1. Chapter 15 – The Genetic Code and Translation

2. Beadle and Tatum Neurospora crassa
Haploid orange bread mold
Haploid?
Prototrophs – grow on minimal media; auxotrophs can only grow on media that has been supplemented with substance that it can’t synthesize
Irradiated spores to induce mutations

3. Beadle and Tatum
Irradiated spores first grown on complete media, then transferred to minimal media
Showed strains that had acquired mutations
Series of tubes with minimal media PLUS one additional ingredient
If strain grew, indicated mutation affected production of that substance

5. Sib and Horowitz Biochemical pathway for arginine production
I – grew in presence of ornithine, citrulline, arginine
II – grew in presence of citrulline and arginine
III – grew in presence of arginine
Which type codes for which enzyme?

6. One gene/One enzyme Each gene codes for one enzyme
Many proteins are composed of multiple polypeptides, each coded for by a different gene
One gene/one polypeptide

7. Protein Structure Polymer of 20 amino acids Amino acid structure
Central carbon atom
Hydrogen atom
Amino group
Carboxyl group
R/radical group
What defines one amino acid
Each R group has different sizes/properties

11. Amino acid linkage
Two amino acids covalently linked together by process of dehydration synthesis
Formed bond = peptide bond
Long chain called a polypeptide

12. Levels of Protein Structure
Primary structure
Amino acid sequence
Secondary structure
Hydrogen bonds form alpha helices or beta sheets

13. Levels of Protein Structure
Tertiary structure
Interactions between R groups
Quaternary structure
Only present in proteins composed of multiple polypeptide chains
How polypeptide subunits are associated to make a complete protein

14. Genetic code Nucleotide sequence must code for specific amino acids
Francis Crick – 3 nucleotides code for one amino acid
codon
64 codons
61 code for an amino acid
“degenerate”
More than one codon can code for the same amino acid

16. Genetic code tRNA Wobble 30-50 tRNA for 20 amino acids
Isoaccepting tRNA have different anticodons but still carry the same amino acid
Wobble
1st nucleotide in anticodon pairs with the 3rd codon of mRNA
Flexibility in bonding allows one tRNA to recognize more than one codon
Still codes for same amino acid

18. Reading frame Determined by the START/initiation codon
AUG – also codes for methionine
No overlapping or skipping of bases
Termination/stop codons
Also called nonsense codons
Universality
With rare exception, genetic code is read the same by all organisms

19. tRNA charging Attachment of appropriate amino acid
Aminoacyl-tRNA synthetase (20 different)
Recognize specific sequences in certain regions of tRNA, and binds the appropriate amino acid to 3′ acceptor arm of tRNA
Forms aminoacyl-tRNA

20. Translation Occurs at ribosomes
Attaches to 5′ end of mRNA and moves toward 3′ end
Binding determined by Shine-Dalgarno sequence in prokaryotic cells/ modifications in eukaryotic cells

21. Initiation of translation
Ribosome has two subunits – small (30S) and large (50S)
Complete ribosome (70S)
Initiation factors bind to small unit, prohibiting small unit from binding with large subunit
Now free to bind to mRNA
Aminoacyl-tRNAmet attaches to initiation/ start codon
Large ribosomal subunit attaches

23. Ribosome Has three sites for tRNA
A (aminoacyl) site
P (peptidyl) site
E (exit) site
Initiator tRNAmet enters P site; all other tRNA first enter the A site
A→P→E

24. Elongation Next codon is exposed in the A site
Charged tRNA enters A
Peptide bond forms between the P site amino acid and A site amino acid
Peptidyl transferase
Translocation
ribosome moves down mRNA molecule
P site tRNA enters E site
A site tRNA with growing polypeptide enters P site
A site now has next codon exposed; ready for next tRNA
During next shift, E site tRNA is released into cytoplasm to be re-charged with another specific amino acid

26. Termination A STOP codon enters A site Release factors enter A site
No appropriate tRNA
Release factors enter A site
Ribosome subunits dissociate
Polypeptide is released from last tRNA

27. Post-translation modifications
Methionine cleaved off; chain possibly cleaved
Carbohydrates attached (forms glycoproteins)
Folding into proper 3D shape
Aided by chaperone proteins