CHAPTER 12 From DNA to Protein: Genotype to Phenotype

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

2. Genotype to Phenotype • Genes are made up of DNA (genotype). • Genes cannot directly produce a phenotype. • Genes must be expressed (phenotype) as polypeptides.

3. 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, and • Its fourth base is uracil rather than thymine. • Adenine pairs with uracil

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

5. DNA, RNA, and the Flow of Information - Summary • A gene is transcribed to produce messenger RNA (mRNA). • mRNA is complementary to one of the DNA strands • Transfer RNA ((tRNA) translates sequence of bases in mRNA into appropriate sequence of amino acids. • Amino acids join together (peptide bonds) to form proteins. • Review Figure 12.3

6. figure 12-03.jpg Figure 12.3 Figure 12.3

7. Transcription – synthesis of RNA from DNA • Transciption requires the enzyme RNA polymerase, RNA nucleotides, and a DNA template. • Transcription occurs in the nucleus. • The product, a RNA transcript, is sent to the cytoplasm where translation occurs.

8. Transcription • Transcription is divided into three processes: • Initiation, • Elongation, and • termination

9. Initiation • Transcription begins at a promoter – a special sequence of DNA . • The promoter determines the direction, which strand to read, and direction to take • RNA polymerase binds to the promoter. • Once the polymerase is attached to the promoter DNA, the DNA strands unwind and transcription begins.

10. Elongation • As RNA polymerase moves along the DNA, it continues to unwind DNA about 20 bases pairs at a time. • One side of the unwound DNA acts as template for RNA synthesis • RNA transcript is formed by complementary base pairings.

11. Termination • Specific DNA base sequences terminate transcription. • Pre-mRNA is released. • Review Figure 12.4 • The resulting RNA transcript may be mRNA, tRNA, or rRNA.

12. Messenger RNA (mRNA) carries a genetic message from DNA to the protein synthesizing machinery of the cell (ribosomes)

13. figure 12-04a.jpg Figure 12.4 – Part 1 Figure 12.4 – Part 1

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

15. The Genetic Code • The genetic code consists of triplets of nucleotides (codons). • Since there are four bases, there are 64 possible codons (43) • There are more codons than different amino acids.

16. The Genetic Code • AUG codes for methionine and is the start codon. • UAA, UAG, and UGA are stop codons. • Stop codons indicate the end of translation. • The other 60 codons code only for particular amino acids.

17. 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

18. The Universal Genetic Code The genetic code appears to be nearly universal. • Provides a common language for evolution. • Implications for genetic engineering.

19. figure 12-05.jpg Figure 12.5 Figure 12.5

20. Preparation for Translation: Linking RNA’s, Amino Acids, and Ribosomes • Translation occurs at the ribosomes. • Translate the message from sequence of nucleotides to sequence of amino acids

21. Components of Translation • Ribosomes – small and large subunits • mRNA (messenger RNA) • tRNA (transfer RNA) • tRNA transfer an amino acid. • tRNA has a sequence of 3 bases known as the anticodon that is complementary to mRNA codon • amino acids are linked in codon-specified order per mRNA.

22. figure 12-07.jpg Figure 12.7 Figure 12.7

23. Example of Process • The DNA coding region for proline is GGG which is transcribed to • The mRNA codon CCC which binds to • The tRNA with the anticodon GGG

24. Activating Enzymes link tRNA and amino acids • A family of activating enzymes – aminoacyl-tRNA synthetases_ attach specific amino acids to their appropriate tRNA’s to from charged tRNA • The amino acid is attached to the 3’ end of tRNA with a high energy bond • Review Figure 12.8

25. Ribosomes • The ribosome consist of a large and a small subunit. • When not active in translation, the ribosomes exist as separate units. • They can come together and separate as needed. • Review Figure 12.9

26. figure 12-09.jpg Figure 12.9 Figure 12.9

27. Three Phases of Translation • Initiation • Elongation • Termination

28. Translation: Initiation • A sequence of mRNA (initiation factors) binds to the small subunit of a ribosome. • Aminoacyl tRNA bearing UAC binds to the start codon. • Large subunit of ribosome joins the complex Review Figure 12.10

29. figure 12-10.jpg Figure 12.10 Figure 12.10

30. Elongation – A Four Step Process • A charged tRN moves into the ribosome and occupies the A site. Its anticodon matches the mRNA codon • The polypeptide chain is transferred. • Ribosome moves along the mRNA. Empty tRNA is ejected via the E site. • tRNA with peptide chains moves to P site. A is empty. Repeat

31. figure 12-11a.jpg Figure 12.11 – Part 1 Figure 12.11 – Part 1

32. figure 12-11b.jpg Figure 12.11 – Part 2 Figure 12.11 – Part 2

33. Translation: Termination • The presence of a stop codon (UAA, UAG, or UGA) in the A site of the ribosome causes translation to terminate. • Both tRNA and the polypeptide are released from the P site. • The ribosomes separate • Review Figure 12.12

34. figure 12-12.jpg Figure 12.12 Figure 12.12

35. 4 Sites for tRNA Binding • T (transfer) site is where tRNA + amino acids first attaches to the ribosome. • The A (amino acid) site is there the tRNA anticodon binds to mRNA codon • The P (polypetide) site is where the amino acids are bonded together. • The E (exit) site is where the tRNA will leave the ribosome to pick up additional amino acids.

36. Regulation of Translation • Antibiotics can interfere with translation • Erythromycin plugs the exit channel so the polypeptide chain cannot leave the ribosome. • Review Table 12.2

37. Regulation of Translation • In a polysome, more than one ribosome moves along the mRNA at one time. • Multiple copies of the same protein is made for a single mRNA. • Review Figure 12.13

38. figure 12-13.jpg Figure 12.13 Figure 12.13

39. Posttranslational Events • The functional protein may vary from the polypeptide chain that is originally released. • Signals contained in the amino acid sequences of proteins direct them to cellular destinations. • And polypeptides may be altered by the addition of chemical groups that affect function of the protein. • Review Figure 12.14

40. figure 12-14.jpg Figure 12.14 Figure 12.14

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

42. 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

43. figure 12-15a.jpg Figure 12.15 – Part 1 Figure 12.15 – Part 1

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

45. Posttranslational Events • Covalent modifications of proteins after translation include: • proteolysis – polypeptide chain is cut • Glycosylation – additions of sugars to proteins • Phosphorylation – add phosphate groups to protiens. • Review Figure 12.16

46. figure 12-16.jpg Figure 12.16 Figure 12.16

47. Mutations: Heritable Changes in Genes • Mutations in DNA are often expressed as abnormal proteins. • However, the result may not be easily observable phenotypic changes. • Some mutations appear only under certain conditions, such as exposure to a certain environmental agent or condition.

48. Mutations: Heritable Changes in Genes • Point mutations (silent, missense, nonsense, or frame-shift) result from alterations in single base pairs of DNA.

49. Mutations: Heritable Changes in Genes • Chromosomal mutations (deletions, duplications, inversions, or translocations) involve large regions of a chromosome. • Review Figure 12.18

50. figure 12-18.jpg Figure 12.18 Figure 12.18