukaryotes with which you may not be familiar.
If you recall, the phase of translation in which the protein
making machinery is assembled is called the initiation step,
and this phase has special importance to the process of translation in general.
The reasons for this are many, but include fidelity.
That is, we want translation to begin at the right spot on the messenger RNA,
and not just at any AUG codon that the ribosome might find.
The speed at which translation occurs is also important,
because initiation is the rate-limiting step in translation.
The rate at which initiation occurs
determines how quickly a given amount of protein is made.
Initiation is the primary phase for regulating translation.
Understanding how this important initiation process actually
happens is important if we're going to understand how ribosomes actually work.
Translation initiation in eukaryotes is
a stepwise ordered assembly process that is known to occur by one of two mechanisms.
The first mechanism is that which is used by the vast majority of messenger RNAs.
This is a cap-dependent, scanning-dependent mechanism,
which as its name implies,
depends on the presence of a 7-methylguanosine cap on the messenger RNA.
The second known mechanism for translation initiation is
a cap independent mechanism called internal initiation of translation.
We're going to talk about this more in detail, but for now,
what I want you to understand is that internal initiation is driven by
specific RNAs called internal ribosome entry sites or IRESes.
The lecture is going to be focused around three specific questions.