Download Protein Sythesis

DNA does not leave the nucleus. Why?
1) To dangerous!
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If distorted, cleaved, or damaged  useless
Possible death of organism
2) Proteins required in large amounts

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Only 2 strands of DNA
Can be many strands of different RNA chains
(coding for different proteins)
 Increases flexibility in carrying out various activities.
3) Leaving/re-entering the nucleus would be
time-consuming.

Transcription (copy from one medium to
another)

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DNA  RNA
Translation (convert into a different
language)

RNA  polypeptide
Dogma: “proclaimed as true without perfect proof”
Science is a study which examines ‘truth’ down to its core.
As a scientist, you can never prove anything as perfectly true. You can observe and study
millions of different species or processes within species... up to the finest parts that
make up matter. But a real scientist will never stop trying to find out more.
The more you learn, the more there is to know.
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Ribonucleic Acid (RNA)
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How does RNA differ from DNA again?
Three Kinds

mRNA: messenger
 Longer the gene, the longer
the mRNA

tRNA: transfer
 Transfers appropriate AA
to ribosome to build a PP
 Short: 70-90 ribonucleotides
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rRNA: ribosomal
 Structural component of
Ribosome.
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There are _____ amino acids.
How many nucleotides must code for one amino acid?
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If only one nucleotide coded for one AA...
 41 = 4 possible combinations
 Ex// thymine  cysteine
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If two nucleotides coded for one AA...
 ________ possible combinations
 Ex// AC  cysteine
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If three nucleotides code for one AA...
 _________ possible combinations
 Ex// CAU  histidine
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Codon: sequence of three bases in DNA or
complementary mRNA that serves as a code for a
particular amino acid.
More than one codon can code for a single amino acid
(redundancy).
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Start Codon: signals initiation of translation.
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Only one in the genetic code
Is usually AUG  methionine
Stop Codon: does not code for an amino acid:
signals termination of translation.
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Three of these in the genetic code
UAA, UAG, UGA
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Pg. 241, #1--13
Initiation
 RNA polymerase binds to specific part of DNA
that is to be transcribed  helix opens.
 Specific. How?
Transcription factors help polymerase to bind  these
factors help control which genes are transcribed.
 Bound upstream of the gene: promoter region.

 Usually high in T and A
 Less energy to unwind helix
 RNA polymerase only recognizes the promoter region
(therefore, not random!)
 This region is not transcribed
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Elongation

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RNA polymerase builds mRNA in 5’ – 3’ direction
Does not require a primer
RNA poly. Uses one strand of DNA for mRNA
synthesis
 Template strand: strand of DNA used by RNA
polymerase to build complimentary mRNA.
 mRNA complimentary to this DNA strand
 Coding strand: not being used for transcription.
 mRNA identical to the DNA strand (except for uracil)
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Termination

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mRNA synthesized until the end of gene is reached
Terminator sequence: sequence of bases at end of a
gene that signals the RNA polymerase to stop
transcribing
 Differs between prokaryotes and eukaryotes
 mRNA dissociates with DNA
 RNA polymerase free to bind to another promoter
region.
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
The mRNA made by transcription is called the
primary transcript.
Modifications need to be made to RNA before
it can leave the nucleus


CAPPING AND TAILING
SPLICING
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5’ cap: 7-methyl guanosine
Added to start of primary transcript
 Protects it from digestion by nucleases and
phosphatases
 Helps bind mRNA to ribosome
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
Poly-A tail: approx. 200 adenine
ribonucleotides is added to 3’ end by poly-A
polymerase.
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Slowly degraded over time
When ‘used up,’ mRNA is degraded

DNA of eukaryotic gene comprises of two
intermingling regions

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Exons: coding regions
Introns: noncoding regions
 If these regions are translated, protein will not function
properly.
 Removed from primary transcript by spliceosomes
 Introns stay within nucleus  degraded  nucleotides
recycled.


Primary transcript  capped/tailed and
introns exiced  mRNA transcript.
No quality control enzyme!

Errors not as detrimental to the cell (multiple copies
of mRNA)
 If error made during transcription, protein usually
useless and will be degraded.
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Page 249
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#1-6, 8-9.
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Initiation
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Ribosome recognizes the 5’ cap
in eukaryotes and binds to
mRNA.
Elongation
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Ribosome moves along mRNA
in 5’-3’ direction.
Amino acids are added to the
growing PP
Codon  AA.
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Reading Frame: how the mRNA is read.
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Three possibilities
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tRNA: single stranded nucleic acid

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Transfers AAs to ribosome
Anticodon: sequence of three bases on one arm of
the tRNA.
 Recognizes the codon of mRNA (complimentary)
 If mRNA codon is AUA, anticodon is _____.
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tRNA + amino acid = aminoacyl-tRNA.
Aminnoacyl-tRNA synthetase: enzyme
responsible for adding AA to tRNA.

At least 20 of these enzymes.
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Every tRNA transfers only ONE specific AA.
Therefore, there must be AT LEAST ____ different
tRNA molecules.
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Third base may differ between two codons, but may
code for the same AA.

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Ex// UAU and UAC  tyrosine.
If tRNA’s anticodon is UAU, can still bind to UAC
codon.
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Between 20-64 tRNAs: depends on organism.
tRNA can recognize more than one codon by unusual pairing
Advantage: correct amino acid may be added despite errors
made from DNA  mRNA.
WOBBLE HYPOTHESIS: the proposal that tRNA can
recognize more than one codon by unusual base
pairing b/n 1st base of anticodon and 3rd base of codon.


Start codon: _______. Every protein initially begins
with the AA _______________.
Ribosome has two sites for tRNA
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A (acceptor) site: site that tRNA brings amino acid to.
P (peptide) site: peptide bonds are formed between
adjoining AAs.
tRNA carrying
Methionine enters
P site first.
Next appropriate AA
will enter the A site.
Met + AA peptide bind.

a)
Methionine is the
start codon.
b)
Second tRNA enters
A site.
c)
Ribosome
TRANSLOCATES
one codon over, and
next tRNA enters A
site.
d)
Bond, translocation,
and addition of new
tRNA.
e)
Repeat d. Stop codon
does not code for an
AA.
f)
Release-factor protein
helps dismantle the
ribosome-mRNA
complex. New PP is
released!

Ribosome eventually reaches a stop codon.
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Release Factor Protein: aids in the release of the
PP chain from the ribosome.
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Ribosome stalls
Subunits fall off of mRNNA.
Glycosylation: sugars may be added to PP.
Phosphorylation: addition of phosphates.
PP may also be cleaved (cut) afterward

Ex// methionine may not be the first AA anyore.
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