1. HAPPY MOLE DAY!!! 6.02 X 1023 (6:02 PM on Oct 23) Announcements
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HAPPY MOLE DAY!!!
6.02 X 1023
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2. From DNA to Protein: Genotype to Phenotype

3. From DNA to Protein: Genotype to Phenotype
One Gene, One Polypeptide
DNA, RNA, and the Flow of Information
Transcription: DNA-Directed RNA Synthesis
The Genetic Code
Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
Translation: RNA-Directed Polypeptide Synthesis
Regulation of Translation
Posttranslational Events
Mutations: Heritable Changes in Genes

4. One Gene, One Polypeptide
Our focus in this chapter:
How do we get from a Genotype (se/se) to a Phenotype (fruit fly with sepia eyes)?
Genotype Phenotype
se/se

5. One Gene, One Polypeptide
Don’t forget that the environment has some control over the phenotype (e.g., fur color with Siamese cats). However, during the next two lectures we’ll be focusing on the genetic contribution to the phenotype.
When we speak of genetics, we think of genes.
A gene is defined as a DNA sequence.

6. One Gene, One Polypeptide
In the 1940s, Beadle and Tatum showed that when an altered gene resulted in an altered phenotype, that altered phenotype always showed up as an altered enzyme.
They experimented with strains of the bread mold Neurospora crassa: a wild-type, and several mutant strains, which were created by hitting the wild-type with X-rays (mutagen).

7. One Gene, One Polypeptide
Their results suggested that mutations cause a defect in only one enzyme in a metabolic pathway.
This led to the one-gene, one-enzyme hypothesis.
In the example below, three genes, and therefore three enzymes, are involved in the metabolic pathway. If any of the genes (A,B, or C) are mutated, arginine will not be produced.

8. One Gene, One Polypeptide
The one gene – one enzyme hypothesis has undergone several modifications. Some enzymes are composed of different subunits coded for by separate genes. Remember the quaternary structure of a protein?
This suggests, instead of the one-gene - one enzyme hypothesis, a one-gene - one-polypeptide relationship.
150/chemistry/hemoglobin.jpg

9. DNA, RNA, and the Flow of Information
The EXPRESSION OF A GENE takes place in two steps:
Transcription makes a single-stranded RNA copy of a segment of the DNA.
Translation uses information encoded in the RNA to make a polypeptide.
cellcycle/mcell-transcription-translation_eng_zoom.gif

10. DNA, RNA, and the Flow of Information
Francis Crick’s central dogma stated that DNA codes for RNA, and RNA codes for protein.

11. DNA, RNA, and the Flow of Information
Certain viruses use RNA rather than DNA as their information molecule during transmission.
These viruses transcribe from RNA to RNA; they make a complementary RNA strand and then use this “opposite” strand to make multiple copies of the viral genome by transcription.
HIV and certain tumor viruses (called retroviruses) have RNA as their infectious information molecule; they convert it to a DNA copy inside the host cell and then use it to make more RNA.

12. DNA, RNA, and the Flow of Information
RNA (ribonucleic acid) differs from DNA in three ways:
RNA consists of only one polynucleotide strand.
The sugar in RNA is ribose, not deoxyribose.
RNA has uracil instead of thymine.
RNA can base-pair with single-stranded DNA (adenine pairs with uracil instead of thymine) and also can fold over and base-pair with itself.
U – A in RNA
T – A in DNA
Modified -

13. DNA, RNA, and the Flow of Information
Messenger RNA, or mRNA moves from the nucleus of eukaryotic cells into the cytoplasm, where it serves as a template for protein synthesis.
Transfer RNA, or tRNA, is the link between the code of the mRNA and the amino acids of the polypeptide, specifying the correct amino acid sequence in a protein.

14. Figure 12.3 From Gene to Protein

15. Transcription: DNA-Directed RNA Synthesis
TRANSCRIPTION (DNA to RNA)
In normal prokaryotic and eukaryotic cells, transcription requires the following:
A DNA template for complementary base pairing
The appropriate ribonucleoside triphosphates (ATP, GTP, CTP, and UTP) to act as substrates
The enzyme RNA polymerase

16. Figure 12.4 (Part 1) DNA is Transcribed in RNA

17. Transcription: DNA-Directed RNA Synthesis
Just one DNA strand (the template strand) is used to make the RNA.
For different genes in the same DNA molecule, the roles of these strands may be reversed.
The DNA double helix partly unwinds to serve as template.
As the RNA transcript forms, it peels away, allowing the already transcribed DNA to be rewound into the double helix.
From Purves et al.

18. Transcription: DNA-Directed RNA Synthesis
The first step of transcription, initiation, begins at a promoter, a special sequence of DNA.
There is at least one promoter for each gene to be transcribed.
The RNA polymerase binds to the promoter region when conditions allow.
The promoter sequence directs the RNA polymerase as to which of the double strands is the template and in what direction the RNA polymerase should move.

19. Transcription: DNA-Directed RNA Synthesis
After binding, RNA polymerase unwinds the DNA about 20 base pairs at a time and reads the template in the 3¢-to-5¢ direction (elongation).
The new RNA elongates from its 5¢ end to its 3¢ end; thus the RNA transcript is antiparallel to the DNA template strand.
Transcription errors for RNA polymerases are high relative to DNA polymerases. Less of a problem because many copies are available and mRNA is short-lived.
Particular base sequences in the DNA specify termination.

20. DNA Transcribed Into RNA
Layered Figure DNA is Transcribed into RNA

21. Figure 12.4 (Part 2) DNA is Transcribed in RNA

22. Figure 12.4 (Part 3) DNA is Transcribed in RNA

23. THE GENETIC CODE (translating bases into amino acids)
A genetic code relates genes (DNA) to mRNA and mRNA to the amino acids of proteins.
mRNA is read in three-base segments called codons (e.g., CCC).
The number of different codons possible is 64 (43), because each of the three positions in the codon can be occupied by one of four different bases.
The 64 possible codons code for only 20 amino acids and the start and stop signals.

24. The Genetic Code
HOW DO WE KNOW THIS?
In the early 1960s, molecular biologists broke the genetic code.
Nirenberg prepared an artificial mRNA in which all bases were uracil (poly U).
When incubated with additional components, the poly U mRNA led to synthesis of a polypeptide chain consisting only of phenylalanine amino acids.
UUU appeared to be the codon for phenylalanine.
Other codons were deciphered from this starting point.

25. Figure 12.6 Deciphering the Genetic Code

26. Figure 12.5 The Universal Genetic Code

27. The Genetic Code
AUG, which codes for methionine, is called the start codon, the initiation signal for translation.
Three codons (UAA, UAG, and UGA) are stop codons, which direct the ribosomes to end translation.
After subtracting start and stop codons, the remaining 60 codons code for 19 different amino acids.
This means that many amino acids have more than one codon. Thus the code is redundant.
However, the code is not ambiguous. Each codon is assigned only one amino acid.

28. Till next time…

29. Announcements
Reminder:
UUU - phenylalanine
CCC – proline
GGG – glycine
AAA – lysine
Start Codon – AUG
Stop Codons – UAA, UGA, UAG

30. Figure 12.4 (Part 1) DNA is Transcribed in RNA
Recall Transcription: Getting the genetic information from DNA and placing it on mRNA.

31. Figure 12.5 The Universal Genetic Code

32. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
ON TO TRANSLATION… mRNA to PROTEIN
cellcycle/mcell-transcription-translation_eng_zoom.gif

33. Transcription & Translation
jpitocch/genbio/codonsAA.JPG

34. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
The codon in mRNA and the amino acid in a protein are related by way of an adapter— a specific tRNA molecule.
tRNA has three functions:
It carries an amino acid.
It associates with mRNA.
It interacts with ribosomes.

35. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
A tRNA molecule has 75 to 80 nucleotides and a three- dimensional shape (conformation).
The shape is maintained by complementary base pairing and hydrogen bonding.
The three-dimensional shape of the tRNAs allows them to combine with the binding sites of the ribosome.

36. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
At the 3¢ end of every tRNA molecule is a site to which its specific amino acid binds covalently.
Midpoint in the sequence are three bases called the anticodon.
The anticodon is the contact point between the tRNA and the mRNA.
The anticodon is complementary (and antiparallel) to the mRNA codon.
The codon and anticodon unite by complementary base pairing.

37. Figure Transfer RNA

38. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
Amino acids are attached to the correct tRNAs by activating enzymes called aminoacyl-tRNA synthetases.
The enzyme has a three-part active site that binds:
A specific amino acid
ATP
A specific tRNA, charged with a high-energy bond
The high-energy bond provides the energy for making the peptide bond (during translation).

39. Charging a tRNA Molecule
Layered Figure 12.8: Charging a tRNA Molecule

40. The reactions have two steps: Enzyme + ATP + AA ® enzyme—AMP—AA + PPi
Figure Charging a tRNA Molecule
The reactions have two steps:
Enzyme + ATP + AA ® enzyme—AMP—AA + PPi
Enzyme—AMP—AA + tRNA ® enzyme + AMP + tRNA—AA

41. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
Each ribosome has two subunits: a large one and a small one.
In eukaryotes the large one has three different associated rRNA molecules and 45 different proteins.
The small subunit has one rRNA and 33 different protein molecules.
When they are not translating, the two subunits are separate.

42. Figure 12.9 Ribosome Structure

43. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
The large ribosomal subunit has four binding sites:
The T site where the tRNA first lands
The A site where the tRNA anticodon binds to the mRNA codon
The P site where the tRNA adds its amino acid to the polypeptide chain
The E site where the tRNA goes before leaving the ribosome

44. Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
The small ribosomal subunit plays a role in validating the three-base-pair match between the mRNA and the tRNA.
If hydrogen bonds have not formed between all three base pairs, the tRNA is ejected from the ribosome.
image/dna-protein/all/small_N1fka.gif

45. Translation: RNA-Directed Polypeptide Synthesis
Translation begins with an initiation complex: a charged tRNA with its amino acid and a small subunit, both bound to the mRNA.
This complex is bound to a region upstream of where the actual reading of the mRNA begins.
The start codon (AUG) designates the first amino acid in all proteins.
The large subunit then joins the complex.
The process is directed by proteins called initiation factors.

46. Figure 12.10 The Initiation of Translation

47. Translation: RNA-Directed Polypeptide Synthesis
Ribosomes move in the 5¢-to-3¢ direction on the mRNA.
The peptide forms in the N–to–C direction. Remember the amino group and carboxylic acid group on amino acids?
The large subunit catalyzes two reactions:
Breaking the bond between the tRNA in the P site and its amino acid
Peptide bond formation between this amino acid and the one attached to the tRNA in the A site
This is called peptidyl transferase activity.
chem/V.27/amino_acid_structure_2.jpg

48. Figure 12.11 Translation: The Elongation Stage

49. Translation: RNA-Directed Polypeptide Synthesis
After the first tRNA releases methionine, it dissociates from the ribosome and returns to the cytosol.
The second tRNA, now bearing a dipeptide, moves to the P site.
The next charged tRNA enters the open A site.
The peptide chain is then transferred to the P site.
These steps are assisted by proteins called elongation factors.

50. Translation: RNA-Directed Polypeptide Synthesis
When a stop codon—UAA, UAG, or UGA— enters the A site, a release factor and a water molecule enter the A site, instead of an amino acid.
The newly completed protein then separates from the ribosome.

51. Figure 12.12 The Termination of Translation

52. Posttranslational Events
Two posttranslational events can occur after the polypeptide has been synthesized:
The polypeptide may be moved to another location in the cell, or secreted.
Most polypeptides are modified by the addition of chemical groups, folding, or trimming. Often essential to the functioning of a protein.
Three types of modifications:
Proteolysis (cleaving)
Glycosylation (adding sugars)
Phosphorylation (adding phosphate groups)

53. Figure 12.16 Posttranslational Modifications to Proteins

54. Figure 12.14 Destinations for Newly Translated Polypeptides in a Eukaryotic Cell

55. Mutations
MUTATIONS!

56. Mutations: Heritable Changes in Genes
Mutations are heritable changes in DNA—changes that are passed on to daughter cells.
Multicellular organisms have two types of mutations:
Somatic mutations are passed on during mitosis, but not to subsequent generations.
Germ-line mutations are mutations that occur in cells that give rise to gametes.
All mutations are alterations of the DNA nucleotide sequence and are of two types:
Point mutations are mutations of single genes.
Chromosomal mutations are changes in the arrangements of chromosomal DNA segments.

57. Mutations: Heritable Changes in Genes
Point mutations result from the addition or subtraction of a base or the substitution of one base for another.
Point mutations can occur as a result of mistakes during DNA replication or can be caused by environmental mutagens.
Because of redundancy in the genetic code, some point mutations, called silent mutations, result in no change in the amino acids in the protein.
Recall: CCC, CCU, CCA, and CCG all code for the amino acid proline.

58. Silent Mutation

59. Mutations: Heritable Changes in Genes
Some mutations, called missense mutations, cause an amino acid substitution.
An example in humans is sickle-cell anemia, a defect in the b-globin subunits of hemoglobin.
The b-globin in sickle-cell differs from the normal by only one amino acid.
Missense mutations may reduce the functioning of a protein or disable it completely.

60. Missense mutation

61. Mutations: Heritable Changes in Genes
TWO OTHER TYPES OF MUTATIONS
Nonsense mutations are base substitutions that substitute a stop codon.
The shortened proteins are usually not functional.
A frame-shift mutation consists of the insertion or deletion of a single base in a gene.
This type of mutation shifts the code, changing many of the codons to different codons.
These shifts almost always lead to the production of nonfunctional proteins.

62. Nonsense mutation

63. Frame-shift mutation

64. Mutations: Heritable Changes in Genes
DNA molecules can break and re-form, causing four different types of mutations:
Deletions are a loss of a chromosomal segment.
Duplications are a repeat of a segment.
Inversions result from breaking and rejoining when segments get reattached in the opposite orientation.
Translocations result when a portion of one chromosome attaches to another.

65. Figure 12.18 Chromosomal Mutations (Part 1)

66. Figure 12.18 Chromosomal Mutations (Part 2)

67. Mutations: Heritable Changes in Genes
Spontaneous mutations are permanent changes, caused by any of several mechanisms:
Nucleotides occasionally change their structure (called a tautomeric shift).
Bases may change because of a chemical reaction.
DNA polymerase sometimes makes errors in replication which can escape being repaired.
Meiosis is imperfect. Nondisjunction and translocations can occur.

68. Mutations: Heritable Changes in Genes
Induced mutations are permanent changes caused by some outside agent (mutagen).
Mutagens can alter DNA in several ways:
Altering covalent bonds in nucleotides
Adding groups to the bases
Radiation damages DNA:
Ionizing radiation (X rays) produces free radicals.
Ultraviolet radiation is absorbed by thymine and causes interbase covalent bonds to form.

69. Figure 12.19 Spontaneous and Induced Mutations (Part 1)

70. Figure 12.19 Spontaneous and Induced Mutations (Part 2)

71. Mutations: Heritable Changes in Genes
Mutations have both benefits and costs.
Germ line mutations provide genetic diversity for evolution, but usually produce an organism that does poorly in its environment.
Somatic mutations do not affect offspring, but can cause cancer.

72. Mutations: Heritable Changes in Genes
Mutations are rare events and most of them are point mutations involving one nucleotide.
Different organisms vary in mutation frequency.
Mutations can be detrimental, neutral, or occasionally beneficial.
Random accumulation of mutations in the extra copies of genes can lead to the production of new useful proteins.