Gene Expression I: Transcription and RNA Processing 2 November, 2005 Text Chapter 17

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Gene Expression ITranscription and RNA
Processing2 November, 2005Text Chapter 17
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• Usually, dominant alleles are recipes for
  functional proteins.

• Recessive alleles are altered recipes that
  produce non-functional proteins.

Think about flower color in pea
plants. Substrate (colorless)
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.
Enzyme P
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.
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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.
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Genes are the instructions for making proteins.
In prokaryotes, this is a two-step process.
Eukaryotes add an RNA processing step.
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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.
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Codons are read as non-overlapping 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-
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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.
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Transcription
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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.
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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.
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Most eukaryotic genes are spliced - introns are
removed, and exons (coding regions) are joined
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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,
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