Biology Lecture No. 15: Splicing & Eukaryotic Genes
Monday March 5 , 2012
-The synthesis and function of a typical tRNA molecule requires complementary base-pairing with itself,
DNA, other RNA and one particular amino acid.
Lactose & Lies:
-The Lac operon probably has nothing to do with the molecule lactose. Even though galactosidease
concentration increases, lactose simply induces and is not the primary galactoside responsible for the
intended function of this operon.
-This is easy to explain as it is highly unlikely that E. coli would ever encounter lactose in its environment,
the gut of adult mammals, which seldom in evolutionary history ever consumed milk.
-Other evidence shows that the Lac A gene, which codes for transacetylase, does not transfer a single
acetyl group to lactose. This doubts that lactose is at all responsible for the “Lac” operon’s existence in
E. coli, but rather that another galactoside that may need acetylation from time to time, could be
Regulation Of Eukaryotic Gene Expression:
-How does a caterpillar undergo metamorphosis and become a butterfly when they in fact both share
the same genome? How can the same DNA be used to create two completely different organisms? The
answer lies with the regulation of eukaryotic genes.
-So far, what has been discussed relates to transcription and translation of genes as described in
prokaryotic organisms as well as in energy-transducing organelles such as the chloroplast and
-Genes of the nucleus are very different in their expression than those in mitochondria and
chloroplasts. The algae Vaucheria litorea is an example of a eukaryote that is coenocytic as it does not
possess many cell walls and resembles a tube containing many diverse, cellular components.
Nuclear Gene Structure (Eukaryotes):
-In the nucleus of Vaucheria litorea, genes are very similar to prokaryotes with coding regions consisting
stop codons, 5’ UTR, 3’ UTR as well as promoters and proximal regions (such as the CAP/cAMP binding
site) that are used in gene expression.
-It is in eukaryotes that an additional structure called an enhancer may be found as well as the tendency
to possess introns (anywhere from zero to hundreds of these). Eukaryotic Promoters & Transcription Factors:
-All promoters are position-dependent. It is incredibly difficult to manipulate their location and expect
them to keep their function. They are direction-dependent as well and any irregular inversion would
cause them to not work. It is upon the direction of the promoter sequence that RNA polymerase is
-This is due to the fact that promoters bind other proteins. Such protein-coding genes have a region
known as the TATA box (any particular sequence that is common to promoters and protein-coding genes
-The TATA box attracts the attention of the TATA-binding protein (TBP) and it is after TBP is recognized
by the promoter that many other transcription factors accumulate on TBP.
-All of these transcription factors make the promoter more attractive to RNA polymerase and
transcription of the gene commences. You can also regulate transcription here, by regulating the
availability of the transcription factors.
The Function Of The Enhancer:
-The collection of transcription factors that accumulates on TBP and the promoter may be somewhat
unstable. This is why activator proteins bind to the region proximal to the promoter and on to the
enhancer as well. When this happens, the enhancer is able fold onto the promoter, like a “sandwich.”
-In this sense, the enhancer is merely the binding site for activator proteins that stabilize this complex of
transcription factors and thus, enhances (increases) transcription.
-There are also regions that stabilize proteins known as silencers but they engage in the stability of
inhibitory complexes of proteins which prevent transcription.
-Mainly eukaryotic in nature, enhancers are position independent as well as immune to inversion. As
long as an enhancer for a particular gene can bend into position, they are still able to complete their
-Dramatic changes in position and/or inversion have little or no effect on the intended function of
Differential Gene Regulation & Combination Of Activators:
-In order to undergo dramatic changes in physicality and phenotype, different collections of genes need
to be expressed.
-This can be achieved with a different sweet of activator proteins to bind on to the enhancers and they
will enhance a different form of gene expression.
-This can be another function of enhancers; that they are responsible for regulating a particular
collection of activator proteins in the cell and assist in regulating which sequences get transcribed. Eukaryotic Transcription:
-When messages are made in eukaryotes, just like in prokaryotes, polymerases read from the 3’ to 5’
end and produce the message (mRNA) from the 5’ to 3’ end.