Gene to Protein
Genes to Proteins
 “Inborn Errors of Metabolism” – diseases that reflect a person’s inability to make a particular enzyme
 Garrod’s hypothesis - genes dictate phenotypes through enzymes that catalyze specific chemical reactions
 Beadle and Tatum’s hypothesis – one gene-one polypeptide
Transcription
 Transcription – synthesis of RNA (specifically mRNA) under the direction of the DNA
 Location
o Prokaryotes: cytoplasm
o Eukaryotes: nucleus
 Transcription unit – DNA that is transcribed into an RNA molecule
Transcription Step
Initiation
Prokaryotes
 Promoter – DNA sequence where
RNA polymerase attaches and
initiates transcription
 RNA polymerase binds to
promoter
Elongation
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Termination
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RNA polymerase reads DNA in
nucleotide triplets called codons
RNA polymerase adds nucleotides
to the 3’ end of the growing
mRNA strand
Terminator – DNA sequence that
signals the end of transcription by
detaching the polymerase and
mRNA
Eukaryotes
 RNA polymerase binds to
transcription factors and promoter
complex to form transcription
initiation complex
 TATA box – nucleotide sequence
containing only TATA about 25
nucleotides upstream from
starting point
 RNA polymerase reads DNA in
nucleotide triplets called codons
 RNA polymerase adds nucleotides
to the 3’ end of the growing premRNA strand
 Some eukaryotes - multiple RNA
polymerases can attach to one
gene sequence transcribing one
sequence one after the other
 Polymerase transcribes a
polyadenylation sequence
consisting of only AAUAA after
which proteins cut the pre-mRNA
from the polymerase
RNA Modification (eukaryotes)
 5’ cap
o 5’ end of mRNA is capped by a modified
guanine nucleotide
o Protects the mRNA
o Helps with ribosomal attachment
 Poly (A) Tail
o 50-250 adenine nucleotides are added to 3’
end of mRNA
o Helps the mRNA exit the nucleus
o Stabilizes mRNA
o Protects the mRNA
o The longer the tail, the longer the shelf-life
of the mRNA
 RNA splicing
o Pre-mRNA composed of introns and exons
 Introns – noncoding segments of
DNA
 Exons – segments of DNA that code
for proteins
o Small nuclear ribonucleoproteins (snRNP),
ribozymes and other proteins form a
splicesome to remove introns and connect
exons for translation
o Modified pre-mRNA is now mRNA
 Alternative RNA splicing
o One gene gives rise to multiple proteins
o Different exons are spliced out to create
different proteins
Translation
 tRNA
o Transfers amino acids to the ribosome
o Anticodon – nucleotide triplet on a tRNA molecule
 Wobble – third nucleotide can vary but still
code for the same amino acid
 Ex) GGU, GGC, GGA all code for glycine
o Made in the nucleus
o Aminoacyl-tRNA synthetase – joins a specific
amino acid to tRNA using ATP

Ribosomes
o Two subunits (large and small subunits) made in the nucleolus (in eukaryotes)
o Made up of proteins and rRNA
o A site (aminoacyl-tRNA binding site) – holds the tRNA carrying the next amino acid to be added to the
polypeptide chain
o P site (peptidyl-tRNA binding site) – holds the tRNA attached to the growing polypeptide
o E site (exit site) – discharged tRNA leaves this site
Translation Step
Initiation
Description
 Small ribosomal unit binds to
mRNA and the initiator tRNA
(containing met) at the AUG
codon
 Initiation factors help attach
the large ribosomal unit to the
mRNA, initiator tRNA and
small ribosomal unit (uses
GTP energy)
Elongation
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Termination
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Picture
(Codon Recognition)
Anticodon of incoming tRNA
pairs with the codon on the
mRNA
2 GTP are hydrolyzed to
increase accuracy
(Peptide Bond Formation)
The polypeptide is detached
from the tRNA in the P site
and is attached to the amino
acid in the A site
(Translocation)
The tRNA in the A site is
moved to the P site while the
tRNA that was in the P site is
moved to the E site where it
exits the ribosome
1 GTP is required to move the
tRNA to different sites
When a stop codon reaches
the A site, a release factor
attaches
Release factor hydrolyzes the
bond between the tRNA and
the peptide to separate them
The ribosomal units and other
parts dissociate
Polyribosomes
o Multiple ribosomes on one mRNA molecule
o Enable a cell to make many copies of a polypeptide very quickly
o Occurs in both prokaryotes and eukaryotes
Post-translational Modifications
 Attachment of sugars, lipids, phosphate groups, etc.
 Signal peptides
o Sequence of ~20 amino acids at the beginning of the polypeptide
o Signal-recognition particle (SRP) recognizes the signal peptide and attaches to it
o SRP brings the ribosome to the membrane and binds with the SRP receptor protein to create a
translocation complex
o SRP leaves while the polypeptide is translated across the membrane
o An enzyme cuts the signal peptide off
o The ribosome is detached from the membrane
Mutations
 Mutation – any change in the nucleotide sequence of an organism’s DNA
 Point mutations
o Change in one base pair in a gene (affects ~1 nucleotide of the chromosome)
o Substitution – replacement of one nucleotide for another
o Insertion – addition of nucleotides
o Deletion – removal of nucleotides
o Frame-shift – the number of nucleotides added/deleted is not in a multiple of three
o Missense – altered codon still codes for an amino acid
o Nonsense – altered codon codes for a stop signal
 Chromosomal mutations
o Changes that affect multiple genes (covers a large area of the chromosome)
o Inversion – a section of a chromosome flips the order of genes
o Translocation – a section of one chromosome separates and attaches to a non-homologous chromosome
o Duplication – a section of one chromosome repeats
o Deletion – a section of a chromosome is removed
 Mutagens – physical and chemical agents that interact with DNA to cause mutations (x-rays, ultraviolet light, etc.)
Prokaryotic vs. Eukaryotic Protein Synthesis
Prokaryotes
Eukaryotes
Transcription and translation occurs in cytoplasm
Transcription occurs in nucleus and translation occurs in
cytoplasm
Transcription and translation can be simultaneous
Translation occurs after transcription
No mRNA processing
mRNA processing
No transcription factors
Transcription factors
Polyribosomes