1. Protein Synthesis

2. Making proteins from genes
One gene-one enzyme hypothesis (Beadle and Tatum)
Later revised to the one gene-one polypeptide hypothesis
Transcription
synthesis of RNA under the direction of DNA (mRNA)
Translation
synthesis of a polypeptide under the direction of mRNA

3. DNARNAProtein ‘U’ (uracil) replaces ‘T’ in RNA
The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’
Triplet code/codons
More than one triplet for each Amino Acid

4. The Genetic Code

5. Initiation of Transcription
Transcription factors attach to TATA box
Contains sequences of only adenine and thymine
RNA polymerase attaches to begin transcription

6. Elongation and Termination
RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end
Termination
RNA polymerase reaches terminator sequence
Polyadenylation signal--AAUAAA

7. Transcription Helpers
RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code
Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins
Terminator region: sequence that signals the end of transcription
Transcription unit: stretch of DNA transcribed into an RNA molecule

8. mRNA modification 5’ cap: modified guanine molecule Protection
recognition site for ribosomes
3’ tail: poly (A) tail (up to 250 adenine nucleotides)
Recognition
transport

9. RNA Splicing Exons (expressed sequences) kept
Introns (intervening sequences) spliced out
Splicing done by snRNPs which bind together to form a large compound known as a spliceosome
Made of snRNA
RNA that acts as an enzyme= ribozyme

10. Basics of Translation mRNA moved through ribosome
“code” read in triplets
tRNA transfers correct amino acid to the polypeptide chain

11. tRNA Has an anticodon (nucleotide triplet)
Codes for a specific amino acid

12. Picking the right Amino Acid
Aminoacyl-tRNA synthetase
Joins a specific amino acid to its tRNA molecule
Wobble
Relaxation in the base-pairing rules
Third nucleotide in a codon can be different in many cases and still code for the same amino acid

13. Ribosomes rRNA (ribosomal RNA) P site A site E site
site of mRNA codon & tRNA anticodon coupling
P site
holds the tRNA carrying the growing polypeptide chain
A site
holds the tRNA carrying the next amino acid to be added to the chain
E site
discharged tRNA’s leave the ribosome

14. Initiation of Translation
Union of mRNA, tRNA, and small ribosomal subunit
Start codon = AUG (codes for methionine)
Large subunit then attaches

15. Elongation Codon recognition Peptide bond formation Translocation
Anticodon of tRNA binds with the complementary mRNA codon in the A site
Peptide bond formation
Large subunit forms a peptide bond between the amino acid in the A site and the growing polypeptide
Translocation
tRNA in A site moves to P site… the one in the P site moves to the E site

16. Termination
When a stop codon (UAG, UAA, or UGA) is reached, the A site accepts a “release factor”
Release factor cuts the polypeptide free from the tRNA molecule
Ribosome dissociates

18. Polyribosomes Translation of mRNA by many ribosomes
many copies of a polypeptide very quickly

19. Where does it happen? Protein synthesis always begins in the cytosol
Will continue there unless it receives a signal peptide telling it to bind to the ER
SRP’s (signal-recognition particles)
Binds to a ribosome and brings it to the membrane of the ER
Attaches to a receptor molecule

20. Roles of RNA

21. Mutations Point mutation- The change of just one nucleotide
Base-pair substitution
One nucleotide substituted for another
Sometimes a “silent mutation” if it still codes for the same amino acid
Missense mutation- still codes for an amino acid, but not the right one
Nonsense mutation- changes to a stop codon and terminates translation

22. Frame-shift Mutations
Insertion
Nucleotide is added
Deletion
Nucleotide is lost
Causes the codons from that point on to be incorrectly grouped
Mutagen- anything that causes a mutation (chemicals, X-rays, carcinogens, etc.)