From Gene to Protein
Gene Expression
• Process by which DNA directs the synthesis of
a protein
• 2 stages
transcription
translation
• All organisms
One gene  one protein
1. Transcription of DNA
• Gene
– Composed of DNA
– 1000s of base pairs long
– Sequence of G, A, T, C is a code
ACATACATGTAGCAACGAAACCTGAACATGCGAACATTGACACAACAACAAUGUAGCCA
A portion of the insulin gene
• A gene is transcribed into mRNA
– One DNA strand is a template
– Follow base complementation using G, A, U, C
Which strand is the template strand?
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This mRNA is referred to as a “transcript”
Single stranded
Uracil nucleotide (not thymine)
ONLY gene of interest is transcribed
mRNA will leave the nucleus for translation
step
• mRNA (transcript) leaves nucleus
• Translated to protein in cytoplasm
• What organelle translates mRNA to
protein?
• How many amino acids?
• How many amino acids long is a
protein?
• How could 10 identical protein
molecules be translated from 1
mRNA?
• What happens to mRNA when the
cell has finished with it?
• Can the cell transcribe more than
one gene at a time?
2. Translation of mRNA
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Nucleotides (G, A, U, C) translated to amino acids
Ribosomes assemble amino acids to form polypeptide
What is the difference between a polypeptide and a protein?
What is the length difference between a chromosome and an
mRNA transcript?
• Why do researchers use frog embryos to study translation?
Differences between prokaryotes and
eukaryotes
• Proks
– No nucleus
– Translation can begin while transcription is still in
progress
• Euks
– Nuclear envelope
– Transcript is modified before leaving nucleus
• Primary transcript mRNA  mRNA
– Translation in cytoplasm
Central Dogma
DNA  mRNA  protein
(F. Crick 1956)
The genetic code
Triplet code
43 =
– # combinations of 3 using G, A, T, C
– Can code for all 20 aa
• Each triplet codes for a certain amino acid
Codon table (1960’s Nirenberg and Khorana)
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3 stop codons
1 start codon
Redundant
Nearly universal
Which strand of DNA is the template?
Identify the codons (triplets)
Code is non-overlapping
Compare the mRNA to the non-template strand
How many codons are needed for a 20 amino acid
long protein?
How many nucleotides?
The reading frame
Amino acids?
AUG UGG GUU GGC UCA
met –trp- phe-gly-ser
Transcription in more detail
1. Initiation of transcription
A. RNA polymerase binds to template strand of DNA
 upstream ----------------------------------------------downstream 
The promoter is upstream from
the start site for transcription
B. Transcription factors (proteins) are required
for RNA pol to bind
C. mRNA begins to be synthesized
(RNA pol does not need a primer)
2. Elongation of
transcript
– RNA pol synthesizes
5’  3’
– More than one
transcript made
3. Termination
RNA pol detaches
Translation summary
Translation in more detail
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 Cytoplasm
The players:
Transfer RNA tRNA
Ribosomes
mRNA
Amino acids
1. tRNA
2o structure is a cloverleaf
CONCEPT CHECK
How many bases is a tRNA?
How many bases is an anticodon?
Where is the amino acid site with respect to the anticodon?
Why does the tRNA look to be double stranded?
What does the “t” mean in tRNA?
• A charged tRNA
carries an amino acid
• Contains an anticodon
– Complementary to a
codon
Note the CCA
2. Ribosome
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rRNA + protein
Large and small subunits
3 binding sites on ribosome
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P site holds tRNA
A site for tRNA with next amino acid
E site allows tRNA to leave
Note: large and small
subunits
Ribosomes
adds each aa from tRNA to
growing polypeptide chain
up to 100,000s per cell
Activity overview of
protein synthesis
Stages of translation
1. Initiation
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Ribosome scans
mRNA for AUG
start
tRNA brings met
2. Elongation
Amino acids added to chain
Peptide bonds between amino acids
3. Termination
stop codon
Polypeptide released
• Polyribosomes
• One mRNA
translated by
many ribosomes
Wobble hypothesis
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How many sense codons?
Stop codons?
BUT……………only 45 tRNAs
Some tRNAs recognize more than one codon
3rd position in mRNA codon “wobbles”
Us and Cs can be matched with G in anticodon
Rules of base pairing relaxed in the 3rd position
After translation  ER  Golgi
Post – translational modification