Download File - Mr. Shanks` Class

F. PROTEIN SYNTHESIS [or translating the message]
1. The code mRNA
a. the message on mRNA is written in 3-letter triplets
called codons
there are 20 different amino acids, so how many codons do we need?
given the words are 3 letters long and we have a choice of 4 letters,
how many words do we make?
b. there are 43 or 64 possible codons
c. pg 248 shows the codons – you will have to learn the
patterns but NOT memorize this with the
exception of the START and the STOP codons
2nd BASE
1st BASE
U
C
3rd BASE
A
G
U
UUU
UUC
UUA
UUG
Phe
Phe
Leu
Leu
UCU
UCC
UCA
UCG
Ser
Ser
Ser
Ser
UAU
UAC
UAA
UAG
Tyr
Tyr
stop
stop
UGU
UGC
UGA
UGG
Cys
Cys
stop
Trp
U
C
A
G
C
CUU
CUC
CUA
CUG
Leu
Leu
Leu
Leu
CCU
CCC
CCA
CCG
Pro
Pro
Pro
Pro
CAU
CAC
CAA
CAG
His
His
Gln
Gln
CGU
CGC
CGA
CGG
Arg
Arg
Arg
Arg
U
C
A
G
A
AUU
AUC
AUA
AUG
Ile
Ile
Ile
Met
ACU
ACC
ACA
ACG
Thr
Thr
Thr
Thr
AAU
AAC
AAA
AAG
Asn
Asn
Lys
Lys
AGU
AGC
AGA
AGG
Ser
Ser
Arg
Arg
U
C
A
G
G
GUU
GUC
GUA
GUG
Val
Val
Val
Val
GCU
GCC
GCA
GCG
Ala
Ala
Ala
Ala
GAU
GAC
GAA
GAG
Asp
Asp
Glu
Glu
GGU
GGC
GGA
GGG
Gly
Gly
Gly
Gly
U
C
A
G
d. notice that UUU & UUC both code for phe and CUU,
CUC, CUA & CUG all code for leu this indicates that the
3rd letter is the least important in the code
e. only two amino acids have a unique code UGG  trp and
AUG  met
f. all proteins must start with met as AUG is the start code; this
may be removed later
g. there is no amino acid that is coded by UAA, UAG or UGA
and so the protein breaks here and these are called STOP
codes
2. The ribosomes [rRNA]
a. the rRNA holds together the large subunit [60s] and the
small subunit [40s] to form the active ribosome [80s]
b. the 5’ [CAP region] adheres to the active ribosome
c. the ribosome will move the mRNA through it starting at
the 5’ end and going toward the 3’ end
3. The tRNA
a. each tRNA has a cross-like structure [pg 251]
b. at the base there is an anticodon which is the complement of the
codon on themRNA
eg
the mRNA has the codon AUG for met 
the tRNA—met has the anticodon UAC
c. at the 3’ end of the tRNA is the acceptor site that holds the
amino acid
d. it requires 1 ATP to attach each amino acid to the correct
tRNA to make an amino-acyl tRNA
e. there are 61 possible tRNA for the 20 amino acids, therefore a
cell needs 61 enzymes for this
show animation,
then complete notes
STEPS IN PROTEIN SYNTHESIS
1. The mRNA, rRNA and tRNA are transcribed from DNA in
the nucleus. They randomly drift around in the cytoplasm
where the large and small subunits of the ribosome are
also drifting.
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
mRNA
UAC
tRNA
large
subunit of
ribosome
rRNA
small
subunit of
ribosome
2. The mRNA leaves the nucleus and encounters the small
subunit of the ribosome.
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
3. The rRNA attaches the large ribosome subunit to the small
subunit that is attached to the mRNA.
The tRNA’s are activated by the attachment of an amino
acid, by the use of an ATP.
Gly
Gln
Met
Ile
CCG
Pro
UAC
GUC
Val
UAG
GGU
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
P-site A-site
CAU
4. The tRNA with Met attached, randomly enters the
P-site on the ribosome.
Because its anti-codon matches the codon in the P-site,
this tRNA is held in place by hydrogen bonds.
Gly
Met
Gln
Ile
CCG
UAC
Met
Pro
GUC
Val
UAG
GGU
CAU
UAC
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
P-site A-site
5. The tRNA for the next amino acid randomly
moves into the A-site.
Pro
Gly
Gln
GGU
Ile
CCG
Met
Pro
GUC
Val
UAG
UAC GGU
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
P-site A-site
CAU
6. The tRNA that brought Met now is released after Met is bonded
to the Pro.
The mRNA is now moved through the ribosome until the tRNA
with Met-Pro is in the P-site.
The tRNA for the next code now fits into the A-site
Gly
Ile
Gln
UAC
UAG
CCG
Met- Pro
Ile
GUC
GGU UAG
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
P-site A-site
Val
CAU
7. The processes of step 5 & step 6 are repeated until one of the
STOP codes is in the A-site. At this point all of the
amino acids have been bonded into a polypeptide chain
connected to the last tRNA in the P-site.
UAG
UAC
GGU
CCG
Met-Pro-Ile-Gly-Gln-Val
CAU
CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC
P-site A-site
GUC
8.
Because there was no tRNA to match UAG, the polypeptide
chain is released. and the ribosome falls apart.
The mRNA will be broken down.
C AUG CCA AUC GGC C
CC GAG CU
AG GUA UAG GGG ACC
mRNA
Met-Pro-Ile-Gly-Gln-Val
large
subunit of
ribosome
polypeptide chain
small
subunit of
ribosome
Now attempt the assignment at the bottom of the page
From the piece of DNA below, make the complimentary strand of DNA, transcribe the appropriate
strand, package it for the cytoplasm and then translate it as described in the steps below.
In row A; make the complimentary DNA strand;
In row C, transcribe the DNA into RNA;
In column 1 and 28, place the appropriate 3’ or 5’;
In column 2 & 27, ‘package’ the RNA in row C;
In row D, underline the ‘words’ in the RNA;
In row E, translate the RNA into amino acids using the 3letter codes for them
1
2
3 4 5 6 7 8 9
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
2
7
2
8
A 5
C C A T G C G A T G
C G C G T T C C T G A G A G C
3
B 3
G G T A C G C T A C C G C A A G G A C T C T C G
5
C 53
CAP
C C A U G C G A U G G C G U U C C U G A G A G C polyA 3
D
E
met
arg
trp
arg
ser