BIOL 1010 Lecture Notes - Start Codon, Primary Transcript, Snrnp

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26 Nov 2012
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Lecture 31
Central dogma of Biology
Information from DNA is transmitted to DNA when it is replicated
Information from DNA is transmitted from DNA to RNA to proteins
during phenotypic expression
(transcription, translation)
The dictionary of the genetic code
In the genetic code, nucleotide
triplets specify amino acids.
Codon- a three nucleotide
sequence in mRNA that specifies which
amino acid will be added to a growing
polypeptide or that signals termination
Steps of Transcription
Transcription occurs in three stages
o Initiation
o Elongation
o Termination
Initiation
RNA polymerase binds directly or
indirectly at regions of the DNA called
promoters; transcription begins here
o Promoters include the transcription
start point and sequences before the
start point as well.
o The promoter determines which DNA strand will be used as a template
o In prokaryotes, RNA polymerase binds directly to the promoter
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o In eukaryotes, other proteins (transcription factors) bind to the promoter
first, followed by RNA polymerase
Eukaryotic promoter
In eukaryotes, promoter is about 100 nucleotides long and contains:
o 1. The initiation site
o 2. Specific nucleotide sequences recognized by DNA- binding proteins
called transcription factors that help bring RNA polymerase to the
promoter
RNA polymerase II cannot recognize promoter unless specific transcription
factors are bound first
3. TATA box: a short nucleotide sequence in the promoter that contains T and A,
located about 25 nucleotides from the start point-
o Specific transcription factors bind here
o RNA polymerase recognizes the complex
and binds
The initiation of transcription at a Eukaryotic
promoter
In eukaryotic cells, the enzyme that transcribes
protein-coding genes into pre-mRNA is RNA
polymerase II
This enzyme initiates RNA synthesis at
promoters that commonly include TATA box, a
nucleotide sequence typically something like
TATAAAA
(A) Within the promoter, the TATA box is
located about 25 nucleotides from the
transcriptional start point
(B) RNA polymerase II cannot recognize the
TATA box and other landmarks of the
promoter on its own
Another protein, a transcription factor that
recognizes the TATA box, binds to the DNA before the RNA polymerase can
do so
(C) Additional transcription factors (purple) join the polymerase on the DNA
Protein-protein interaction are critical in the formation of the eukaryotic
transcription initiation complex
The DNA double helix unwinds, and RNA synthesis begins at the start point
on the template strand
Elongation of the RNA strand
Once transcription begins the RNA polymerase moves along the DNA and
performs two primary functions
o In untwists and opens a short segment of DNA exposing about ten
nucleotide bases. One of the exposed strands is template for base-
pairing of nucleotides
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