Gene expression

1. Gene expression Transcription and Translation

2. 1. Important Features • a. DNA contains genetic template for proteins. • b. DNA is found in the nucleus • c. Protein synthesis occurs in the cytoplasm - ribosome. • d. "Genetic information" must be transferred to the cytoplasm where proteins are synthesized.

3. 2. Processes of Protein Synthesis • a. Transcription - genetic template for a protein is copied and carried out to the cytoplasm • b. Translation - template serves as a series of codes for the amino acid sequence of the protein

4. Cells Use RNA to Make Protein • The RNA Players – rRNA, mRNA, tRNA • During polypeptide synthesis, ribosomal RNA (rRNA) is the site of polypeptide assembly. • Messenger RNA (mRNA) directs which amino acids are assembled into polypeptides. • Transfer RNA (tRNA) transports and positions amino acids.

5. Central Dogma of Gene Expression

6. Transcription – in the nucleus (if you have one) • DNA sequence is transcribed into RNA sequence • only one of two DNA strands (templateor antisense strand) is transcribed • non-transcribed strand is termed coding strand or sense strand same as RNA (except T’s are U’s) • In both bacteria and eukaryotes, the polymerase adds ribonucleotides to the growing 3’ end of an RNA chain. • synthesis proceeds in 5’3’ direction (template runs 3’5’)

7. Transcription • TATA box • Signals beginning of section to be transcribed. • Transcription factor • Binds to TATA box so that RNA polymerase can then bind • Promoter • Transcription starts at RNA polymerase binding sites called promoters on DNA template strand, which includes the TATA box and about 25 other nucleotides that will not be transcribed. • Initiation • Other transcription factors bind, assembling a transcription initiation complex. • RNA polymerase begins to unwind DNA helix.

8. Transcription • Elongation • Transcription bubble moves down DNA at constant rate leaving growing RNA strands protruding from the bubble. • Termination • Stop sequences, or terminators, at the end of the gene cause phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA.

9. Transcription Bubble

10. Transcription Tell the story of transcription with your group – one person starts: “First….,” and says one sentence. The next group member picks up where the first left off, and so on.

11. RNA Processing • In eukaryotes, RNA is modified after transcription • DNA sequence specifying a protein is broken into coding segments (exons) scattered among longer noncoding segments (introns). • Small nuclear ribonuclearproteins (snRNPs) associate with proteins to form spliceosomes. • Intron sequences are cut out of primary transcript before it is used in polypeptide synthesis - they are not translated • remaining exon sequences are spliced together to form final processed mRNA

12. RNA Processing • 5’ cap – G-P-P-P – protects mRNA from degradation and serves as an “attach here” sign for ribosomes • PolyA tail – A-A-A-A-A – inhibits degradation and stabilizes mRNA as it moves out of nucleus

13. Each person in group tells one way that RNA is modified after transcription in eukaryotes RNA Processing

14. Now TRANSLATION!!!!

15. Translation • Begins when initial portion of mRNA molecule binds to rRNA in a ribosome • mRNA is in triplet code – 3 bases = codon • tRNA molecule with complimentary anticodon binds to exposed codon on mRNA. • The codon determines which amino acid the tRNA carries • AUG is always the start codon – it codes for the amino acid Methionine (Met)

17. Translation • Elongation • Once mRNA binds to small subunit, large subunit attaches • A site = where tRNAs Arrive • P site = where Peptide bonds are fomed • E site = where tRNAs Exit

18. Translation • Termination • stop signal coded by one of three nonsense codons: UAA - UAG – UGA • Polypeptide released from ribosome

20. Translation Tell the story of translation with your group – one person starts: “First….,” and says one sentence. The next group member picks up where the first left off, and so on.

21. A A T T C G A C T A T G T C G T A A template Strand U C G A U A A U C mRNA

22. A A T T C G A C T A T G T C G T A A template Strand U C G A U A A U C Nucleus mRNA Cytoplasm Ribosome

23. A A T T C G A C T A T G T C G T A A Template Strand U C G A U A A U C mRNA U C G A U A A U C Nucleus Cytoplasm

24. A A T T C G A C T A T G T C G T A A Template Strand AA1 AGC U C G A U A A U C tRNA’s Nucleus Cytoplasm

25. A A T T C G A C T A T G T C G T A A template Strand AA1 tRNA’s AGC U C G A U A A U C Nucleus Cytoplasm

26. A A T T C G A C T A T G T C G T A A template Strand AA1 • AA2 AGC AAG U C G A U A A U C Nucleus ATP Cytoplasm tRNA’s

27. A A T T C G A C T A T G T C G T A A Template Strand AGC AA3 AA1 AA1 U U U U C G A U A A U C Nucleus ATP • AA2 Cytoplasm AAG

28. A A T T C G A C T A T G T C G T A A template Strand AGC AA3 AA1 AA1 U U U U C G A U A A U C Nucleus • AA2 Cytoplasm AAG

29. A A T T C G A C T A T G T C G T A A Template Strand AGC AA3 AA1 AA1 AAG U U U U C G A U A A U C Nucleus • AA2 Cytoplasm

30. A A T T C G A C T A T G T C G T A A Template Strand AGC AA3 AA1 AA1 AAG U U U U C G A U A A U C Nucleus • AA2 Cytoplasm

31. The Genetic Code 1. A triplet code comprised of three nucleotide bases in a sequence. 2. How many triplet codes? 20 common amino acids in a protein 4 diff. bases on DNA A,T,C, & G | | | | 4 diff. bases on RNA U,A,G, & C 4 things put together in combinations of 3 = 43= 64 Therefore - 64 different DNA triplet codes or RNA codons

32. The 64 triplet codes • 60 code for amino acids • 4 act as "stop" and "start codes • Degenerate Code- more than one triplet code for some amino acids e.g., GGG GGU GGC GGA All code for the amino acid glycine