Chapter 9 From DNA to Protein

1. Chapter 9From DNA to Protein

2. 9.1 The Aptly Acronymed RIPs • A tiny amount of ricin, a natural protein found in castor-oil seeds, can kill an adult human – there is no antidote • Ricin is a ribosome-inactivating protein (RIP • Other RIPs include shiga toxin, made by Shigella dysenteriae bacteria, and enterotoxins made by E. coli bacteria, including the strain O157:H7

3. Some RIPs

4. 9.2 DNA, RNA, and Gene Expression • Transcription converts information in a gene to RNA DNA → transcription → mRNA • Translation converts information in an mRNA to protein mRNA → translation → protein

5. The Nature of Genetic Information • Each DNA strand consists of a chain of four kinds of nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C) • The sequence of the bases in the strand is the genetic code • Genes

6. Converting a Gene to an RNA • Transcription • DNA is transcribed to RNA • Most RNA is single stranded • RNA uses uracil in place of thymine • RNA uses ribose in place of deoxyribose

7. A DNA Nucleotide base (guanine) 3 phosphate groups sugar (deoxyribose) A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate

8. An RNA Nucleotide base (guanine) 3 phosphate groups sugar (ribose) An RNA nucleotide: guanine (G), or guanosine triphosphate

9. DNA deoxyribonucleic acid RNA ribonucleic acid adenine A adenine A sugar– phosphate backbone guanine G guanine G cytosine C cytosine C nucleotide base base pair thymine T uracil U DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring. RNAs have various functions. Some serve as disposable copies of DNA’s genetic message; others are catalytic. Still others have roles in gene control. Nucleotide bases of DNA Nucleotide bases of DNA Figure 9-3 p151

10. RNA in Protein Synthesis • Messenger RNA (mRNA) • Contains information transcribed from DNA • Ribosomal RNA (rRNA) • Main component of ribosomes, where polypeptide chains are built • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

11. Converting mRNA to Protein • Translation • mRNA is translated to protein

12. Gene Expression • A cell’s DNA sequence (genes) contains all the information needed to make the molecules of life • Gene expression • A multistep process including transcription and translation • Gene -> trait

13. http://video.pbs.org/video/1506740590/

14. 9.3 Transcription: DNA to RNA • RNA polymerase • A new RNA strand is complementary

15. DNA Replication and Transcription • In transcription • Uracil (U) nucleotides pair with A nucleotides • RNA polymeraseadds nucleotides to the transcript

16. The Process of Transcription • Promoter (site in DNA close to the start of a gene)

17. RNA polymerase gene region RNA promoter sequence in DNA RNA polymerase binds to a promoter in the DNA. The binding positions the polymerase near a gene. In most cases, the base sequence of the gene occurs on only one of the two DNA strands. Only the DNA strand complementary to the gene sequence will be translated into RNA. 1 DNA winding up DNA unwinding The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA. The DNA winds up again after the polymerase passes. The structure of the “opened” DNA at the transcription site is called a transcription bubble, after its appearance. 2 direction of transcription Zooming in on the gene region, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template strand, so it is an RNA copy of the gene. 3 Stepped Art Figure 9-4 p152

18. DNA molecule RNA transcripts Figure 9-5 p153

19. Post-Transcriptional Modifications • In eukaryotes, RNA is modified before it leaves the nucleus as a mature mRNA • Introns • Nucleotide sequences that are removed from a new RNA • Exons • Sequences that stay in the RNA

20. Example • Introns and Exons: • JUSTICESCALIAEUDIFKFNDI88ADMITS2DHFJDHEDOESNOTFEELSKFJKDCJDIFLQULIFIEDTORULEKDKFNDOINFHTEEDHFJDFHUDWONTHEACCURACYOFTHISSCIENCE • Exons only (Introns removed) • JUSTICESCALIAADMITSHEDOESNOTFEELQUALIFIEDTORULEONTHEACCURACYOFTHISSCIENCE

21. Alternative Splicing • Alternative splicing

22. gene promoter exon intron exon intron exon DNA transcription exon intron exon intron exon new transcript RNA processing exon exon exon 5′ 3′ finished RNA cap poly-A tail Figure 9-6 p153

23. 9.4 RNA and the Genetic Code • Base triplets in an mRNA encode a protein-building message • Ribosomal RNA (rRNA) and transfer RNA (tRNA) translate that message into a protien

24. mRNA – The Messenger • mRNA carries protein-building information to ribosomes • Codon

25. Figure 9-7a p154

26. Genetic Code • Genetic code • Consists of 64 mRNA codons (triplets) • Twenty kinds of amino acids are found in proteins • Some amino acids can be coded by more than one codon • Some codons signal the start or end of a gene • AUG (methionine) is a start codon • UAA, UAG, and UGA are stop codons

27. Figure 9-7b p154

28. From DNA to RNA to Amino Acids a gene region in DNA transcription codon codon codon mRNA translation amino acid sequence methionine (met) tyrosine (tyr) serine (ser)

29. rRNA and tRNA – The Translators • tRNAs deliver amino acids to ribosomes • tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon • Ribosomes, which link amino acids into polypeptide chains, consist of two subunits of rRNA and proteins

30. tRNA Structure anticodon anticodon amino acid attachment site

31. Translation: Ribosome and tRNA

32. 9.5 Translation: RNA to Protein • Translation converts genetic information carried by an mRNA into a new polypeptide chain • The order of the codons in the mRNA determines the order of the amino acids in the polypeptide chain

33. Translation • Translation occurs in the cytoplasm of cells • Translation occurs in three stages: • Initiation • Elongation • Termination

34. Initiation • An initiation complex is formed • A small ribosomal subunit binds to mRNA • The anticodon of initiator tRNA base-pairs with the start codon (AUG) of mRNA • A large ribosomal subunit joins the small ribosomal subunit

35. Elongation • The ribosome assembles a polypeptide chain as it moves along the mRNA • Initiator tRNA carries methionine, the first amino acid of the chain • The ribosome joins each amino acid to the polypeptide chain with a peptide bond

36. Termination • When the ribosome encounters a stop codon, polypeptide synthesis ends • Release factors bind to the ribosome • Enzymes detach the mRNA and polypeptide chain from the ribosome

37. start codon (AUG) 2 Ribosome subunits and an initiator tRNA converge on an mRNA. A second tRNA binds to the second codon. A peptide bond forms between the first two amino acids. initiator tRNA 1 first amino acid of polypeptide peptide bond A peptide bond forms between the second and third amino acids. 4 The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon. 3 A peptide bond forms between the third and fourth amino acids. The process repeats until the ribosome encounters a stop codon in the mRNA. 6 The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth codon. 5 Stepped Art Figure 9-11 p156

38. Transcription polysomes ribosome subunits tRNA Convergence of RNAs mRNA Translation polypeptide Figure 9-12a p157

39. Practice • The DNA base sequence of the gene coding for a short polypeptide is • T A C G C T A G G C G A T T G A C T • Transcribe this DNA. • Translate into the amino acid sequence.

40. Polysomes • Many ribosomes may simultaneously translate the same mRNA, forming polysomes mRNA polysomes newly forming polypeptide

41. 9.6 Mutated Genes and Their Protein Products • If the nucleotide sequence of a gene changes, it may result in an altered gene product, with harmful effects • Mutations • Small-scale changes in the nucleotide sequence of a cell’s DNA that alter the genetic code

42. Mutations and Proteins • A mutation that changes a UCU codon to UCC is “silent” – it has no effect on the gene’s product because both codons specify the same amino acid • Other mutations may change an amino acid in a protein, or result in a premature stop codon that shortens it – both can have severe consequences for the organism

43. Common Mutations • Base-pair-substitution • May result in a premature stop codon or a different amino acid in a protein product • Example: sickle-cell anemia • Deletion orinsertion • Can cause the reading frame of mRNA codons to shift, changing the genetic message • Example: thalassemia

44. Hemoglobin and Anemia • Hemoglobin is a protein that binds oxygen in the lungs and carries it to cells throughout the body • The hemoglobin molecule consists of four polypeptides (globins) folded around iron-containing hemes – oxygen molecules bind to the iron atoms • Defects in polypeptide chains can cause anemia, in which a person’s blood is deficient in red blood cells or in hemoglobin

45. Mutations in the Beta Globin Gene

46. Figure 9-13a p158

47. Figure 9-13b p158

48. Figure 9-13c p158

49. Figure 9-13d p158

50. Figure 9-13e p158