From DNA to Protein

1. From DNA to Protein Chapter 9 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

2. 9.1 Ricin and Your Ribosomes • Ricin • Naturally occurring protein that is highly toxic • A dose smaller then a few grains of salt can kill an adult (die of low blood pressure and respiratory failure) • One of Ricin’s two polypeptide chains is an enzyme that inactivated ribosomes • The second polypeptide chain binds to the plasma membrane allowing the cell to take up Ricin by endocytosis • Ribosomes • Assembly amino acids into proteins • Proteins are critical to all life processes

3. Proteins • All proteins consist of polypeptide chains • A linear sequence of amino acids • Each chain corresponds to the nucleotide base sequence of a gene

4. The Path From Genes to Proteins 1. Transcription • Enzymes use the base sequence of a gene in the DNA as a template to make a strand of RNA 2. Translation • Information in the RNA strand is decoded (translated) into a sequence of amino acids

5. Prokaryotes and Eukaryotes • In prokaryotic cells (no nucleus) • Transcription and translation occur in cytoplasm • In eukaryotic cells • Genes are transcribed in the nucleus • Resulting mRNA is translated in the cytoplasm

6. Key Concepts:INTRODUCTION • Life depends on enzymes and other proteins • All proteins consist of polypeptide chains • Chains are sequences of amino acids that correspond to sequences of nucleotide bases in DNA called genes • The path leading from genes to proteins has two steps: transcription and translation

7. 9.2 The Nature of Genetic Information • Genetic information consists of the nucleotide base sequence of DNA • The linear order, or sequence, of the four bases in the strand is the genetic information • Genetic information occurs in subsets called genes • Genes  part of the DNA sequence that specifies an RNA or protein production

8. Transcription: DNA to RNA • Two DNA strands unwind in a specific region • RNA polymerase assembles a strand of RNA • Covalently bonds RNA nucleotides (adenine, guanine, cytosine, uracil) according to the nucleotide sequence of the exposed gene

9. Three Types of RNA • Messenger RNA (mRNA) • Carries protein-building codes from DNA to ribosomes • Ribosomal RNA (rRNA) • Forms ribosomes (where polypeptide chains are assembled) • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

10. RNA and DNA Compared

11. RNA Base Pairing

12. phosphate group base (uracil) sugar (ribose) Fig. 13.2, p.198

13. Gene Transcription Definition: The process by which the information in a gene becomes converted to an RNA or protein product RNA polymerase  enzyme that carries out transcription Promoter  in DNA, a sequence to which RNA polymerase binds

14. Fig. 13.3, p.198

15. Fig. 13.3, p.198

16. newly forming RNA transcript gene region RNA polymerase, the enzyme that catalyzes transcription DNA template winding up DNA template unwinding Fig. 13.3, p.198

17. Fig. 13.3, p.198

18. Fig. 13.3, p.198

19. RNA Modification: Alternative Splicing • Before mRNA leaves the nucleus: • Introns are removed during RNA processing • Some exons are removed along with introns; remaining exons are spliced together in different combinations • Most are not removed during RNA processing • Alternative splicing • RNA processing event in which some exons are removed or joined in various combinations • Poly-A tail is added to 3’ end of new mRNA

20. The Poly-A Tail • The longer its poly-A tail, the more time an mRNA transcript (and its protein-building message) will remain intact in the cytoplasm

21. Post-Translational RNA Modification

22. unit of transcription in DNA strand exon intron exon intron exon transcription into pre-mRNA cap poly-A tail 5' 3' snipped out snipped out mature mRNA transcript Fig. 13.4, p.199

23. Key Concepts:TRANSCRIPTION • During transcription, the two strands of the DNA double helix are unwound in a gene region • Exposed bases of one strand become the template for assembling a single strand of RNA (a transcript) • Messenger RNA is the only type of RNA that carries DNA’s protein-building instructions

24. RNA and the Genetic Code • Messenger RNA (mRNA) carries DNA’s protein-building information to ribosomes for translation • mRNA’s genetic message is written in codons • Sets of three nucleotides along mRNA strand • The genetic code  • The concept that a set of three nucleotides specifies a particular amino acid

25. Codons • Codons specify different amino acids • A few codon signals stop during translation • Sixty-four possible codons constitute a highly conserved genetic code

26. Genetic Code: RNA Triplets

27. From DNA to Polypeptide

28. DNA mRNA mRNA codons threonine proline glutamate glutamate lysine amino acids Fig. 13.5, p.200

29. Variation in Genetic Code • Variant codons occur among prokaryotes, prokaryote-derived organelles (such as mitochondria), and some ancient lineages of single-celled eukaryotes

30. Key Concepts:CODE WORDS IN THE TRANSCRIPTS • The nucleotide sequence in RNA is read three bases at a time • Sixty-four base triplets that correspond to specific amino acids represent the genetic code, which has been highly conserved over time

31. tRNA and rRNA Function in Translation • Transfer RNA (tRNA) • Anticodon binds to mRNA codon • Also binds amino acid specified by codon • Different tRNAs carry different amino acids • tRNAs deliver free amino acids to ribosomes during protein synthesis

32. tRNA

33. rRNA • Ribosomal RNA (rRNA) and proteins make up the two subunits of ribosomes

34. Three Stages of Translation • mRNA-transcript information directs synthesis of a polypeptide chain during translation • Translation proceeds in three stages • Initiation • Elongation • Termination

35. Initiation • One initiator tRNA, two ribosomal subunits, and one mRNA come together as an initiation complex • Methionine (M) • tRNA carries M  M is the first amino acid of the new polypeptide chain

36. Initiation

37. Elongation • tRNAs deliver amino acids to the ribosome in the order specified by mRNA codons • Ribosomal rRNA catalyzes the formation of a peptide bond between amino acids

38. Elongation Peptide Bond Forms between Met and Valine

39. Elongation

40. Elongation

41. Termination • Translation ends when RNA polymerase encounters a STOP codon in mRNA • New polypeptide chain and mRNA are released • Ribosome subunits separate from each other

42. Termination

43. Initiation A mature mRNA leaves the nucleus and enters cytoplasm, which has many free amino acids, tRNAs, and ribosome subunits. An initiator tRNA binds to a small ribosomal subunit and the mRNA. mRNA small ribosomal subunit initiator tRNA large ribosomal subunit A large ribosomal subunit joins, and the cluster is now called an initiation complex. Fig. 13.8, p.202

44. Elongation 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). 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. 13.8, p.202

45. The second RNA 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) Termination 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 4.8). Fig. 13.8, p.202

46. Key Concepts:TRANSLATION • During translation, amino acids become bonded together into a polypeptide chain in a sequence specified by base triplets in messenger RNA • Transfer RNAs deliver amino acids one at a time to ribosomes • Ribosomal RNA catalyzes the formation of peptide bonds between the amino acids

47. Transcription-Translation Concepts • Many ribosomes may simultaneously translate the same mRNA, this is called  polysomes • Transcription and translation both occur in the cytoplasm • Compared to DNA, RNA is not very stable • An mRNA may last only a few minutes before it gets disassembled by enzymes in the cytoplasm • Translation is  Energy intensive (use ATP)

48. Mutated Genes and Their Protein Products • Mutations are permanent, small-scale changes in the base sequence of a gene • Common mutations include • Insertions  • one or more base pairs are inserted into the DNA • Deletions  • one or more base pairs are lost • Base-pair substitutions • A single base-pair is changed

49. Common Gene Mutations

50. Transposable Elements • Segments of DNA that can insert themselves anywhere in a chromosome