1. Protein Synthesis
AP Biology
Ch. 17

2. “One Gene-One Polypeptide” Theory
George Beadle and Edward Tatum (late 40’s to early 50’s)
used X-rays to induce mutations in Neurospora crassa (bread mold)
Auxotrophs—nutritional mutants unable to synthesize certain amino acid and vitamins. They traced the defect to the enzymes involved in their synthesis.
Conclusion: One gene encodes one enzyme.
Gene products encode both protein and RNA

3. Central Dogma of Molecular Biology
In eukaryotic cells, a message (mRNA) carries the genetic information (DNA) from the nucleus to the cytoplasm  Transcription
An adapter (tRNA) translates the nucleic acid (mRNA) into amino acid (protein)  Translation
Information flow is unidirectional
GCTGCTAACGTCAGCTAGCTCGTAGCGCTAGCGCTTGCGTAGCTAAAGTCGAGCTCGCTTGCGTAGCTAAAGTCGAGCTGCTGCTAACGTCAGCTAGCTCGTAG
RNA
Proteins

5. Structure of RNA
Single strand of nucleotides [instead of 2 strands in DNA]
Nucleotides contain 5-carbon sugar ribose [instead of deoxyribose in DNA]
4 different nitrogenous bases
Adenine (A)
Uracil (U) [instead of thymine in DNA]
Cytosine (C)
Guanine (G)

6. Forms of RNA 3 Main Types of RNA
mRNA (messenger RNA) brings DNA message out of nucleus to the cytoplasm
Each 3 bases on mRNA is a “codon”
tRNA (transfer RNA) –The anticodon that matches with the codon from mRNA to determine which amino acid joins the protein chain
rRNA (ribosomal RNA) – make up the ribosomes—RNA that lines up tRNA molecules with mRNA molecules

9. Transcription: Synthesis and Processing of RNA
RNA Polymerase is an enzyme that synthesizes RNA using one strand of the DNA as a template
Separates 2 strands of DNA and links RNA nucleotides as they base-pair along the DNA template in an overall is 5’ to 3’ direction

10. Transcription: Initiation
RNA polymerase attaches to promotor sequence (TATA box) of DNA sequence
Prokaryotes—enzyme binds directly to sequence
Eukaryotes—enzyme binds after transcription factors bind to the site

11. Transcription: Elongation
RNA polymerase
Synthesizes in 5’ 3’ direction, anti-parallel to DNA template
Unwinds about 20 base pairs as it reads and synthesizes complementary RNA

12. Transcription: Termination
RNA polymerase terminates transcription at special DNA sequences, terminator
Prokaryote—stops right at the end of termination signal
Eukaryote—continues past termination signal, to a poly(A) tail (AAUAAA) in the pre-mRNA

13. Transcription The code on DNA tells how mRNA is put together.
Example: DNAACCGTAACG
mRNAUGGCAUUGC
Each set of 3 bases is called a triplet or codon
(in mRNA)
UGG CAU UGC

14. RNA Splicing Pre-mRNA never leaves the nucleus
Introns—non-coding regions
Exons—coding regions
snRNPs join with other proteins to form spliceosome, which releases introns

16. Translation Genetic information in mRNA is read by codons
64 different codons
Genetic code is redundant—more than one codon codes for many of the 20 amino acids
AUG—universal start codon
UAA, UAG, UGA—stop codons

17. Structure of tRNA

18. Aminoacyl-tRNA Formation

19. Structure of a Ribosome

20. Translation: Initiation
Small ribosomal subunit binds to start codon (AUG) on mRNA
Initiator tRNA (UAC) base-pairs with mRNA
Large ribosomal unit binds so initiator tRNA is in the P site
GTP provides energy for intiation process

21. Translation: Elongation

22. Translation: Termination
Review entire
process

24. Polyribosomes

25. Coupled Transcription and Translation in Bacteria

26. Point Mutations

27. Substitution Mutations
Missense mutation: altered codon still codes for an amino acid, although maybe not the right one
Nonsense mutation: altered codon is a stop codon and translation is terminated prematurely
Leads to nonfunctional proteins

28. Insertions and Deletions
Frameshift mutation: addition or loss of one or more nucleotide pairs in a gene shifts the reading frame for translation and incorrect protein is made

29. The end

30. 5’ 3’ Ribosome U U C U G G A U G U G G U U C U G G mRNA A U C C A
Reading direction
mRNA A U C
Met C A
Trp
Incoming tRNA charged with amino acid A U C
Met

31. 5’ 3’ Ribosome mRNA U U C U G G A U G U G G U U C U G G A C A G U
Reading direction
mRNA C A
Trp U
Outgoing “empty” tRNA
Met A G
Phe

32. 5’ 3’ Ribosome mRNA U U C U G G A U G U G G U U C U G G G A C A C U
Reading direction
mRNA C A
Trp
Met G
Phe U C A
Trp

33. 5’ 3’ Ribosome mRNA U U C U G G A U G U G G U U C U G G A C G U
Reading direction
mRNA C A
Trp
Phe G U
Met

34. 5’ 3’ Ribosome mRNA U U C U G G A U G U G G U U C U G G U A C
Incoming tRNA charged with amino acid
Ribosome 5’ 3’
U U C U G G A U G U G G U U C U G G
Reading direction
mRNA A C
Trp
Anticodon G
Phe
Outgoing “empty” tRNA U
Met
Amino acid