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Protein
Synthesis
Protein Synthesis
 The instructions contained in the DNA must be
copied into mRNA.
 This is transcription.
 Transcription takes place in the nucleolus.
 mRNA copies are then transported to the ribosomes
for translation or protein synthesis.
 Ribosomes are in the cytoplasm.
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 During transcription, the portion of DNA that is to be
copied unzips:
 hydrogen bonds are broken by a helicase enzyme
 exposes the bases of the complementary DNA strands
 Only one of the strands is copied – the transcribing
strand.
 Activated RNA nucleotides line up opposite their
exposed complimentary bases:




U with A
A with T
G with C
C with G
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 The adjacent nucleotides are then joined together by
a RNA polymerase, which:
 uses the energy released when the two extra
phosphate groups are removed from the activated
nucleotides
 Note:
 the mRNA has a base sequence that is complementary
to that of the transcribing strand
 the triplets of bases that code for an amino acid in the
mRNA are called codons
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1. Helicase enzyme
breaks the
hydrogen bonds
3. The nontranscribing strand
of DNA
4. Polymerase
enzyme bonds the
nucleotides
2. The transcribing
strand acts as the
template
A
T
G
G
T
A
C
A
T
C
T
T
A
C
T
A
U
G
G
U
A
C
A
U
C
U
U
A
C
U
T
A
C
C
A
T
G
T
A
G
A
A
T
G
A
6. mRNA leaves the
nucleus via the
nuclear pore
5. Complementary
nucleotides line up
(AT, CG, UA, GC)
Note: once produced, the mRNA molecule is released and leaves the nucleus
via the nuclear pores. It travels to the ribosome for translation, i.e. reading5 of
the message and production of protein.
 As described, the process of transcription has
similarities to DNA replication:
 The DNA molecule unwinds and unzips.
 It acts as the template for synthesis of the RNA and
complementary base pairing ensures that the message
is transcribed accurately.
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Q. How is this different from DNA replication?
You may find it helpful to consider: exposure of the DNA to act as a
template; which strands are copied; the nucleotides involved
and any differences in their composition, e.g. pentose sugar
involved; complementary pairing; other features, e.g. nature of
the resulting polynucleotide.
 Only one strand, the transcribing strand, is copied.
 DNA nucleotides are composed of deoxyribose,
phosphate and adenine, guanine, thymine or
cytosine.
 RNA nucleotides are composed of ribose, phosphate
and adenine, guanine, uracil or cytosine.
 DNA is double stranded whilst RNA is single
stranded.
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Translation
 Translation takes place in the cytoplasm at the
ribosomes.
 Ribosomes are made in the nucleolus from protein
and ribosomal RNA (rRNA).
 There are two subunits:
 a large subunit and a small subunit
 with a groove between them in which the mRNA fits
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tRNA
 Transfer RNA (tRNA) transports amino acids to the
ribosome for incorporation into protein.
 Each tRNA is a single polynucleotide that is folded
into a clover-leaf shape.
 Hydrogen bonds between complementary bases
stabilise the shape.
 tRNA has:
 an anticodon that is complementary to the codon on
the mRNA
 three unpaired bases to which a specific amino acid
can be attached
 This ensures that the right amino acid gets to the
right place.
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U A C
U
A
C
Anticodon. The
bases are
complementary to
those on the mRNA.
Three bases for the
attachment of a specific
amino acid.
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 During translation, mRNA enters the groove between
the small and large subunit.
 The ribosome is large enough for two codons to be
side by side.
 The very first codon is always the same:
 initiation or start codon AUG
 codes for methionine
 The anticodon of a tRNA carrying methionine
hydrogen bonds to the first codon.
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 A second tRNA, carrying the second amino acid
coded for, hydrogen bonds to the second codon.
 A peptide bond is formed between the two amino
acids and the ribosome moves three bases (one
codon) along the mRNA so that another codon is
exposed.
 A third tRNA brings another amino acid which is
added to the dipeptide and so the process:
 continues until a termination or stop codon (UAA, UAG
or UGA) is reached
 Add descriptions/explanations to the diagrams
showing translation.
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A U G
U A C
G U A
C A U
C A U
C
U
U
A C U
G A A
U G A
G U A
Leu
Met
Val
His
Thr
• mRNA lies in the groove of the ribosome, which has places for
two codons (triplets of bases) coding for specific amino acids.
• The start codon always encodes for methionine.
• In the cytoplasm, tRNA transferases load tRNA with its amino
acid.
• tRNA (complementary anticodon) aligns itself with its codon.13
A U G
U A C
Met
G U A
C A U
Val
C A
U
G U
His
C
U
U
A G
A
Leu
A C U
A
U
G A
Thr
• A second tRNA binds to the next codon.
• Again, complementary base pairing ensures that only the
correct tRNA binds.
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A U G
U A C
Met
G U A
C A U
Val
C A
C
U
G
U
His
U
U
A G
A
Leu
A C U
A
U
G A
Thr
• The two amino acids are held very close to each other.
• A peptide bond is produced between the carboxylic acid group
and amino group of the two amino acids.
• Condensation reaction occurs – removal of water.
• Ribosome has enzyme that does this.
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A
U A
U
C
G
G U
C A
Val
Met
A
U
C A U
G U A
His
C
U
G
U
A
Leu
A
A
U
C U
G A
Thr
• Ribosome moves along, exposing the next codon on the
mRNA.
• Complementary base pairing between the codon and
anticodon of the next tRNA happens.
• Peptide bond is formed between, in this case, valine and
histidine.
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A
U A
U G
C
C
G U
A U
A
C A U
G U A
C
G
U
A
U
A
A C U
U
Met
Val
His
G A
Leu
Thr
• Ribosome moves along, exposing the next codon on the
mRNA.
• Complementary base pairing occurs between the newly
exposed codon and the anticodon.
• Process continues until a stop codon, e.g. UGA is reached.
• No tRNA for this, so the protein and mRNA are released. 17
 The protein may need further processing before it is
active:
 insulin requires considerable processing first
 some enzymes require activation by cyclic AMP
 See activation of enzymes.
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 The process of transcription and translation are
summarised on the next slide.
 Key elements:
 unzipping of DNA by helicase
 complementary mRNA synthesised using an RNA
polymerase
 mRNA leaves nucleus and goes to ribosome
 spaces for two codons
 complementary base pairing between codon and
anticodon of tRNA
 peptide bond inserted between amino acids
 peptide grows by one amino acid at a time, until stop
codon reached
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Single-stranded messenger RNA leaves nucleus
and travels to the ribosome for translation
A
T
G
G
T
A
C
A
T
C
T
T
A
C
T
A
U
G
G
U
A
C
A
U
C
U
U
A
C
U
T
A
C
C
A
T
G
T
A
G
A
A
T
G
A
A U G GU A C A U C U U A C U
UA C CA U
GU AG A A
U GA
Met
Val
His
Leu
Thr
A UG G U A C A U C U U A C U
GU A G A A
UA C CA U
U GA
Met
Val
His
Amino acids
transferred to
ribosome by
transfer RNA
Leu
Thr
Complementary base pairing allows transcription and
correct sequencing of amino acids at the ribosome
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