Protein Synthesis
(From Nucleus to
Cytoplasm)
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The Central Dogma

www.video.sina.com.cn
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tRNA
TRANSCRIPTION
DNA
mRNA
TRANSLATION
Amino Acids
PROTEIN
Ribosomes
REPLICATION
rRNA
Protein
The Central Dogma (in equation
form)
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 Initial Problem:
DNA codes for proteins BUT DNA is
confined to the nucleus and the ‘equipment’ to
carry out protein synthesis is in the cytoplasm.
(The equipment includes ribosomes, tRNA,
and amino acids).
Solution???
DNA can be “copied” into an RNA molecule
(messenger RNA (mRNA)), which can travel
from the nucleus to the cytoplasm carrying the
instructions for building a protein.
This DNA to mRNA copying process is known
as transcription (see fig. 25.7 on p. 512).
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Visual Examples of Transcription
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Transcription
 During this process, DNA serves as a template
(guide) for the production of mRNA
 The enzyme DNA helicase serves to unwind and
‘unzip’ the portion of the DNA double helix (ie. The
GENE) that is to be transcribed (copied into
mRNA).
 Once this occurs, free-floating RNA nucleotides
(within the nucleus along with free-floating DNA
nucleotides) bond to the exposed bases of DNA
through complementary base-pairing (forming
hydrogen bonds).
** RNA nucleotides bind to only ONE exposed DNA strand.
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



A (mRNA) binds with T (DNA).
U (mRNA) binds with A (DNA).
C (mRNA) binds with G (DNA).
G (mRNA) binds with C (DNA).
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 The first three mRNA bases are always AUG (called
the start codon), which means that the first three
DNA bases that are transcribed are TAC.
 The last three mRNA bases are always one of UAG,
UAA, or UGA (STOP codons), which means that the
last three DNA bases transcribed are always one of
ATC, ATT, or ACT.
 The enzyme RNA Polymerase then works to join
added mRNA nucleotides to each other (sugar-Psugar-P etc…) through dehydration synthesis
(producing water).
 So, the mRNA nucleotides form spontaneous Hbonds with exposed DNA bases but then need
enzymatic aid (RNA polymerase) to form the actual
mRNA chain.
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 DNA helicase continues to unwind/unzip the
DNA until the gene that requires copying has
been fully exposed.
 The signal for DNA helicase to stop is when it
encounters ATC, ATT, or ACT.
A poly-adenine tail is added to one end of
the finished mRNA while a guanine-based
‘cap’ is added to the other side to protect
the molecule from cytoplasmic enzymes.
Once this is finished, the mRNA moves into
the cytoplasm, through a nuclear pore, and
the DNA joins back together by reforming its
complementary base-pairing H-bonds.
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START Codon
STOP Codon
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The Genetic Code
 Query: How is a sequence of nitrogenous bases
on mRNA going to be used to code for a sequence
of amino acids and hence, a protein???
 First of all, DNA is the universal code (ie. Every
living thing has DNA, and DNA codes for proteins
(through the use of mRNA)).
 There are 20 amino acids in nature.
 There are 4 different nitrogenous bases in both
DNA and mRNA, and they serve as a ‘code’ for the
amino acid sequence of proteins.
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 The code reads as a TRIPLET CODE (43 = 64
possibilities), meaning that three nitrogenous
bases, as a group, code for ONE amino acid.
 A singlet code could only code for 41 = 4
amino acids (yet, there are 20) – thus proving
inadequate.
 A doublet code could only code for 42 = 16
amino acids.
 A group of three mRNA nucleotides (bases) is
called a CODON. In total, there are 64 (43)
different codons.
 A group of three DNA nucleotides (bases) is
called a DNA triplet.
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 There are 61 different codons that correspond to
the 20 different amino acids (one of these 61 is
AUG, which is the ‘start’ codon, that codes for the
amino acid methionine).
 The other three codons are called STOP codons,
which terminate the formation of the
polypeptide/protein chain.
 These STOP codons DO NOT code for an amino
acid.
 The Genetic code is sometimes referred to as
being redundant, because most amino acids are
coded for by 2-6 different codons.
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 AAA /GCU /ACC /GGU /UAC /GUC /UAG mRNA sequence
 Questions: i. How many codons present? 7
ii. How many a. acids coded for? 6
iii. What is the a. acid sequence?
lysine, alanine, threonine, glycine,
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tyrosine, valine, STOP
 iv. What was the DNA sequence of
triplets that coded for this mRNA?
TTT CGA TGG CCA ATG CAG ATC
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Translation
 The process of turning mRNA into a protein (ie.
Translating the language of nitrogenous bases into
the language of amino acids).
 Recall that mRNA is constructed in the nucleus
through the process of transcription, and is sent out
of the nucleus through a nuclear pore.
 Once mRNA enters the cytoplasm, it immediately
associates with a ribosome (either a ‘free’ ribosome
or a Rough ER ribosome).
 The ribosome attaches to the mRNA at the guaninebased ‘cap’ that not only served as protection from
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enzymes, but acted as a ‘start here’ signal.

i.
Translation requires two other types of RNA:
rRNA (Ribosomal RNA)
-- joins with ribosomal proteins (from
nucleolus) to form ribosomes.
-- produced in the nucleolus.
-- one ribosome has two subunits:
a. Large subunit (3 rRNAs and proteins)
b. Small subunit (1 rRNA and proteins)
-- the two subunits remain close together but
do not actually attach until just prior to
translation.
-- rRNA is not involved in any coding or
translating, it is purely structural.
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ii. tRNA (Transfer RNA)
-- located in the cytoplasm and serve as ‘carriers’
of singular amino acids to the mRNA/ribosome
complex.
-- tRNAs carry one amino acid at one end, and a
specific ANTICODON at the other end.
-- this anticodon will ‘match-up’ with a
complementary codon on mRNA (through
complementary base-pairing) (fig. 25.8 p. 513).
-- tRNA molecules are very specific (ie. A tRNA
with a certain anticodon will ALWAYS be carrying the
same amino acid).
-- remember, though, that the translation table is
translated from mRNA codons, not the tRNA
anticodons.
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Eg. If the mRNA codon being translated is ACG,
what anticodon and amino acid will the tRNA
molecule, specific to this codon, be carrying?
Ans. Anticodon = UGC
Amino Acid = Threonine (need table)
Eg 2. Codon = CAA, find anticodon and a. acid.
Anticodon = GUU, A. acid = Glutamine
Eg 3. DNA triplet = TTC, find mRNA codon, tRNA
anticodon, and amino acid.
mRNA codon = AAG, anticodon = UUC
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Amino acid = Lysine
 HINT: The anticodon will be the same as
the original DNA triplet except that a U
will replace a T.
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Three Steps of Translation
1. INITIATION
-- the ‘cap’ of mRNA binds to the ribosome and
the ribosome moves along the mRNA,
‘reading’ it, until it comes upon the ‘start’
codon, AUG.
-- the tRNA with anticodon UAC binds to the AUG
codon (through complementary base-pairing)
at the A-site, and delivers the first amino acid
Methionine.
-- see handout (crude handout, that is)
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A-site
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2. ELONGATION (lengthening of the amino acid
chain).
-- firstly, the ribosome is large enough to
accommodate two tRNAs at the same time 
the ‘incoming’ tRNA (at the A-site) and the
‘outgoing’ tRNA (at the P-site).
-- after the first tRNA binds to the mRNA codon,
the ribosome shifts one codon (3 bases), thus
exposing a new codon in the A-site which can
then be bonded to by a new tRNA with the
complementary anticodon.
-- after this next tRNA binds, the ribosome shifts
again and ‘bumps’ the tRNA in the P-site off of
the ribosome.
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-- before it is bumped off of the ribosome’s P-site,
the outgoing tRNA molecule always passes the
amino acid chain, via dehydration synthesis, to
the tRNA that is shifting from the A-site to the Psite.
-- the liberated or bumped tRNA (now without an
amino acid) will eventually pick up the same
amino acid that it just ceded and will rejoin the
group of tRNAs waiting to be ‘chosen’.
-- amino acids are readily available in the
cytoplasm.
-- the ribosome continually shifts to accept more
tRNA molecules so that the protein chain can
grow one amino acid at a time.
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P-site
A-site
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iii. TERMINATION
-- occurs once the codon appearing in the A-site of
the ribosome is a STOP codon.
-- this STOP codon is recognized by the ribosome
complex and a RELEASE FACTOR protein is
summoned from the cytoplasm into the A-site.
-- there is NO tRNA molecule for these codons.
-- once the release factor protein binds to the STOP
codon, the ribosome dissociates into its two subunits
and falls off the mRNA (which is recycled).
-- the peptide/protein chain is released by the tRNA in
the P-site into the lumen of the Rough ER (if for
export), or into a transition vesicle bound for the
Golgi for modifications (if it is to remain in the cell).
-- see fig. 25.9 p. 514, fig 25.11 p. 516
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-- read translation summary on pp. 512-516.
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By simply eating protein, you can build new,
custom proteins and maybe look like this:
Yes, and once again, I
am Hans; and I am
Franz; and we want to
PUMP…YOU UP!
Especially you ‘girly’
men!
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