Transcript Transcription - smithlhhsb121

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The replication of DNA takes place in S phase
of interphase
However, DNA is also used during G1 to
assemble proteins
This process is broken down into two distinct
segments: transcription and translation
The entire human genome is found in every
cell, but only a portion is activated
This portion has to convey the message
outside the nucleus
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It is not possible to translate a gene directly
to a protein
RNA has to be used as an intermediary
Quite similar to DNA, with a few key
differences
Structurally, it contains a ribose sugar
◦ This has a hydroxyl group in the 2’ position instead
of a hydrogen
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Additionally, RNA tends to be single stranded,
and contains uracil in the place of thymine
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DNA is not capable of leaving the nucleus
A complement of one of the strands is
transcribed as RNA
◦ Called messenger RNA (mRNA)
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RNA polymerase (II in eukaryotes) serves the
duel purpose of splitting the strand and
attaching the complementary nucleotides
◦ Again, in the 5’->3’ direction
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This binding is not random
A region of DNA, called the promoter,
initiates transcription
The order of nucleotides in the promoter
determines which strand gets transcribed
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A series of proteins, called transcription
factors, bind to the promoter
◦ For example, a TATA box is a part of the promoter
region
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The group of TFs and RNA pol is called a
transcription initiation complex
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Transcription proceeds until a terminator is
reached
The actual detachment process is still a bit
“iffy”
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The form of mRNA detaching from the DNA is
not ready to be read yet
It must go through two processes
1. Alteration of endings
2. Splicing of regions
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The 5’ end gets “capped” by a modified
version of guanine
This 5’ cap holds the pre-mRNA together and
acts as a recognition molecule for ribosomes
On the 3’ end, a region of 30 to 200 adenine
nucleotides is added – called a poly(A) tail
◦ Again, to assist in holding the pre-mRNA together
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A large portion of the pre-mRNA will not
leave the nucleus
Portions are cut out, and the flanking regions
are joined together
The regions that are kept are exons, ones
that are not are introns
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Short regions of nucleotides at the end of
introns signal their removal
Small nuclear ribonucleoproteins, snRNPs,
join up with other proteins to form a
spliceosome
◦ This carries out the splicing process
◦ http://www.youtube.com/watch?v=WsofH466lqk&f
eature=related
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Now the mRNA is ready to be translated
Recall that the message is “read” three
nucleotides at a time
This is the codon triplet
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The codon is interpreted by transfer RNA
(tRNA)
Each version has a nucleotide triplet of its
own
This serves as the complement to the codon
of mRNA
◦ Hence, it is the anticodon
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All 64 permutations are covered by tRNA
molecules, carrying the 20 different amino
acids
◦ However, there are only about 45 difference tRNA
molecules
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In many cases, the first two nucleotides are
sufficient for recognition
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However, each tRNA carries one, and only
one, amino acid
The attachment of tRNA to amino acid is
mediated by an aminoacyl-tRNA synthetase
enzyme
◦ 20 different versions, one for each amino acid
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The interaction of tRNA and mRNA takes
place in a ribosome
Consists of two protein subunits and
ribosomal RNA (rRNA)
Within the ribosome are three binding sites
◦ P site (peptidyl-tRNA site) where the tRNA holding
the polypepetide chain
◦ A site (aminoacyl-tRNA site) where next tRNA in line
is held
◦ E site (exit site) where used tRNAs are discharged
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Begins with initiation
Small subunit of ribosome attaches to leader
segment of mRNA
Initiator tRNA bearing methionine then
attaches, followed by attachment of large
ribosomal subunit
Although methionine is initially attached, it is
sometimes removed
The next stage, elongation, is now set
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This is broken into three distinct steps
1. Codon recognition – mRNA codon hydrogen
bonds with tRNA anticodon in A site
2. Peptide bond formation – rRNA molecule catalyzes
formation of peptide bond (amino acid-amino
acid bond) between adjacent amino acids in A and
P sites
3. Translocation – amino acid in A site is moved to P
site, and is now carrier of growing polypeptide
chain (AA in P site moved to E site for removal)
http://www.youtube.com/watch?v=5bLEDdPSTQ&feature=related
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Translation is finalized during termination
Three stop codon triplets UAA, UAG and UGA,
do not code for any amino acid
Rather, a protein called a release factor binds
to stop codon, causes a water molecule to
attach
The polypeptide chain is released and is
ready for the folding process that will make it
a functional protein
http://www.youtube.com/watch?v=SMtWvDbfHLo&feature=relmfu
http://www.youtube.com/watch?v=TfYf_rPWUdY&feature=relmfu