1. Transcription and Translation
Chapter 14

2. Central Dogma DNA mRNA Protein DNA DNA – Replication – nucleus
DNA mRNA – Transcription – Nucleus
mRNA Protein – Translation – Ribosome in the cytoplasm

3. Central Dogma

4. RNA vs. DNA

5. DNA vs. RNA DNA RNA Nucleus only Double Stranded ATCG
Deoxyribose Sugar
Nucleus and cytoplasm (ribosome)
Single Stranded
AUCG
Ribose sugar
3 main types
mRNA - messenger
tRNA - transfer
rRNA - ribosomal

6. Triplet Code: Codons Universal code: Strong evidence for evolution
Codon Chart pg. 273
3 Nucleotides = 1 amino acid

7. Transcription 14_08TranscriptionIntro_A.swf
DNA  mRNA
Copying of DNA’s message to mRNA
Occurs in the nucleus
Pre-mRNA is processed into mRNA and then leaves the nucleus for the cytoplasm (ribosome)

8. Detailed Transcription
Initiation: RNA polymerase binds to DNA at promoter region.
Promoter is before the gene that is to be transcribed (eukaryotes need transcription factors to help RNA polymerase bind to the TATA box)
Determines which strand of DNA to use
Elongation - adds nucleotides to mRNA strand based on DNA strand in a 5’ 3’ direction (adding only to the 3’ end).
Termination – RNA polymerase “falls off” the DNA strand when the termination sequence (terminator) is reached.
AAUAAA in eukaryotes, this is now pre-mRNA

9. Transcription Transcription Initiation Eukaryotes
Transcription Process
Transcription Initiation Eukaryotes

10. RNA Processing Pre-mRNA RNA
5’ cap – guanine and phosphate cap on the 5’ end of mRNA
3’ poly-A tail – 50 to 250 Adenines are added to the 3’ end of the mRNA
Both the 5’ cap and 3’ poly-A tail facilitate the export of mRNA from the nucleus
Both protect the mRNA from degradation by hydrolytic enzymes in the cytoplasm
Both help ribosomes attach to the 5’ ends of the mRNA strand

11. RNA processing

12. Alternative RNA splicing
Introns – non-coding sections of mRNA
Don’t leave the nucleus – only on pre-mRNA
Exons – coding sections (expressed) of mRNA
Exit the nucleus – final mRNA
Keeps the exons  gets rid of the introns
snRNPs – cut the introns
Ribozyme – RNA that functions like an enzyme
Spliceosomes – join remaining exons together to form final mRNA

14. Translation 14_18TranslationIntro_A.swf
mRNA  protein
Process of mRNA converting to a protein
Occurs in the cytoplasm – ribosome

15. tRNA Translator of mRNA’s message is tRNA – transfer RNA
80 nucleotides long
Hairpin shape – L shaped
One end contains an anticodon which pairs with the codon on the mRNA
Codons determine which amino acid is coded for by the DNA
The other end contains an amino acid attachment site
Aminoacyl-tRNA synthetase attaches the correct amino acid to the tRNA

16. tRNA

17. Ribosomes
Pair codons on mRNA with anticodons on tRNA to form polypeptides
Made of large and small subunits
rRNA – ribosomal RNA
Made in the nucleolus
Contain multiple binding sites
mRNA binding site
P site – peptidyl – tRNA site
A site – aminoacyl – tRNA site
E site – exit site

18. Ribosomes

19. Making a protein Initiation Small subunit binds to mRNA
Start codon AUG – methionine at P site
Elongation
A site recognizes codon and pairs with correct tRNA
Peptide bond forms between the carboxyl end of the polypeptide at the P site and amino acid at the A site
Amino acid in the A site translocates to the P site
Termination
Stop codon is reached at the A site
UAA, UAG, UGA
Release factors free the polypeptide from the ribosome

20. Making a Protein ProteinSynthesis.mpg
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Amino
acids
tRNA with
amino acid
attached
tRNA
Anticodon
Trp
Phe
Gly A G C U
Codons 5 3
Figure 17.13

21. Proteins
Fold spontaneously into primary, secondary, and tertiary structures.
Chaperone proteins assist in folding.
Some polypeptides become quaternary with multiple subunits
Signal peptide – directs proteins through the endomembrane system

22. Targeting Polypeptides to locations

23. Prokaryotes vs. Eukaryotes
Locations:
Free cytosol Ribosomes: Stay in the cell somewhere (free floating, mitochondria, etc)
Rough ER Ribosomes: SRP sends ribosome to go to ER, becomes protein in endomembrane system, or cell membrane protein or is secreted from cell (all in vesicles)
Nucleus
Pre-mRNA  mRNA
transcription
Larger ribosomes
translation
No nucleus
Tanscription and translation same location
Smaller ribosomes

24. Mutations Point Mutations – one base altered Base-pair substitution
Silent mutation – no effect
Missense mutation – changes an amino acid
Nonsense mutation – creates a stop codon
Insertion – extra base
Deletion – removal of a base
Frameshift mutations – nonfunctional proteins