1. Gene Activity: How Genes Work
Chapter 14
Gene Activity: How Genes Work

2. Genes are segments of DNA that specify amino acids in a protein
14.1 The Function of Genes
Genes are segments of DNA that specify amino acids in a protein
Sir Archibald Garrod proposed a link between genes and proteins
Genes Specify Enzymes
Beadle and Tatum (1940) experimented with Neurospora (mold similar to Sordaria)
they induced mutations that made the mold only able to grow on enriched medium

3. Fig. 14.1 Beadle & Tatum experiment

4. One gene, one enzyme experiment

5. Genes Specify Enzymes, cont.
14.1 The Function of Genes
Genes Specify Enzymes, cont.
Beadle and Tatum (1940), cont.
concluded that mold must have been missing an enzyme needed to produce missing nutrient
therefore, they proposed the “one gene, one enzyme” hypothesis

6. Genes Specify Enzymes, cont.
14.1 The Function of Genes
Genes Specify Enzymes, cont.
Pauling and Itano (1949)
demonstrated that sickle cell hemoglobin is different from normal hemoglobin using electrophoresis
showed that a mutation in DNA resulted in a change in the structure of a protein
“one gene, one enzyme” revised to “one gene, one polypeptide”

7. Fig. 14.2 Sickle cell disease

8. Fig. 14.2c R group and properties

9. From DNA to RNA to Protein
14.1 The Function of Genes
From DNA to RNA to Protein
gene: segment of DNA that specifies the amino acid sequence of a protein
genes pass information to RNA (ribonucleic acid), which is more directly responsible for building proteins

10. DNA to RNA to Protein, cont.
14.1 The Function of Genes
DNA to RNA to Protein, cont.
RNA differs from DNA

11. Fig RNA structure

12. DNA to RNA to Protein, cont.
14.1 The Function of Genes
DNA to RNA to Protein, cont.
there are three major classes of RNA, each with its own size, shape, and function
messenger RNA (mRNA): takes message from DNA to ribosomes
transfer RNA (tRNA): carries amino acids to ribosomes
ribosomal RNA (rRNA): makes up ribosomes (along with ribosomal proteins)

13. DNA to RNA to Protein, cont.
14.1 The Function of Genes
DNA to RNA to Protein, cont.
steps of protein synthesis (gene expression)
transcription: DNA serves as template to make RNA
translation: mRNA transcript codes for sequence of amino acids
this is Crick’s Central Dogma of Molecular Biology

14. Overview of gene expression

15. Overview of gene expression

16. The genetic code is a triplet code
each codon consists of three nucleotide bases and codes for one amino acids
three nucleotides is necessary to code for 20 amino acids

17. Finding the Genetic Code
Nirenberg and Matthei (1961) used an enzyme to produce an RNA molecule made entirely of uracil
a protein composed only of phenylalanine resulted from the RNA, therefore UUU = phe
in a similar way, the other amino acids were assigned mRNA codons

18. Finding the Genetic Code
Nirenberg and Matthei (1961)
all uracil RNA made a protein that was all phenylalanine
therefore UUU = phe
in a similar way, the other amino acids were assigned mRNA codons

19. Fig. 14.5 Messenger RNA codons

20. Finding the Genetic Code, cont.
properties of the genetic code
1. it is degenerate (redundant); most amino acids have more than one codon
2. it is unambiguous; each codon has only one meaning
3. it has start and stop signals
start codon is AUG; codes for MET
UAA, UAG, UGA stop translation (no amino acid encoded)

21. The genetic code (easier to read)

22. 14.2 The Genetic Code The Code Is Universal
the code is used by all living things
evidence for the common ancestry of living things
there are some exceptions to the universality, but they generally involve just a few codons

23. Overview of transcription

24. 14.3 First Step: Transcription
During transcription, a segment of DNA serves as a template for the production of RNA
Messenger RNA Is Formed
initiation
a promoter signals the start of a gene, direction of transcription, strand to be transcribed
promoters are specific nucleotide sequences on DNA (often TATA...)
attract RNA polymerase

25. Initiation of transcription

26. 14.3 First Step: Transcription
Messenger RNA Is Formed, cont.
initiation, cont.
transcription factors bind to the promoter and help RNA polymerase recognize and bind to DNA
elongation
RNA polymerase opens DNA strands and synthesizes mRNA along the template strand

27. 14.3 First Step: Transcription
Messenger RNA Is Formed, cont.
elongation, cont.
RNA polymerase, cont.
reads 3’-5’
makes mRNA 5’-3’
mRNA grows at a rate of nucleotides/second
only newest portion of mRNA is bound to DNA; most of the new strand dangles off to the side

28. Fig Transcription

29. 14.3 First Step: Transcription
Messenger RNA Is Formed, cont.
elongation, cont.
many RNA polymerase molecules might be working at the same time
template/non-template strands of DNA attach back together after RNA polymerase passes

30. Numerous RNA transcripts

31. Fig. 14.7b RNA polymerase

32. 14.3 First Step: Transcription
Messenger RNA Is Formed, cont.
termination
specific nucleotide sequences signal “stop” and cause RNA polymerase to release DNA
“stop” sequence is called a terminator, usually AAUAAA on the mRNA in eukaryotes
(do not confuse this with a stop codon)
the product is primary RNA

33. Transcription animation

34. 14.3 First Step: Transcription
RNA Molecules Are Processed
occurs before leaving nucleus
5’ cap added to tell ribosome where to attach
3’ poly-A tail added to aid transport out of nucleus and inhibit degradation
introns removed by spliceosomes
introns: in between exons; 95%
exons: coding regions that will be expressed

35. RNA processing

36. RNA splicing

37. End result of splicing

38. Prokaryotes versus eukaryotes

39. 14.4 Second Step: Translation
During translation, the mRNA message is translated to an amino acid sequence
The Role of Transfer RNA
tRNA molecules transfer amino acids to ribosomes
tRNA has an anticodon that is complementary to a codon
tRNA also has an amino acid binding site, making it “bilingual”

40. Fig. 14.9 Structure of transfer RNA

41. 14.4 Second Step: Translation
The Role of Transfer RNA, cont.
most cells have 40 different tRNA molecules
this is less than the 61 amino-acid-coding codons and more than the 20 different amino acids
perhaps some tRNAs can pair with more than one codon as long as first two positions are correct
called wobble hypothesis

42. 14.4 Second Step: Translation
The Role of Transfer RNA, cont.
aminoacyl-tRNA synthetases attach correct amino acid to each tRNA
requires ATP
20 different types (1 for each amino acid)
this means they can accommodate different kinds of tRNA

43. 14.4 Second Step: Translation
The Role of Ribosomal RNA
structure of a ribosome
consists of a small and large subunit
made in nucleolus
60% rRNA, 40% protein
has 3 binding sites for tRNA
A site: amino acid site
P site: peptide site
E site: exit site
has 1 binding site for mRNA

44. Fig. 14.10a,b Ribosome structure

45. 14.4 Second Step: Translation
The Role of Ribosomal RNA, cont.
function of a ribosome
tRNA binding sites facilitate base pairing between tRNA anticodons and mRNA codons
rRNA joins amino acids as a polypeptide is synthesized
multiple ribosomes can translate same mRNA simultaneously; complex called a polyribosome

46. Fig c,d Polyribosome

47. 14.4 Second Step: Translation
Translation Requires Three Steps
initiation (in prokaryotes)
brings all the translation components together
small ribosomal subunit attaches to mRNA at 5' end at initiation sequence (AUG)
tRNA with anticodon UAC (carrying amino acid MET) attaches to initiator

48. Fig Initiation

49. 14.4 Second Step: Translation
Translation Requires Three Steps
initiation, cont.
tRNA, mRNA, MET and small subunit form initiation complex
large ribosomal subunit attaches to initiation complex
tRNA is at P site
A site is for next tRNA
E site is for tRNAs leaving ribosome

50. Fig Elongation

51. 14.4 Second Step: Translation
Translation Requires Three Steps
elongation
polypeptide increases in length one amino acid at a time
next tRNA with complementary anticodon binds to A site
peptide transferred to this tRNA as ribozyme joins the amino acids at A and P site (forms peptide bond)

52. 14.4 Second Step: Translation
Translation Requires Three Steps
elongation, cont.
ribosome moves forward
tRNA at A site (with peptide) shifted to P site
tRNA at P site (“empty”) shifted to E site and exits
the next tRNA binds at A site and process repeats

53. 14.4 Second Step: Translation
Translation Requires Three Steps
termination
translation components separate
occurs at a stop codon (UAG, UAA, UGA)
no tRNA with anticodon for these
release factors cleave polypeptide from last tRNA and cause ribosome to split, releasing mRNA and new protein
protein is properly folded

54. Fig Termination

55. Translation

56. Translation

57. Translation

58. Translation

59. Translation

60. Translation

61. Translation

62. Translation

63. Translation

64. Translation

65. Translation

66. Translation

67. Translation

68. Translation

69. Translation

70. Translation animation

71. Fig Summary

72. Central Dogma of Molecular Biology
Nucleus
Replication
Enzymes
Translation
DNA
Protein
RNA
Structural
Components
of the Cell
Transcription
Cytoplasm (ribosomes)