From DNA to Protein

1. From DNA to Protein Chapter 14

2. 14.1 DNA, RNA, and Gene Expression • What is genetic information and how does a cell use it?

3. The Nature of Genetic Information • Each strand of DNA consists of a chain of four kinds of nucleotides: A, T, G and C • The sequence of the four bases in the strand is the genetic information

4. Converting a Gene to an RNA • Transcription • Enzymes use the nucleotide sequence of a gene to synthesize a complementary strand of RNA • DNA is transcribed to RNA • Most RNA is single stranded • RNA uses uracil in place of thymine • RNA uses ribose in place of deoxyribose

5. adenine A adenine A DNA RNA deoxyribonucleic acid ribonucleic acid NH 2 NH 2 N N C C C C N N nucleotide base HC HC N CH CH C C N N N sugar– phosphate backbone guanine G guanine G O O N C C N C C NH NH HC HC N N C C C C N N NH 2 NH 2 cytosine C cytosine C NH 2 NH 2 C C HC HC N N C O O HC C HC N N thymine T base pair uracil U O O CH 3 C C C NH HC NH O C O C HC HC N N 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. Nucleotide bases of DNA Nucleotide bases of RNA Fig. 14-3, p. 217

6. 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

7. Converting mRNA to Protein • Translation • The information carried by mRNA is decoded into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein • mRNA is translated to protein • rRNA and tRNA translate the sequence of base triplets in mRNA into a sequence of amino acids

8. 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, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body

9. 14.2 Transcription: DNA to RNA • RNA polymerase assembles RNA by linking RNA nucleotides into a chain • A new RNA strand is complementary in sequence to the DNA strand from which it was transcribed

10. DNA Replication and Transcription • DNA replication and transcription both synthesize new molecules by base-pairing • In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotides • Uracil (U) nucleotides pair with A nucleotides • RNA polymerase adds nucleotides to the transcript

11. Base-Pairing in DNA Synthesis and Transcription

12. The Process of Transcription • RNA polymerase and regulatory proteins attach to a promoter • RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins free RNA nucleotides into a complementary strand of mRNA

13. Transcription

14. Fig. 14-5b, p. 219

15. Animation: Gene transcription details

16. 14.3 RNA and the Genetic Code • Base triplets in an mRNA are words in a protein-building message • Two other classes of RNA (rRNA and tRNA) translate those words into a polypeptide chain

17. 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

18. Alternative Splicing • After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly-A tail at the 3' end

19. Animation: Pre-mRNA transcript processing

20. mRNA – The Messenger • Codon • A sequence of three mRNA nucleotides that codes for a specific amino acid

21. Genetic Information • From DNA to mRNA to amino acid sequence

22. Genetic Code • Genetic code • Consists of 64 mRNA codons (triplets) • 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

23. Codons of the Genetic Code

24. 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

25. tRNA

26. 14.4 Translation: RNA to Protein • Translation converts genetic information carried by an mRNA into a new polypeptide chain

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

28. 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

29. 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

30. Termination • When the ribosome encounters a stop codon, polypeptide synthesis ends • Release factors bind to the ribosome

31. Elongation C An initiator tRNA carries the amino acid methionine, so the first amino acid of the new polypeptide chain will be methionine. A second tRNA binds the second codon of the mRNA (here, that codon is GUG, so the tRNA that binds carries the amino acid valine). A peptide bond forms between the first two amino acids (here, methionine and valine). Fig. 14-12c, p. 223

32. D The first tRNA is released and the ribosome moves to the next codon in the mRNA. A third tRNA binds to the third codon of the mRNA (here, that codon is UUA, so the tRNA carries the amino acid leucine). A peptide bond forms between the second and third amino acids (here, valine and leucine). Fig. 14-12d, p. 223

33. E The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth mRNA codon (here, that codon is GGG, so the tRNA carries the amino acid glycine). A peptide bond forms between the third and fourth amino acids (here, leucine and glycine). Fig. 14-12e, p. 223

34. Termination F Steps d and e are repeated over and over until the ribosome encounters a stop codon in the mRNA. The mRNA transcript and the new polypeptide chain are released from the ribosome. The two ribosomal subunits separate from each other. Translation is now complete. Either the chain will join the pool of proteins in the cytoplasm or it will enter rough ER of the endomembrane system (Section 4.9). Fig. 14-12f, p. 223

35. 14.5 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

36. 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 or insertion • Can cause the reading frame of mRNA codons to shift, changing the genetic message • Example: Huntington’s disease

37. Common Mutations

38. Animation: Base-pair substitution

39. What Causes Mutations? • Transposable elements • Segments of DNA that can insert themselves anywhere in a chromosomes • Spontaneous mutations • Uncorrected errors in DNA replication • Harmful environmental agents • Ionizing radiation, UV radiation, chemicals

40. Mutations Caused by Radiation • Ionizing radiation damages chromosomes, nonionizing (UV) radiation forms thymine dimers

41. Inherited Mutations • Mutations in somatic cells of sexually reproducing species are not inherited • Mutations in a germ cell or gamete may be inherited

42. Animation: Protein synthesis summary

43. Animation: Structure of a ribosome

44. Animation: Structure of a tRNA

45. Animation: Transcription

46. Animation: Uracil-thymine comparison