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Gene Expression I:
Transcription and RNA Processing
3 November, 2004
Text Chapter 17
•Usually, dominant alleles are recipes for functional proteins.
•Recessive alleles are altered recipes that produce nonfunctional proteins.
Think about flower color in pea plants.
Substrate (colorless)
Enzyme P
Product (purple)
The P allele is a recipe for a functional enzyme. The p allele is a
recipe for a non-functional enzyme. Purple is dominant because one
copy of a functional recipe is enough.
In the analogous situation in snapdragons, one copy is not enough,
And an intermediate phenotype is seen.
At the molecular level, both functional and non-functional proteins
are present. This is more like codominance.
Neurospora crassa is a fungus that can grow on minimal medium. This
means that it can synthesize all of the amino acids, including arginine,
from simple precursors.
The biochemical pathway specific to arginine synthesis consists of three
enzymes. Arginine auxotrophs are deficient in one of these enzymes.
Genes are the instructions for making proteins.
In prokaryotes, this is a two-step process. Eukaryotes add an RNA
processing step.
mRNA molecules
are complementary
to the template
strand of the DNA.
Codons are 3-letter
genetic words that
specify amino acids.
Proteins are linear
polymers of amino
acids.
Codons are read as nonoverlapping three-letter
words.
The code is not delimited
(there is no punctuation).
This means that in any
RNA, there are three
possible reading frames.
auaugauucuucgauaaca
a uau gau ucu ucg aua aca
au aug auu cuu cga uaa ca
aua uga uuc uuc gau aac a
auAUGauucuucgauaaca
Met-Ile-Leu-Arg-*
The Genetic Code
The genetic code is redundant - most amino acids are coded
by more than one of the 64 possible codons.
The genetic code is not ambiguous - no codon codes for
more than one amino acid.
The genetic code is universal - all organisms use the same
code, indicating that the code evolved once, early in the
history of life.
An important implication of the universal code is that genes
code for the same protein sequence in any organism.
Transcription
Factors
Transcription begins when
protein transcription
factors bind at the
promoter. These proteins
allow RNA polymerase to
bind and unwind the DNA
at the transcription start
site.
Eukaryotes modify the pre-mRNA after transcription.
These modifications include a modified nucleotide cap at the 5’
end, and a poly-A tail added to the 3’ end of the completed
transcript.
Most eukaryotic genes are spliced - introns are removed, and exons
(coding regions) are joined
Introns are removed when
spliceosome RNA pairs with
signal sequences at the ends
of the intron. The
spliceosome then joins the
exons that flank the removed
intron.
Many mRNAs can
potentially code for a
number of different
proteins, depending on
which exons end up in the
final mRNA,