Terminacja

Terminacja

transkrypcji

transkrypcji

The work of the ribosome ends when it reaches
one of the three stop codons that are used:
UAG, UAA or UGA.

The stop codons were originally identified by
mutations in bacteriophage T4.

The first one identified was UAG and was called
the "amber codon".

The history of the amber codon:

Here's the story…

In Benzer's lab at Caltech the search was on
for a mutation that would allow a certain kind
of phage mutant to grow.

Seymour said that whoever identified the
mutation, he would name it after him (in some
versions of the story, it would be named after
the discoverer's mother).

The graduate student who isolated the
mutation was a young man named Harris
Bernstein.

The name "Bernstein" in German means
"amber".

And so the UAG codon, known as a nonsense
codon (later known as a stop codon), was
named the amber codon.

Later, the other two stop codons were called
"ochre" (UAA) and "opal" (UGA) (sometimes
called, "umber") to maintain the color
metaphor.

What happened to the graduate student?

He became a famous molecular biologist and
is currently on the faculty in our own medical
school.

The event of termination is essentially the
same in both prokaryotes and eukaryotes.
The only difference is the number of factors
required. This figure shows a schematic of
termination:

When the a site of the ribosome finds itself
over one of the stop codons (in this case, the
amber codon), there will be no corresponding
aminoacyl-tRNA.

Instead, there are release factors (RFs) that
enter the A site.

In prokaryotes there are three release factors
(RF-1, RF-2, and RF-3), while in eukaryotes
there is only one (eRF).

Here's is the lactose mRNA again, to show
where all of the stop codons are in each of the
protein reading frames:

Termination results in the release of the
ribosome from the mRNA and the removal of
the peptide from its link to the tRNA.