1. Chapter 17 Warm-Up
Explain the contribution that Beadle and Tatum made to understanding the role of DNA.
Compare and contrast DNA to RNA.
What is the difference between replication, transcription and translation?

2. Chapter 17 Warm-Up Describe the steps in transcription.
Contrast transcription in prokaryotes vs. eukaryotes.
How many nucleotides are in an mRNA molecule to code for a protein with 200 amino acids?

3. Chapter 17 Warm-Up How does mRNA differ from pre-mRNA?
What is the difference between introns and exons?
Describe how spliceosomes modify mRNA.

4. Chapter 17 Warm-Up Describe the steps of translation.
If the DNA sequence is: 3’ T A C G A T C A G 5’
the cDNA would be:
the mRNA is:
the tRNA is:
the amino acid sequence is:
How does the cell determine the ultimate destination of a polypeptide being synthesized?

5. Chapter 17 Warm-Up
What is a frameshift mutation? How can they impact protein synthesis?
Contrast a missense vs. nonsense mutation.

6. Chapter 17 Warm-Up
Refer to page 327. Fill in the chart comparing prokaryotic and eukaryotic gene expression:
Prokaryotes
Eukaryotes

7. From Gene to Protein
Chapter 17

8. What you need to know:
The key terms: gene expression, transcription, and translation.
The major events of transcription.
How eukaryotic cells modify RNA after transcription.
The steps to translation.
How point mutations can change the amino acid sequence of a protein.

9. Concept 17.1: Genes specify proteins via transcription and translation
How was the fundamental relationship between genes and proteins discovered?

10. Gene Expression: process by which DNA directs the synthesis of proteins (or RNAs)
Old idea: one gene-one enzyme hypothesis
Proposed by Beadle & Tatum – mutant mold experiments
Function of a gene = dictate production of specific enzyme
Newer idea: one gene-one polypeptide hypothesis
Most accurate: one gene-one RNA molecule (which can be translated into a polypeptide)

11. one gene = one RNA molecule
DNA
RNA
Nucleic acid composed of nucleotides
Single-stranded
Ribose=sugar
Uracil
Many different roles!
Nucleic acid composed of nucleotides
Double-stranded
Deoxyribose=sugar
Thymine
Template for individual

12. RNA plays many roles in the cell
pre-mRNA=precursor to mRNA, newly transcribed and not edited
Messenger RNA (mRNA)= the edited version; carries the code from DNA that specifies amino acids
Transfer RNA (tRNA)= carries a specific amino acid to ribosome based on its anticodon to mRNA codon
Ribosomal RNA (rRNA)= makes up 60% of the ribosome; site of protein synthesis
snRNA=small nuclear RNA; part of a spliceosome. Has structural and catalytic roles
srpRNA=a signal recognition particle that binds to signal peptides
RNAi= interference RNA; a regulatory molecule
ribozyme= RNA molecule that functions as an enzyme

13. Flow of genetic information
Central Dogma: DNA  RNA  protein
Transcription: DNA  RNA
Produces messenger RNA (mRNA)
Occurs in the nucleus
Translation: RNA  protein
Ribosome are the site of translation

14. Flow of Genetic Information in Prokaryotes
In prokaryotes, mRNA produced by transcription is immediately translated without more processing
Why do you think that is? (Hint: what is missing?)

15. Flow of Genetic Information in Eukaryotes
In a eukaryotic cell, the nuclear envelope separates transcription from translation
Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA

16. The Genetic Code
How are the instructions for assembling amino acids into proteins encoded into DNA?
There are 20 amino acids, but there are only four nucleotide bases in DNA
So how many bases correspond to an amino acid?

17. Codons: Triplets of Bases
The Genetic Code
Codons: Triplets of Bases
The flow of information from gene to protein is based on a triplet code: a series of non-overlapping, three-nucleotide words
These triplets are the smallest units of uniform length that can code for all the amino acids
Found on the mRNA strand
Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced

18. The Genetic Code
During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript
During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction
Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

19. The Genetic Code
64 different codon combinations
Redundancy: 1+ codons code for each of 20 AAs
Reading frame: groups of 3 must be read in correct groupings
This code is universal: all life forms use the same code.

20. Concept Check
In RNA, _____ codon(s) translate to ______ amino acid(s) a. 1, 1 b. 3, 1 c. 3, 3 d. 1, 20
The sugar in RNA is _____, the sugar in DNA is _______ a. deoxyribose, ribose b. ribose, deoxyribose c. ribose, phosphate d. ribose, uracil
Which of the following is found on RNA but not DNA? a. uracil b. deoxyribose c. phosphate d. adenine
A stretch of chromosome that codes for a trait can be called a(n): a. chromatid b. replication fork c. gene d. base-pair
How many different amino acids are there? A. 3 B. 20 C D. an infinite number

21. Concept 17.2: Transcription is the DNA-directed synthesis of RNA

22. Transcription
Transcription unit: stretch of DNA that codes for a polypeptide or RNA (eg. tRNA, rRNA)
RNA polymerase:
Separates DNA strands and transcribes mRNA
mRNA elongates in 5’  3’ direction
Uracil (U) replaces thymine (T) when pairing to adenine (A)
Attaches to promoter (start of gene) and stops at terminator (end of gene)

23. LE 17-7 Elongation Non-template strand of DNA RNA nucleotides RNA
polymerase 3¢
3¢ end 5¢ 5¢
Direction of transcription
(“downstream”)
Template
strand of DNA
Newly made
RNA

24. Synthesis of an RNA Transcript
The three stages of transcription:
Initiation
Elongation
Termination

25. 1. Initiation
Bacteria: RNA polymerase binds directly to promoter in DNA

26. 1. Initiation Eukaryotes:
TATA box = DNA sequence (TATAAAA) upstream from promoter
Transcription factors must recognize TATA box before RNA polymerase can bind to DNA promoter

27. 2. Elongation
RNA polymerase adds RNA nucleotides to the 3’ end of the growing chain (A-U, G-C)

28. 2. Elongation
As RNA polymerase moves, it untwists DNA, then rewinds it after mRNA is made
As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
Transcription progresses at a rate of 60 nucleotides per second in eukaryotes
A gene can be transcribed simultaneously by several RNA polymerases

29. 3. Termination
RNA polymerase transcribes a terminator sequence in DNA, then mRNA and polymerase detach.
It is now called pre-mRNA for eukaryotes.
Prokaryotes = mRNA ready for use

30. Flow of Genetic Information in Prokaryotes vs. Eukaryotes

31. Concept 17.3: Eukaryotic cells modify RNA after transcription
Enzymes in the eukaryotic nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm

32. Additions to pre-mRNA:
5’ cap (modified guanine) and 3’ poly-A tail ( A’s) are added
Help export from nucleus, protect from enzyme degradation, assist ribosomes attach to the 5’ end

33. RNA Splicing
Pre-mRNA has introns (noncoding sequences) and exons (codes for amino acids)
Introns- stay IN the nucleus
Exons- are EXpressed
Splicing = introns cut out, exons joined together

34. RNA Splicing small nuclear ribonucleoproteins = snRNPs
snRNP = snRNA + protein
Pronounced “snurps”
Recognize splice sites
snRNPs join with other proteins to form a spliceosome
Spliceosomes catalyze the process of removing introns and joining exons
Ribozyme = RNA that acts as enzyme and can splice RNA

35. Why have introns?
Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing
Alternative RNA Splicing: produce different combinations of exons
One gene can make more than one polypeptide!
20,000 genes  100,000 polypeptides

36. Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide

37. Components of Translation
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
mRNA = message
tRNA = interpreter
Ribosome = site of translation

38. tRNA Transcribed in nucleus Specific to each amino acid
Transfer AA to ribosomes
Anticodon: pairs with complementary mRNA codon
Base-pairing rules between 3rd base of codon & anticodon are not as strict. This is called wobble.

39. tRNA Accurate translation requires two steps:
First step: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase
Second step: a correct match between the tRNA anticodon and an mRNA codon
Aminoacyl-tRNA-synthetase: enzyme that binds tRNA to specific amino acid

40. Ribosomes Ribosome = rRNA + proteins made in nucleolus 2 subunits
Large and small

41. Ribosomes Active sites: A site: holds AA to be added
P site: holds growing polypeptide chain
E site: exit site for tRNA

42. Building a Polypeptide
The three stages of translation:
Initiation
Elongation
Termination
All three stages require protein “factors” that aid in the translation process

43. 1. Initiation- brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
Small subunit binds to start codon (AUG) on mRNA
tRNA carrying Met attaches to P site
Proteins called initiation factors bring in the large subunit so the initiator tRNA occupies the P site

44. 2. Elongation- amino acids are added one by one to the preceding amino acid

45. 2. Elongation
Codon recognition: tRNA anticodon matches codon in A site

46. 2. Elongation
Peptide bond formation: AA in A site forms bond with peptide in P site

47. 2. Elongation
Translocation: tRNA in A site moves to P site; tRNA in P site moves to E site (then exits)

48. 2. Elongation Repeat over and over
Codon recognition: tRNA anticodon matches codon in A site
Translocation: tRNA in A site moves to P site; tRNA in P site moves to E site (then exits)
Peptide bond formation: AA in A site forms bond with peptide in P site

49. 3.Termination- when a stop codon in the mRNA reaches the A site of the ribosome
Stop codon reached and translation stops
Release factor binds to stop codon
Causes the addition of a water instead of an amino acid
polypeptide is released
Ribosomal subunits dissociate

50. Polyribosomes
A single mRNA can be translated by several ribosomes at the same time

51. Completing and Targeting the Functional Protein
Often translation is not sufficient to make a functional protein
Polypeptide chains are modified after translation
Completed proteins are targeted to specific sites in the cell

52. Protein Folding
During synthesis, polypeptide chain coils and folds spontaneously
Chaperonin: protein that helps polypeptide fold correctly

53. Targeting Polypeptides to Specific Locations
Types of Ribosomes-(are identical and can switch locations)
Free ribosomes: synthesize proteins that stay in cytosol and function there
Bound ribosomes (attached to ER): make proteins of endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vacuoles, plasma membrane) & proteins for secretion
Uses signal peptide to target location

54. Cellular “Zip Codes”
Signal peptide: 20 AA at leading end of polypeptide determines destination
Signal-recognition particle (SRP): binds to signal peptide, brings ribosome to ER

55. Concept Check- Quiz 2
RNA is synthesized on a DNA template in a process called ______, which utilizes the enzyme _______ a. translation, RNA polymerase b. transcription, DNA polymerase c. transcription, RNA polymerase d. replication, DNA polymerase
Given the following DNA strand, which of the following is its complementary mRNA? G G A C T G A T T a. C C T G A C T A A b. C C U G A C U A A c. G G A C T G A T T d. T T A G T C A G G
Once transcription has been completed, which of the following is NOT necessary for protein synethesis to occur? a. tRNA b. ribosomes c. mRNA d. DNA
Which of the following is NOT a necessary component of translation? a. anticodon b. mRNA c. ligase d. amino acid
Amino acids are joined together into a protein chain by which of the following? A. transfer RNA b. DNA polymerase c. hydrogen bonds d. messenger RNA

56. Concept 17.5: Point mutations can affect protein structure and function

57. The Central Dogma
Mutations happen here
Effects play out here

58. Mutations = changes in the genetic material of a cell
Large scale mutations: chromosomal; always cause disorders or death
nondisjunction, translocation, inversions, duplications, large deletions
Point mutations: alter 1 base pair of a gene
Base-pair substitutions – replace 1 with another
Missense: different amino acid, more common
Nonsense: stop codon, not amino acid
Frameshift – mRNA read incorrectly; nonfunctional proteins
Caused by insertions or deletions
have a disastrous effect on the resulting protein more often than substitutions do

59. Substitution = Missense- different amino acid

60. Substitution = Nonsense- change an amino acid codon into a stop codon

61. Substitution = Silent (no effect)

62. Insertion = Frameshift Mutation

63. Deletion = Extensive missense, premature termination

64. Pulmonary hypertension
Sickle Cell Disease
Symptoms
Anemia
Pain
Frequent infections
Delayed growth
Stroke
Pulmonary hypertension
Organ damage
Blindness
Jaundice
gallstones
Caused by a genetic defect
Carried by 5% of humans
Carried by up to 25% in some regions of Africa
Life expectancy
42 in males 48 in females

65. Sickle-Cell Disease = Point Mutation

66. Mutation occurs in the beta chain – have them look at their amino acid structures and think about why the change may be important

67. Sickle cell hemoglobin forms long, inflexible chains

69. Comparison: Prokaryotes vs. Eukaryotes

70. Prokaryote vs. Eukaryote

71. Prokaryotes vs. Eukaryotes
Transcription and translation both in cytoplasm
DNA/RNA in cytoplasm
RNA poly binds directly to promoter
Transcription makes mRNA (not processed)
No introns
Transcription in nucleus; translation in cytoplasm
DNA in nucleus, RNA travels in/out nucleus
RNA poly binds to TATA box & transcription factors
Transcription makes pre- mRNA  RNA processing  final mRNA
Exons, introns (cut out)

72. A Summary of Protein Synthesis (p. 348)
Most current definition for a gene: A region of DNA whose final product is either a polypeptide or an RNA molecule