1. Translation
Unit 5B.4

2. DNA mRNA protein trait From gene to protein nucleus cytoplasmaa
From gene to protein
nucleus
cytoplasm
transcription
translation
DNA
mRNA
protein
ribosome
trait

3. from nucleic acid language to amino acid language
Translation
from nucleic acid language to amino acid language

4. How does mRNA code for proteins?
TACGCACATTTACGTACGCGG
DNA 4
ATCG
AUGCGUGUAAAUGCAUGCGCC
mRNA 4
AUCG ?
Met Arg Val Asn Ala Cys Ala
protein 20
How can you code for 20 amino acids with only 4 nucleotide bases (A,U,G,C)?

5. mRNA codes for proteins in triplets
TACGCACATTTACGTACGCGG
DNA
codon
AUGCGUGUAAAUGCAUGCGCC
mRNA
AUGCGUGUAAAUGCAUGCGCC
mRNA ?
Met Arg Val Asn Ala Cys Ala
protein

6. WHYDIDTHEREDBATEATTHEFATRAT WHYDIDTHEREDBATEATTHEFATRAT
1960 | 1968
Cracking the code
Nirenberg & Khorana
Crick
determined 3-letter (triplet) codon system
WHYDIDTHEREDBATEATTHEFATRAT
WHYDIDTHEREDBATEATTHEFATRAT
Nirenberg (47) & Khorana (17)
determined mRNA–amino acid match
added fabricated mRNA to test tube of ribosomes, tRNA & amino acids
created artificial UUUUU… mRNA
found that UUU coded for phenylalanine

7. The code Code for ALL life! Code is redundant Start codon Stop codons
strongest support for a common origin for all life
Code is redundant
several codons for each amino acid
3rd base “wobble”
Why is the wobble good?
Strong evidence for a single origin in evolutionary theory.
Start codon
AUG
methionine
Stop codons
UGA, UAA, UAG

8. How are the codons matched to amino acids?3 5
DNA
TACGCACATTTACGTACGCGG 5 3
mRNA
AUGCGUGUAAAUGCAUGCGCC
codon 3 5
UAC
Met
GCA
Arg
tRNA
CAU
Val
anti-codon
amino acid

9. DNA mRNA protein trait From gene to protein nucleus cytoplasmaa
From gene to protein
nucleus
cytoplasm
transcription
translation
DNA
mRNA
protein
ribosome
trait

10. Transfer RNA structure
“Clover leaf” structure
anticodon on “clover leaf” end
amino acid attached on 3 end

11. tryptophan attached to tRNATrp tRNATrp binds to UGG condon of mRNA
Loading tRNA
Aminoacyl tRNA synthetase
enzyme which bonds amino acid to tRNA
bond requires energy
ATP  AMP
bond is unstable
so it can release amino acid at ribosome easily
The tRNA-amino acid bond is unstable. This makes it easy for the tRNA to later give up the amino acid to a growing polypeptide chain in a ribosome.
Trp
C=O
Trp
Trp
C=O OH
H2O OH O
C=O O
activating
enzyme
tRNATrp A C C U G G
mRNA
anticodon
tryptophan attached to tRNATrp
tRNATrp binds to UGG condon of mRNA

12. Protein synthesis/quiz
Ribosomes
Facilitate coupling of tRNA anticodon to mRNA codon
organelle or enzyme?
Structure
ribosomal RNA (rRNA) & proteins
2 subunits
large
small E P A

13. Ribosomes A site (aminoacyl-tRNA site) P site (peptidyl-tRNA site)
Protein synthesis 2
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
E site (exit site)
empty tRNA leaves ribosome from exit site
Met U A C 5' A U G 3' E P A

14. Building a polypeptide1 2 3
How translation works
Building a polypeptide
Initiation
brings together mRNA, ribosome subunits, initiator tRNA
Elongation
adding amino acids based on codon sequence
Termination
end codon
Leu
Val
release
factor
Ser
Met
Met
Met
Met
Leu
Leu
Leu
Ala
Trp
tRNA C A G C U A C 5' U A C G A C U A C G A C G A C 5' A 5' U A A U G C U G A U A U G C U G A A U A U G C U G A A U 5' A A U
mRNA A U G C U G 3' 3' 3' 3' A C C U G G U A A E P A 3'

15. start of a secretory pathway
Destinations:
secretion
nucleus
mitochondria
chloroplasts
cell membrane
cytoplasm
etc…
Protein targeting
Signal peptide
address label
start of a secretory pathway

16. Can you tell the story? RNA polymerase DNA amino acids tRNA pre-mRNA
exon
intron
tRNA
pre-mRNA
5' GTP cap
mature mRNA
aminoacyl tRNA synthetase
poly-A tail 3'
large ribosomal subunit
polypeptide 5'
tRNA
small ribosomal subunit E P A
ribosome

17. The Transcriptional unit (gene?)
enhancer
1000+b
translation
start
translation
stop
exons
20-30b
transcriptional unit (gene)
RNA
polymerase 3'
TAC
ACT 5'
TATA
DNA
transcription
start
UTR
introns
transcription
stop
UTR
promoter
DNA
pre-mRNA 5' 3'
mature mRNA 5' 3'
GTP
AAAAAAAA

18. Protein Synthesis in Prokaryotes
Bacterial chromosome
Protein Synthesis in Prokaryotes
Transcription
mRNA
Psssst… no nucleus!
Cell
membrane
Cell wall

19. Prokaryote vs. Eukaryote genes
Prokaryotes
DNA in cytoplasm
circular chromosome
naked DNA
no introns
Eukaryotes
DNA in nucleus
linear chromosomes
DNA wound on histone proteins
introns vs. exons
Walter Gilbert hypothesis:
Maybe exons are functional units and introns make it easier for them to recombine, so as to produce new proteins with new properties through new combinations of domains.
Introns give a large area for cutting genes and joining together the pieces without damaging the coding region of the gene…. patching genes together does not have to be so precise.
introns come out!
intron = noncoding (inbetween) sequence
eukaryotic
DNA
exon = coding (expressed) sequence

20. Translation in Prokaryotes
Transcription & translation are simultaneous (coupled) in bacteria
DNA is in cytoplasm
no mRNA editing
ribosomes read mRNA as it is being transcribed

21. Translation: prokaryotes vs. eukaryotes
SEE PROCESSING VIDEO
Translation: prokaryotes vs. eukaryotes
Differences between prokaryotes & eukaryotes
time & physical separation between processes
takes eukaryote ~1 hour from DNA to protein
no RNA processing

22. COMPLETING PROTEINS POLYRIBOSOMES (POLYSOMES)
Numerous ribosomes translate same mRNA at same time
3-D folding (1’, 2’, 3’ structure)
Chaparonins