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Protein
Synthesis
DNA – Its Role in Protein Synthesis
Provides directions for the construction of
proteins
Chromosome consist of:


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deoxyribose sugar backbone
phosphate molecule
4 nitrogenous bases
Proteins can be composed of 20 different
amino acids. One nitrogen base cannot
code for a single amino acid
20 amino acids can be accommodated by a
triplet code (Three-base code aka “codon”)
RNA and DNA are similar, except for;
Uracil = Thymine, the sugar ribose, and
single stranded
RNA – controls assembly of amino acids into
proteins
3 Types of RNA
(See pg 300 Fig. 12-12)
Messenger RNA or mRNA = carrier
molecule, reads the DNA code and carries it
to the ribosomes
Ribosomal RNA combines with proteins to
form ribosomes
Transfer RNA transfers each amino acid to
the ribosome as it is specified by coded
messages in mRNA
Primers (aka Promoters)
Initiators – “turn protein synthesis on”
Terminators – “end protein synthesis”
TRANSCRIPTION
DNA molecule “unzips” – by RNA
polymerase
mRNA finds the appropriate pair
cytosine = guanine
adenine = uracil
Once the pairs are joined mRNA moves
away
 two original DNA strands rejoin
– transcription has been completed
TRANSCRIPTION cont.
mRNA carries the nitrogen base code to
the ribosomes and attaches itself
Transcription Animation
http://www.ncc.gmu.edu/dna/mRNAanim.h
tm
TRANSLATION
See Fig. 12-18 in text
Initiator codon (AUG) – turns protein
synthesis on
tRNA (transfer) picks up amino acids
Each tRNA has a specific anticodon
(exposed nitrogen bases) – complimentary to
one mRNA codon
TRANSLATION cont.
mRNA will attach to tRNA where the
codon and anticodon base pairs match

ie; UUU codon of DNA receives an AAA
anticodon of tRNA
tRNA will place its amino acid in the
ribosome and move away
mRNA moves along the ribosome and the
process known as elongation continues
TRANSLATION cont.
Amino acids carried by tRNA form
proteins (the sequence depends on the
message carried)
Translation is complete

Terminator codon turns off Protein synthesis
See pg 303 Fig.12-17
Even though there are only 20 amino acids
that exist, there are actually 64 possible
tRNA molecules:
4 X 4 X 4 = 64 possible combinations
Translation Animation
http://www.dnatutorial.com/ProteinSynthesis.
shtml