Biology
210
GENETICS
8 April,
1998
Chapter
11b (pages 479-505)
Transcription
of Eukaryotic DNA
1.
The "Central Dogma"
Remember
that in eukaryotic cells, transcription occurs in the nucleus, and the
mRNA must be exported to the cytoplasm for translation.
In eukaryotes,
the mRNA is modified at both ends - the 5' end is "Capped" with a modified
GTP, and to the end is added a long "tail" of poly(A) (usually greater
than 150 A's are added). This figure is taken from "The Biology Place";
you can click on it for a link there.
Once again
(this is a review) - in bacteria, transcription and translation take place
at the same time...
In
Eukaryotes, the process is a bit more complicated....
2.
Transcription in Eukaryotes
In bacteria,
all mRNA is made from the same RNA polymerase. However, in eukaryotes,
there are THREE DIFFERENT RNA polymerases:
1.
RNA Polymerase I synthesizes rRNA.
2.
RNA Polymerase II synthesizes mRNA.
The mRNA molecules are
only for one gene (monocistronic), whereas many prokaryotic mRNAs are polycistronic
(code for more than one gene).
3.
RNA polymerase III synthesizes tRNAs
and also snRNAs
(small nuclear RNAs) and small cellular RNAs.
The Eukaryotic
polymerases are much more complex than E.coli RNAP. This figure
(Figure 13-35 from Griffiths et al., 1996) compares the different subunits
for eukaryotic RNA Polymerases with that of E.coli RNAP.
B.
Eukaryotic Transcription Initiation
There
are 4 different types of proteins which must form a complex for initiation
of transcription in eukaryotes.
C.
Eukaryotic RNA processing
In eukaryotes,
the RNA is processed at both ends before it is spliced.
-
At
the 5' end, a cap is added consisting of a modified GTP (guanosine triphosphate).
This occurs at the beginning of transcription. The 5' cap is used as a
recognition signal for ribosomes to bind to the mRNA.
-
At the
3' end, a poly(A) tail of 150 or more adenine nucleotides is added. The
tail plays a role in the stability of the mRNA.
Eukaryotic
mRNAs are spliced by complexes of small nuclear RiboNucleoProteins (snRNPs)
hit
"reload" to see the animation
The
Ovalbumin mRNA is originally 7700 bp, but the final, spliced product is
only 1800 nucleotides. In some eukaryotic genes, the original mRNA
is perhaps a MILLION bp long!! Remember, EXONS are EXpressed, INTRONS
are the INTervening sequences.
Here's
an electron micrograph of the ovalbumin mRNA, hybridized to a complimentary
DNA strand.
This
is a drawing of the above electron micrograph of the ovalbumin mRNA hybridized
to its complimentary strand of cDNA.
In
mRNA the Introns (INTeRvening sequences) are looped out:
3.
Eukaryotic Translational Control
There are 4 principle types of translation
control mentioned in your text (page 503).
1. Inability of an mRNA molecule to be translated unless a
molecular signal is present.
2. Regulation of the lifetime
of a particular mRNA molecule.
3. Regulation of the overall rate
of protein synthesis.
4. Aborted translation of the
principal reading frame because of the presence of smaller open reading
frames upstream in the mRNA.
Back to the Genetics Syllabus
Last modified on: 19 February, 2000 by Dave Ussery