Protein Synthesis

1. Protein Synthesis Transcription and Translation Mader Biology Chapter 14

2. Protein synthesis = Central dogma= DNA To RNA TO protein

3. DNA TO RNA TO PROTEIN • Transcription: DNA transcribes code to mRNA in nucleus • Translation: tRNA transports amino acids to mRNA on ribosome • Protein Synthesis-assembly of amino acids to polypeptides and, ultimately, proteins

4. 3 rna’s used to make proteins • mRNA – carries the “message” of DNA • Travels from nucleus to cytoplasm • Takes message to ribosome for protein synthesis • tRNA – brings amino acids to ribosome for protein synthesis • rRNA – what ribosome is made of

5. Amino acid attachment site A A G 3 5 Anticodon Symbol used in some books The tRNA molecule

6. Nuclear envelope DNA TRANSCRIPTION DNA TRANSCRIPTION mRNA Ribosome Pre-mRNA RNA PROCESSING TRANSLATION mRNA Polypeptide Ribosome (a) Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing. TRANSLATION Polypeptide (b) Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. Transcription and translation • In eukaryotes, separated by nuclear envelope • In prokaryotes, occurs together

7. transcription DNA mRNA

8. Non-template strand of DNA Elongation RNA nucleotides RNA polymerase T A C C A T A T C 3 U 3 end T G A U G G A G E A C C C A 5 A A T A G G T T Template strand of DNA 5 Direction of transcription (“downstream”) Newly made RNA Transcription • Transfer of genetic information from DNA to RNA (mRNA) • Similar to replication: • The DNA strands must separate. • Carried out by RNA polymerase (not DNA polymerase) • Other similar enzymes • Unlike replication: • Only 1 mRNA strand created, not 2 • 3 stages: • Initiation • Elongation • Termination

10. More Transcription • Promoter- region where RNA polymerase attaches and initiates transcription • Often a sequence of TATA – known as the TATA box • Terminator- DNA sequence that signals the end of transcription

11. After Transcription – mRNA processing • DNA has many non-coding segments • Used to be called “junk” DNA (Not really junk) • In RNA splicing, the extra segments are cut out

12. After Transcription – mRNA processing • Introns = non-coding DNA that is removed – splicing • Exons = coding DNA segments that are kept and joined together

13. Mrna processing – cap and tail • mRNA also receives a G3P cap on the 5’ end • And a poly-a tail (many adenine’s in a row) on the 3’ end

14. transcription DNA mRNA

15. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly tRNA C C C G G Anticodon A A A A G G G U G U U U C Codons 5 3 mRNA Translation • mRNA  protein • Takes place on ribosome in cytoplasm

16. Translation: Genetic Information is “read” • DNA = letters A, C, T, G (you know this!) • These bases are encoded as a sequence base triplets, or codons, each of which is translated into a specific amino acid • FOR EXAMPLE: • Theredfoxatetherat = the red fox ate the rat • AUGCCTUGUCGA = AUG CCT UGU CGA (easier, huh?)

17. Translation: Genetic information is “read” • Codons = genetic code • Translates to amino acids (proteins) • Codon chart is universal for all living organisms (see pink sheet) • So, let’s take the DNA sequence: • TACGGT • mRNA sequence? • Amino acid sequence? • *see chart

18. How does translation happen?3 stage process

19. Large ribosomal subunit P site 5 3 U C A Met Met 3 A 5 G U Initiator tRNA GDP GTP E A mRNA 5 5 3 3 Start codon mRNA binding site Small ribosomal subunit Translation initiation complex 1) Initiation • 1) Initiation • Brings together mRNA, tRNA and ribosome • Begins at the start codon, AUG

20. Amino end of polypeptide 1 Codon recognition DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide E mRNA 3 Ribosome ready for next aminoacyl tRNA P A site site 5 GTP 2 GDP 2 E E P A P A 2 GDP Peptide bond formation Translocation. 3 GTP E P A 2) Elongation • Codon of mRNA and anticodon of tRNA complementary bond to one another • Peptide bond formation • Translocation-ribosome moves to next codon

21. Release factor Free polypeptide 5 3 3 3 5 5 Stop codon (UAG, UAA, or UGA) The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. The two ribosomal subunits and the other components of the assembly dissociate. 2 1 3 3) Termination • mRNA stop codons - UAA, UAG, UGA

22. DNA TRANSCRIPTION RNA is transcribed from a DNA template. 1 4 3 2 5 3 Poly-A RNA transcript RNA polymerase 5 Exon RNA PROCESSING In eukaryotes, the RNA transcript (pre- mRNA) is spliced and modified to produce mRNA, which moves from the nucleus to the cytoplasm. RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Cap NUCLEUS Amino acid FORMATION OF INITIATION COMPLEX AMINO ACID ACTIVATION tRNA CYTOPLASM After leaving the nucleus, mRNA attaches to the ribosome. Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. Growing polypeptide mRNA Activated amino acid Poly-A Poly-A Ribosomal subunits Cap 5 TRANSLATION C C A U A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time. (When completed, the polypeptide is released from the ribosome.) A E A C Anticodon A A A U G U G G U U U A Codon Ribosome summary

24. Protein Synthesis and Mutation • Mutation = Permanent, (sometimes) heritable DNA change • Point mutation (base substitutions) • Missense mutation • Nonsense mutation (premature stop) • Silent mutation • Frameshift mutations • Insertions or deletions • Dramatic change in amino acids • Run-ons, premature stops (nonsense mut.)

25. Protein synthesis and mutation • The Creation of Mutation (mutagenesis): • Spontaneous mutation • Occurs in DNA replication (1 in 109 bp) • Polymerase makes mistakes • Chemical mutagens (alcohol, tobacco, drugs) • Increases rate to 1 in 1000-100,000) • Radiation • X rays, gamma rays break DNA, bases • UV light causes knots in DNA strand

26. Mutation: Some Definitions • A heritable change in the genetic material • Mutations may be neutral, beneficial, or harmful • Mutagen: Agent that causes mutations • Spontaneous mutations: Occur in the absence of a mutagen

27. Missense mutation • Changes an amino acid in the sequence

28. Nonsense mutation • Prematurely stops the amino acid sequence

29. Run-on mutation • Continues the sequence that was supposed to end G C Glu

30. Silent mutation • No effect on the amino acids G C

31. Frameshift Mutations • Insertion or deletion – affects CODONS THEBIGCATATETHERAT = THE BIG CAT ATE THE RAT THEBIGCBATATETHERAT = THE BIG CBA TAT ETH ERA T

32. Summary: Mutations POINT mutations-one base pair is changed • Substitutions FRAMESHIFT: • Insertions (additions) • Deletions • Mutagens-physical and chemical agents that interact DNA to cause mutations—exp.: xrays