1. Gene Expression
Chapter 13

2. Learning Objective 1
What early evidence indicated that most genes specify the structure of proteins?

3. Garrod’s Work Inborn errors of metabolism Alkaptonuria Gene mutation
evidence that genes specify proteins
Alkaptonuria
rare genetic disease
lacks enzyme to oxidize homogentisic acid
Gene mutation
associated with absence of specific enzyme

4. Alkaptonuria

5. Learning Objective 2
Describe Beadle and Tatum’s experiments with Neurospora

6. Beadle and Tatum Exposed Neurospora spores Each mutant strain
to X-rays or ultraviolet radiation
induced mutations prevented metabolic production of essential molecules
Each mutant strain
had mutation in only one gene
each gene affected only one enzyme

7. Beadle-Tatum Experiments

8. Learning Objective 3
How does genetic information in cells flow from DNA to RNA to polypeptide?

9. DNA to Protein Information encoded in DNA 2-step process:
codes sequences of amino acids in proteins
2-step process:
1. Transcription
2. Translation

10. Transcription Synthesizes messenger RNA (mRNA)
complementary to template DNA strand
specifies amino acid sequences of polypeptide chains

11. Translation Synthesizes polypeptide chain Codon specified by mRNA
also requires tRNA and ribosomes
Codon
sequence of 3 mRNA nucleotide bases
specifies one amino acid
or a start or stop signal

12. DNA to Protein

13. Learning Objective 4
What is the difference between the structures of DNA and RNA?

14. RNA RNA nucleotides RNA subunits ribose (sugar)
bases (uracil, adenine, guanine, or cytosine)
3 phosphates
RNA subunits
covalently joined by 5′ – 3′ linkages
form alternating sugar-phosphate backbone

15. RNA Structure

16. Learning Objective 5
Why is genetic code said to be redundant and virtually universal?
How may these features reflect its evolutionary history?

17. Genetic Code mRNA codons 64 codons specify a sequence of amino acids
61 code for amino acids
3 codons are stop signals

18. Codons

19. Genetic Code Is redundant Is virtually universal
some amino acids have more than one codon
Is virtually universal
suggesting all organisms have a common ancestor
few minor exceptions to standard code found in all organisms

20. Learning Objective 6
What are the similarities and differences between the processes of transcription and DNA replication?

21. Enzymes Similar enzymes Carry out synthesis in 5′ → 3′ direction
RNA polymerases (RNA synthesis)
DNA polymerases (DNA replication)
Carry out synthesis in 5′ → 3′ direction
Use nucleotides with 3 phosphate groups

22. Antiparallel Synthesis
Strands of DNA are antiparallel
Template DNA strand and complementary RNA strand are antiparallel
DNA template read in 3′ → 5′ direction
RNA synthesized in 5′ → 3′ direction

23. Antiparallel Synthesis

24. Base-Pairing Rules In RNA synthesis and DNA replication are the same
except uracil is substituted for thymine

25. Transcription

26. Learning Objective 7
What features of tRNA are important in decoding genetic information and converting it into “protein language”?

27. Transfer RNA (tRNA) “Decoding” molecule in translation Anticodon
complementary to mRNA codon
specific for 1 amino acid

28. tRNA

29. Transfer RNA (tRNA) tRNA attaches to specific amino acid
covalently bound by aminoacyl-tRNA synthetase enzymes

30. Aminoacyl-tRNA

31. Learning Objective 8
How do ribosomes function in polypeptide synthesis?

32. Ribosomes Bring together all machinery for translation
Couple tRNAs to mRNA codons
Catalyze peptide bonds between amino acids
Translocate mRNA to read next codon

33. Ribosomal Subunits Each ribosome is made of Each subunit contains
1 large ribosomal subunit
1 small ribosomal subunit
Each subunit contains
ribosomal RNA (rRNA)
many proteins

34. Ribosome Structure

35. Animation: Structure of a Ribosome
CLICK TO PLAY

36. Learning Objective 9
Describe the processes of initiation, elongation, and termination in polypeptide synthesis

37. Initiation 1st stage of translation Initiation factors Initiator tRNA
bind to small ribosomal subunit
which binds to mRNA at start codon (AUG)
Initiator tRNA
binds to start codon
then binds large ribosomal subunit

38. Elongation A cyclic process Proceeds in 5′ → 3′ direction along mRNA
adds amino acids to polypeptide chain
Proceeds in 5′ → 3′ direction along mRNA
Polypeptide chain grows
from amino end to carboxyl end

39. Termination Final stage of translation A site binds to release factor
when ribosome reaches stop codon
A site binds to release factor
triggers release of polypeptide chain
dissociation of translation complex

40. Stages of Transcription

41. Learning Objective 10
What is the functional significance of the structural differences between bacterial and eukaryotic mRNAs?

42. Eukaryotes Genes and mRNA molecules
are more complicated than those of bacteria

43. Eukaryotic mRNA After transcription
5′ cap (modified guanosine triphosphate) is added to 5′ end of mRNA molecule
Poly-A tail (adenine-containing nucleotides)
may be added at 3′ end of mRNA molecule

44. Posttranscriptional Modification

45. Introns and Exons Introns Exons noncoding regions (interrupt exons)
removed from original pre-mRNA
Exons
coding regions in eukaryotic genes
spliced to produce continuous polypeptide coding sequence

46. Learning Objective 11
What is the difference between translation in bacterial and eukaryotic cells?

47. Bacterial Cells Transcription and translation are coupled
Bacterial ribosomes
bind to 5′ end of growing mRNA
initiate translation before message is fully synthesized

48. Bacterial mRNA

49. Initiation

50. Elongation

51. Termination

52. Polyribosome
Many ribosomes bound to a single mRNA

53. Learning Objective 12
Describe retroviruses and the enzyme reverse transcriptase

54. Retroviruses Synthesize DNA from an RNA template
HIV-1 (virus that causes AIDS)
Enzyme reverse transcriptase
reverses flow of genetic information

55. Reverse Transcription

56. Learning Objective 13
Give examples of the different classes of mutations that affect the base sequence of DNA
What effects does each have on the polypeptide produced?

57. Base Substitution May alter or destroy protein function
missense mutation
codon change specifies a different amino acid
nonsense mutation
codon becomes a stop codon
May have minimal effects
if amino acid is not altered
if codon change specifies a similar amino acid

58. BASE-SUBSTITUTION MUTATIONS
Normal DNA sequence
Normal mRNA sequence
Normal protein sequence
(Stop)
BASE-SUBSTITUTION MUTATIONS
Missense mutation
(Stop)
Figure 13.20: Mutations.
(a) Missense and nonsense mutations are types of base-substitution mutations. A missense mutation results in a polypeptide of normal length, but with an amino acid substitution. A nonsense mutation results in the production of a truncated (shortened) polypeptide, which is usually not functional.
Nonsense mutation
(Stop)
Fig a, p. 299

59. Animation: Base-Pair Substitution
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60. Frameshift Mutations
Insertion or deletion of one or two base pairs in a gene
destroys protein function
changes codon sequences downstream from the mutation

61. Normal protein sequence
Normal DNA sequence
Normal mRNA sequence
Normal protein sequence
(Stop)
FRAMESHIFT MUTATIONS
Deletion causing nonsense
Figure 13.20: Mutations.
(b) A frameshift mutation results from the deletion (shown) or insertion of one or two bases, causing the base sequence following the mutation to shift to a new reading frame. A frame shift may produce a stop codon downstream of the mutation (which would have the same effect as a nonsense mutation caused by base substitution), or it may produce an entirely new amino acid sequence.
(Stop)
Deletion causing altered amino acid sequence
Fig b, p. 299

62. Animation: Frameshift Mutation
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63. Transposons Movable DNA sequences Retrotransposons
“jump” into the middle of a gene
Retrotransposons
replicate by forming RNA intermediate
reverse transcriptase converts to original DNA sequence before jumping into gene

64. Animation: Protein Synthesis Summary
CLICK TO PLAY