Chapter 9 From DNA to Protein (Sections 9.1 - 9.3)

1. Chapter 9From DNA to Protein (Sections 9.1 - 9.3)

2. 9.1 Ricin and Your Ribosomes • Ricin, a natural protein in castor oil beans, is highly toxic: A dose as small as a few grains of salt can kill an adult • Ricin inactivates ribosomes – organelles that assemble amino acids into proteins • Proteins are critical to all life processes, so cells that cannot make them die very quickly

3. Ricin • One of ricin’s polypeptide chains helps the molecule cross cell membranes • The other chain destroys a cell’s capacity for protein synthesis

4. 2.2 Nature of Genetic Information • DNA contains all of the instructions for building a new individual • The linear order or sequence of the four bases (A, T, G, C) in the DNA strand is the genetic information, which occurs in subsets called genes • gene • Part of a DNA base sequence • Specifies an RNA or protein product

5. Converting a Gene to RNA • Transcription converts information in a gene to RNA • Enzymes use the nucleotide sequence of a gene as a template to synthesize a strand of RNA (ribonucleic acid) • transcription • Process by which an RNA is assembled from nucleotides using the base sequence of a gene as a template

6. Three Types of RNA • Three types of RNA have roles in protein synthesis: • Ribosomal RNA (rRNA) is the main component of ribosomes, the structures upon which polypeptide chains are built • Transfer RNA (tRNA) delivers amino acids to ribosomes in the order specified by a messenger RNA (mRNA)

7. Key Terms • messenger RNA (mRNA) • Type of RNA that carries a protein-building message • ribosomal RNA (rRNA) • Type of RNA that becomes part of ribosomes • transfer RNA (tRNA) • Type of RNA that delivers amino acids to a ribosome during translation

8. RNA Structure • RNA is a single-stranded chain of four kinds of nucleotides • Like DNA, a RNA nucleotide has three phosphate groups, a sugar, and one of four bases, but RNA is slightly different: • The sugar in RNA is ribose, not deoxyribose • RNA uses the base uracil instead of thymine

9. An RNA and a DNA Nucleotide

10. An RNA Nucleotide Fig. 9.2a, p. 138

11. An RNA Nucleotide base (guanine) 3 phosphate groups sugar (ribose) An RNA nucleotide: guanine (G), or guanosine triphosphate (GTP) A Guanine, one of the four nucleotides in RNA. The others (adenine, uracil, and cytosine) differ only in their component bases (blue). Three of the four bases in RNA nucleotides are identical to the bases in DNA nucleotides. Fig. 9.2a, p. 138

12. A DNA Nucleotide Fig. 9.2b, p. 138

13. A DNA Nucleotide base (guanine) 3 phosphate groups sugar (deoxyribose) A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate (dGTP) B Compare the DNA nucleotide guanine. The only difference between the RNA and DNA versions of guanine (or adenine, or cytosine) is the hydrogen atom or hydroxyl group at the 2’ carbon of the sugar (shown in green). Fig. 9.2b, p. 138

14. DNA and RNA Compared

15. DNA and RNA Compared DNA deoxyribonucleic acid RNA ribonucleic acid adenine A adenine A nucleotide base sugar– phosphate backbone guanine G guanine G cytosine C cytosine C base pair thymine T uracil U B Different types of RNA have different functions. Some serve as disposable copies of DNA’s genetic message; some are catalytic; others have roles in gene control. A DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring. Nucleotide bases of RNA Nucleotide bases of DNA Fig. 9.3, p. 139

16. DNA Fig. 9.3a, p. 139

17. DNA DNA deoxyribonucleic acid adenine A nucleotide base sugar– phosphate backbone guanine G cytosine C thymine T base pair A DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring. Nucleotide bases of DNA Fig. 9.3a, p. 139

18. RNA Fig. 9.3b, p. 139

19. RNA ribonucleic acid RNA adenine A nucleotide base sugar– phosphate backbone guanine G cytosine C uracil U B Different types of RNA have different functions. Some serve as disposable copies of DNA’s genetic message; some are catalytic; others have roles in gene control. Nucleotide bases of RNA Fig. 9.3b, p. 139

20. Converting mRNA to Protein • Translation converts information in an mRNA to protein • mRNA carries a protein-building message encoded in the sequence of sets of three nucleotide bases • mRNA is decoded (translated) into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein • translation • Process by which a polypeptide chain is assembled from amino acids in the order specified by an mRNA

21. Gene Expression • Transcription and translation are part of gene expression, a process by which information encoded by a gene is converted into a structural or functional part of a cell or a body • gene expression • Process by which the information in a gene becomes converted to an RNA or protein product

22. Key Concepts • DNA to RNA to Protein • The sequence of amino acids in a polypeptide chain corresponds to a sequence of nucleotide bases in DNA called a gene • The conversion of information in DNA to protein occurs in two steps: transcription and translation

23. 9.3 Transcription • During transcription, DNA acts as a template upon which a strand of RNA (transcript) is assembled from RNA nucleotides • Each new RNA is complementary in sequence to the DNA template: G pairs with C; A pairs with U (uracil) • RNA polymerase adds nucleotides to the end of a growing transcript

24. 3 Steps in Transcription • Transcription begins with a gene on a chromosome: RNA polymerase and several regulatory proteins attach to a specific binding site (promoter)in the DNA

25. 3 Steps in Transcription • 2. RNA polymerase starts moving along the DNA, in the 3' to 5’ direction over the gene, unwinding the double helix to “read” the base sequence of the DNA strand

26. 3 Steps in Transcription • RNA polymerase bonds free RNA nucleotides into a chain, in the order dictated by that DNA sequence, making an RNA copy of the gene

27. 3 Steps in Transcription RNA polymerase gene region 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 Fig. 9.4.1, p. 140

28. 3 Steps in Transcription RNA 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 Fig. 9.4.2, p. 140

29. 3 Steps in Transcription 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 Fig. 9.4.3, p. 140

30. 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 3 Steps in Transcription Stepped Art Fig. 9.4, p. 140

31. ANIMATION: Gene transcription details To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

32. Finishing Transcription • When the polymerase reaches the end of the gene, the DNA and the new RNA strand are released • Typically, many polymerases transcribe a particular gene region at the same time, so many new RNA strands can be produced very quickly

33. Key Terms • RNA polymerase • Enzyme that carries out transcription • promoter • In DNA, a sequence to which RNA polymerase binds

34. Gene Transcription • Three genes next to one another on the same chromosome are being transcribed simultaneously

35. Gene Transcription RNA transcripts DNA molecule Fig. 9.5, p. 141

36. Post-Transcriptional Modifications • Eukaryotes modify their RNA inside the nucleus, then ship it to the cytoplasm • Intronsare nucleotide sequences that are removed from a new RNA, and exonsare sequences that stay in the RNA • Sometimes, some exons are removed and the remaining exons are spliced together (alternative splicing) which enables one gene to encode different proteins

37. Key Terms • intron • Nucleotide sequence that intervenes between exons and is excised during RNA processing • exon • Nucleotide sequence that is not spliced out of RNA during processing • alternative splicing • RNA processing event in which some exons are removed or joined in various combinations

38. Post-Transcriptional Modifications • New transcripts that will become mRNAs are further modified after splicing • A modified guanine “cap” is added to the 5’ end, which will help the mRNA bind to a ribosome • A tail of 50-300 adenines (poly-A tail) is added to the 3’ end

39. Post-Transcriptional Modifications

40. Post-Transcriptional Modifications gene promoter exon intron exon intron exon DNA transcription exon intron exon intron exon new transcript RNA processing exon exon exon finished RNA cap poly-A tail Fig. 9.6, p. 141

41. ANIMATION: Pre-mRNA transcript processing To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

42. Key Concepts • DNA to RNA: Transcription • During transcription, one strand of a DNA double helix serves as a template for assembling a single, complementary strand of RNA (a transcript) • Each transcript is an RNA copy of a gene

43. ANIMATION: Overview of transcription To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

44. ANIMATION: Transcription - A molecular view To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

45. ANIMATION: Transcription - Introns and exons To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

46. ANIMATION: Transcription