Translation
from
nucleic acid language
to
amino acid language
Draw 7 boxes
on your paper
Bacterial chromosome
Translation in
Prokaryotes
Transcription
mRNA
Translation
Psssst…
no nucleus!
protein
Cell
membrane
Cell wall
2007-2008
Translation in Prokaryotes
• Transcription & translation are
simultaneous in bacteria
• DNA is in cytoplasm
• no mRNA editing
• ribosomes read mRNA
as it is being transcribed
Translation: prokaryotes vs. eukaryotes
• Differences between prokaryotes & eukaryotes
• time & physical separation between processes
• takes eukaryote ~1 hour
from DNA to protein
• RNA processing
Box 1: What’s the difference between translation
in prokaryotes and eukaryotes?
Box 2: what’s the same between translation in
prokaryotes and eukaryotes?
Translation in Eukaryotes
2007-2008
From gene to protein
transcription
DNA
translation
mRNA
mRNA leaves
nucleus through
nuclear pores
nucleus
a
a
a
a
a
a
a
a
a
a
protein
a
a
ribosome
a
a
a
a
proteins synthesized
by ribosomes using
instructions on mRNA
cytoplasm
Box 3
•What are 3 differences between
transcription and translation?
How does mRNA code for proteins?
DNA
TACGCACATTTACGTACGCGG
4 ATCG
mRNA
4 AUCG
protein
AUGCGUGUAAAUGCAUGCGCC
?
Met Arg Val Asn Ala Cys Ala
20
How can you code for 20 amino acids with only
4 nucleotide bases (A,U,G,C)?
mRNA codes for proteins in triplets
DNA
TACGCACATTTACGTACGCGG
codon
mRNA
AUGCGUGUAAAUGCAUGCGCC
?
protein
Met Arg Val Asn Ala Cys Ala
Cracking the code
• Crick
• determined 3-letter (triplet) codon
system
WHYDIDTHEREDBATEATTHEFATRAT
Translation
• Codons
• blocks of 3 nucleotides
coded into the
sequence of amino
acids
Genetic information is encoded as a
sequence of nonoverlapping base
triplets, or codons.
In box 4, draw a codon and its
relationship to an amino acid
The code
• Code for ALL life!
• The genetic code is nearly
universal, shared by organisms
from the simplest bacteria to the
most complex animals
• strongest support for a common
origin for all life
Why is the
wobble good?
• Code is redundant
Start codon
• several codons for each amino
acid
AUG
• 3rd base “wobble”
methionine



Stop codons

UGA, UAA, UAG
Many amino acids have more than one
codon (redundancy).
Box 5
•What does it mean that the code is
“redundant”?
Codons must be read in the correct
reading frame for the specified
polypeptide to be produced.
How are the codons matched to amino acids?
DNA
mRNA
3
5
5
3
TACGCACATTTACGTACGCGG
AUGCGUGUAAAUGCAUGCGCC
codon
3
5
tRNA
UAC
amino
acid
Met
GCA
Arg
CAU
Val
anti-codon
Transfer RNA structure
• “Clover leaf” structure
• anticodon on “clover leaf” end
• amino acid attached on 3 end
Ribosomes
• Facilitate coupling of
tRNA anticodon to
mRNA codon
• organelle or enzyme?
• Structure
• ribosomal RNA (rRNA) &
proteins
• 2 subunits
• large
• small
E P A
Ribosomes
• A site (aminoacyl-tRNA site)
• holds tRNA carrying next amino acid to
be added to chain
• P site (peptidyl-tRNA site)
• holds tRNA carrying growing polypeptide
chain
Met
• E site (exit site)
• empty tRNA
leaves ribosome
from exit site
U A C
A U G
5'
E
P
A
3'
Box 6
•Describe the structure and 3 sites of the
ribosome.
Initiation: Initiation involves the small
subunit of the ribosome binding to the
5' end of mRNA. The first tRNA enters
the ribosome at the P site to initiate
translation at the start codon.
Another tRNA brings the next amino
acid into the A-site of the ribosome.
A peptide bond forms between the
first two amino acids. The tRNA is then
moved from the A-site to P-site and
the ribosome moves over one codon.
The first tRNA is released from the E
site.
The process continues along the mRNA
until a “stop” codon is reached.
Building a polypeptide
• Initiation
• brings together mRNA, ribosome
subunits, initiator tRNA
• Elongation
• adding amino acids based on codon
sequence
• Termination
3 2 1
• end codon
Leu
Val
Met
Met
Met
Met Leu
Ala
Leu
Leu
release
factor
Ser
Trp
tRNA
U AC
5'
CU GA A U
mRNA A U G
3'
E P A
5'
UAC GAC
A U G C U GAA U
5'
3'
U A C GA C
A U G C U G AAU
5'
3'
U AC G A C
AA U
AU G C U G
3'
A CC
U GG U A A
3'
•Tell the person next to you
the process of translation.
From gene to protein
transcription
DNA
translation
mRNA
a
a
ribosome
a
a
a
a
a
a
a
a
a
a
protein
a
a
a
a
aa
nucleus
cytoplasm
Protein targeting
• Signal peptide
• address label
Destinations:






start of a secretory pathway

secretion
nucleus
mitochondria
chloroplasts
cell membrane
cytoplasm
etc…
RNA polymerase
DNA
Can you tell
the story?
amino
acids
exon
intron
tRNA
pre-mRNA
5' cap
mature mRNA
aminoacyl tRNA
synthetase
polyA tail
large ribosomal subunit
polypeptide
5'
small ribosomal subunit
tRNA
E P A
ribosome
3'
Phenotypes are determined through
protein activities.
Point mutation leads to Sickle cell anemia
What kind of mutation?
Missense!
Sickle cell anemia
• Primarily Africans
• recessive inheritance pattern
• strikes 1 out of 400 African Americans
Phenylketonuria (PKU) is an autosomal
recessive genetic disorder caused by a mutation in
the gene for an enzyme that is necessary to
metabolize the amino acid phenylalanine. Untreated
PKU can lead to mental retardation, seizures, and
other serious medical problems. Treatment for PKU is
a PHE-restricted diet.
Cystic fibrosis
• Primarily people of
European descent
• strikes 1 in 2500 births
• 1 in 25 whites is a carrier (Aa)
• normal allele codes for a membrane protein
that transports Cl- across cell membrane
• defective or absent channels limit transport of Cl- (& H2O) across
cell membrane
• thicker & stickier mucus coats around cells
• mucus build-up in the pancreas, lungs, digestive tract & causes
bacterial infections
• without treatment children die before 5;
with treatment can live past their late 20s
Chloride channel
Effect on Lungs
normal lungs
airway
Cl-
transports chloride through
protein channel out of cell
Osmotic effects: H2O follows ClCl- channel
H 2O
cells lining
lungs
cystic fibrosis
ClH 2O
bacteria & mucus build up
thickened mucus
hard to secrete
mucus secreting glands
Box 7
How does protein affect phenotype?