CHAPTER 12 From DNA to Protein: Genotype to Phenotype

1. CHAPTER 12From DNA to Protein: Genotype to Phenotype

2. Chapter 12: From DNA to Protein: Genotype to Phenotype One Gene, One Polypeptide DNA, RNA, and the Flow of Information Transcription: DNA-Directed RNA Synthesis The Genetic Code

3. Chapter 12: From DNA to Protein: Genotype to Phenotype Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes Translation: RNA-Directed Polypeptide Synthesis Regulation of Translation

4. Chapter 12: From DNA to Protein: Genotype to Phenotype Posttranslational Events Mutations: Heritable Changes in Genes

5. One Gene, One Polypeptide • Genes are made up of DNA and are expressed in the phenotype as polypeptides. 5

6. One Gene, One Polypeptide • Beadle and Tatum’s experiments with the bread mold Neurospora resulted in mutant strains lacking a specific enzyme in a biochemical pathway. • These results led to the one-gene, one-polypeptide hypothesis. Review Figure 12.1 6

7. figure 12-01.jpg Figure 12.1 Figure 12.1

8. One Gene, One Polypeptide • Certain hereditary diseases in humans had been found to be caused by the absence of certain enzymes. • These observations supported the one-gene, one-polypeptide hypothesis. 8

9. DNA, RNA, and the Flow of Information • RNA differs from DNA in three ways: • It is single-stranded • its sugar molecule is ribose rather than deoxyribose • its fourth base is uracil rather than thymine. 9

10. DNA, RNA, and the Flow of Information • The central dogma of molecular biology is DNA  RNA  protein. Review Figure 12.2 10

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12. DNA, RNA, and the Flow of Information • A gene is expressed in two steps: • First, DNA is transcribed to RNA; • then RNA is translated into protein. Review Figure 12.3 12

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14. DNA, RNA, and the Flow of Information • In retroviruses, the rule for transcription is reversed: RNA  DNA. • Other RNA viruses exclude DNA altogether, going directly from RNA to protein. Review Figure 12.2 14

15. Transcription: DNA-Directed RNA Synthesis • RNA is transcribed from a DNA template after the bases of DNA are exposed by unwinding of the double helix. 15

16. Transcription: DNA-Directed RNA Synthesis • In a given region of DNA, only one of the two strands can act as a template for transcription. 16

17. Transcription: DNA-Directed RNA Synthesis • RNA polymerase catalyzes transcription from the template strand of DNA. 17

18. Transcription: DNA-Directed RNA Synthesis • The initiation of transcription requires that RNA polymerase recognize and bind tightly to a promoter sequence on DNA. 18

19. Transcription: DNA-Directed RNA Synthesis • RNA elongates in a 5’-to-3’ direction, antiparallel to the template DNA. • Special sequences and protein helpers terminate transcription. Review Figure 12.4 19

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21. figure 12-04b.jpg Figure 12.4 – Part 2 Figure 12.4 – Part 2

22. The Genetic Code • The genetic code consists of triplets of nucleotides (codons). • Since there are four bases, there are 64 possible codons. 22

23. The Genetic Code • One mRNA codon indicates the starting point of translation and codes for methionine. • Three stop codons indicate the end of translation. • The other 60 codons code only for particular amino acids. 23

24. The Genetic Code • Since there are only 20 different amino acids, the genetic code is redundant; that is, there is more than one codon for certain amino acids. • However, a single codon does not specify more than one amino acid. Review Figure 12.5 & Table 1 24

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26. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes • In prokaryotes, translation begins before the mRNA is completed. • In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm. 26

27. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes • Translation requires three components: tRNA’s, activating enzymes, and ribosomes. 27

28. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes In translation, amino acids are linked in codon-specified order in mRNA. This is achieved by an adapter, transfer RNA (tRNA), which binds the correct amino acid and has an anticodon complementary to the mRNA codon. Review Figure 12.7 28

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30. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes • The aminoacyl-tRNA synthetases, a family of activating enzymes, attach specific amino acids to their appropriate tRNA’s, forming charged tRNA’s. Review Figure 12.8 30

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32. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes • The mRNA meets the charged tRNA’s at a ribosome. Review Figure 12.9 32

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34. Translation: RNA-Directed Polypeptide Synthesis • An initiation complex consisting of an amino acid-charged tRNA and a small ribosomal subunit bound to mRNA triggers the beginning of translation. Review Figure 12.10 34

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36. Translation: RNA-Directed Polypeptide Synthesis • Polypeptides grow from the N terminus toward the C terminus. • The ribosome moves along the mRNA one codon at a time. Review Figure 12.11 36

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39. Translation: RNA-Directed Polypeptide Synthesis • The presence of a stop codon in the A site of the ribosome causes translation to terminate. Review Figure 12.12 39

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41. Regulation of Translation • Some antibiotics work by blocking events in translation. Review Table 12.2 41

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43. Regulation of Translation • In a polysome, more than one ribosome moves along the mRNA at one time. Review Figure 12.13 43

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45. Posttranslational Events • Signals contained in the amino acid sequences of proteins direct them to cellular destinations. Review Figure 12.14 45

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47. Posttranslational Events • Protein synthesis begins on free ribosomes in the cytoplasm. • Those proteins destined for the nucleus, mitochondria, and plastids are completed there and have signals that allow them to bind to and enter destined organelles. 47

48. Posttranslational Events • Proteins destined for the ER, Golgi apparatus, lysosomes, and outside the cell complete their synthesis on the ER surface. • They enter the ER by the interaction of a hydrophobic signal sequence with a channel in the membrane. Review Figure 12.15 48

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50. figure 12-15b.jpg Figure 12.15 – Part 2 Figure 12.15 – Part 2