2. 9.1 The Griffith Experiment
Mendel’s work left a key question unanswered:
What is a gene?
The work of Sutton and Morgan established that genes reside on chromosomes
But chromosomes contain proteins and DNA
So which one is the hereditary material
Several experiments ultimately revealed the nature of the genetic material

3. 9.1 The Griffith Experiment
In 1928, Frederick Griffith discovered transformation while working on Streptococcus pneumoniae
The bacterium exists in two strains S
Forms smooth colonies in a culture dish
Cells produce a polysaccharide coat and can cause disease R
Forms rough colonies in a culture dish
Cells do not produce a polysaccharide coat and are therefore harmless

4. Fig. 9.1 How Griffith discovered transformation
Thus, the dead S bacteria somehow “transformed” the live R bacteria into live S bacteria

5. 9.2 The Avery and Hershey-Chase Experiments
Two key experiments that demonstrated conclusively that DNA, and not protein, is the hereditary material
Oswald Avery and his coworkers Colin MacLeod and Maclyn McCarty published their results in 1944
Alfred Hershey and Martha Chase published their results in 1952

6. The Avery Experiments
Avery and his colleagues prepared the same mixture of dead S and live R bacteria as Griffith did
They then subjected it to various experiments
All of the experiments revealed that the properties of the transforming principle resembled those of DNA
1. Same chemistry and physical properties as DNA
2. Not affected by lipid and protein extraction
3. Not destroyed by protein- or RNA-digesting enzymes
4. Destroyed by DNA-digesting enzymes

7. The Hershey-Chase Experiment
Viruses that infect bacteria have a simple structure
DNA core surrounded by a protein coat
Hershey and Chase used two different radioactive isotopes to label the protein and DNA
Incubation of the labeled viruses with host bacteria revealed that only the DNA entered the cell
Therefore, DNA is the genetic material

8. Thus, viral DNA directs the production of new viruses
Fig The Hershey-Chase Experiment
Thus, viral DNA directs the production of new viruses

9. 9.3 Discovering the Structure of DNA
DNA is made up of nucleotides
Each nucleotide has a central sugar, a phosphate group and an organic base
The bases are of two main types
Purines – Large bases
Adenine (A) and Guanine (G)
Pyrimidines – Small bases
Cytosine (C) and Thymine (T)

10. Fig. 9.3 The four nucleotide subunits that make up DNA
Nitrogenous base
5-C sugar

11. Erwin Chargaff made key DNA observations that became known as Chargaff’s rule
Purines = Pyrimidines
A = T and C = G
Rosalind Franklin ( )
Fig. 9.4
Rosalind Franklin’s X-ray diffraction experiments revealed that DNA had the shape of a coiled spring or helix

12. In 1953, James Watson and Francis Crick deduced that DNA was a double helix
They came to their conclusion using Tinkertoy models and the research of Chargaff and Franklin
Fig. 9.4
James Watson ( )
Francis Crick ( )

13. Fig. 9.4 The DNA double helix
Dimensions suggested by X-ray diffraction
The two possible basepairs

14. 9.4 How the DNA Molecule Replicates
The two DNA strands are held together by weak hydrogen bonds between complementary base pairs
A and T
C and G
ATACGCAT
If the sequence on one strand is
The other’s sequence must be
TATGCGTA
Each chain is a complementary mirror image of the other
So either can be used as template to reconstruct the other

15. There are 3 possible methods for DNA replication
Fig. 9.5
Daughter DNAs contain one old and one new strand
Old and new DNA are dispersed in daughter molecules
Original DNA molecule is preserved

16. These three mechanisms were tested in 1958 by Matthew Meselson and Franklin Stahl
Fig. 9.6

17. Thus, DNA replication is semi-conservative
Fig. 9.6

18. How DNA Copies Itself
The process of DNA replication can be summarized as such
The enzyme helicase first unwinds the double helix
The enzyme primase puts down a short piece of RNA termed the primer
DNA polymerase reads along each naked single strand adding the complementary nucleotide

19. Fig. 9.7 How nucleotides are added in DNA replication
Template strand
New strand
Sugar-
phosphate
backbone C G T A HO 3’ O 5’ OH P
Template strand
New strand
Pyrophosphate G C T A HO 3’ O 5’ OH P
DNA polymerase T O OH P

20. DNA polymerase can only build a strand of DNA in one direction
The leading strand is made continuously from one primer
The lagging strand is assembled in segments created from many primers
Fig. 9.8

21. RNA primers are removed and replaced with DNA
Ligase joins the ends of newly-synthesized DNA
Fig. 9.9
Mechanisms exist for DNA proofreading and repair

22. 9.5 Transcription
The path of genetic information is often called the central dogma
DNA
RNA
Protein
A cell uses three kinds of RNA to make proteins
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)

23. 9.5 Transcription
Gene expression is the use of information in DNA to direct the production of proteins
It occurs in two stages
Fig. 9.10

24. 9.5 Transcription The transcriber is RNA polymerase
Fig. 9.11
The transcriber is RNA polymerase
It binds to one DNA strand at a site called the promoter
It then moves along the DNA pairing complementary nucleotides
It disengages at a stop signal

25. 9.6 Translation
Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein
The rules that govern translation are called the genetic code
mRNAs are the “blueprint” copies of nuclear genes
mRNAs are “read” by a ribosome in three-nucleotide units, termed codons
Each three-nucleotide sequence codes for an amino acid or stop signal

26. The genetic code is (almost) universal
Fig. 9.12
The genetic code is (almost) universal
Only a few exceptions have been found

27. Sites play key roles in translation
Ribosomes
The protein-making factories of cells
They use mRNA to direct the assembly of a protein
Fig. 9.13
A ribosome is made up of two subunits
Each of which is composed of proteins and rRNA
Sites play key roles in translation

28. Hydrogen bonding causes hairpin loops
Transfer RNA
Hydrogen bonding causes hairpin loops
Fig. 9.14
tRNAs bring amino acids to the ribosome
They have two business ends
Anticodon which is complementary to the codon on mRNA
3’–OH end to which the amino acid attaches
3-D shape

29. Making the Protein mRNA binds to the small ribosomal subunit
The large subunit joins the complex, forming the complete ribosome
mRNA threads through the ribosome producing the polypeptide
Fig. 9.16

30. The process continues until a stop codon enters the A site
Fig How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released

31. 9.7 Architecture of the Gene
In eukaryotes, genes are fragmented
They are composed of
Exons – Sequences that code for amino acids
Introns – Sequences that don’t
Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript
This transcript is then heavily processed to produce the mature mRNA transcript
This leaves the nucleus for the cytoplasm

32. Protect from degradation and facilitate translation
Fig Processing eukaryotic mRNA
Protect from degradation and facilitate translation
Different combinations of exons can generate different polypeptides via alternative splicing

33. Small ribosomal subunit Large ribosomal subunit
Cytoplasm
Nuclear membrane
tRNA
Amino acid
4. tRNA molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mRNA.
6. The polypeptide chain grows until the protetin is completed.
Completed polypeptide
7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated.
5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site. 5’ 3’
Ribosome
Ribosome moves toward 3’ end
Fig How protein synthesis works in eukaryotes
DNA
RNA polymerase
Primary
RNA transcript 5’ 3’
1. In the cell nucleus, RNA polymerase transcribes RNA from DNA 5’
mRNA
Cap
Nuclear pore
Poly-A tail
3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA
Small ribosomal subunit
Large ribosomal subunit 5’ 3’
Cap
Poly-A tail
2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA
Introns
mRNA
Exons

34. 9.7 Architecture of the Gene
Most eukaryotic genes exist in multiple copies
Clusters of almost identical sequences called multigene families
As few as three and as many as several hundred genes
Transposable sequences or transposons are DNA sequences that can move about in the genome
They are repeated thousands of times, scattered randomly about the chromosomes

35. 9.8 Turning Genes Off and On
Genes are typically controlled at the level of transcription
In prokaryotes, proteins either block or allow the RNA polymerase access to the promoter
Repressors block the promoter
Activators make the promoter more accessible
Most genes are turned off except when needed

36. The lac Operon
An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit
The lac operon contains
Three structural genes
Encode enzymes involved in lactose metabolism
Two adjacent DNA elements
Promoter
Site where RNA polymerase binds
Operator
Site where the lac repressor binds

37. The lac Operon
In the absence of lactose, the lac repressor binds to the operator
RNA polymerase cannot access the promoter
Therefore, the lac operon is shut down
Fig. 9.19

38. The lac Operon
In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor
This induces a change in the shape of the repressor which makes it fall off the operator
RNA polymerase can now bind to the promoter
Transcription of the lac operon is ON

39. Fig. 9.19

40. The lac Operon
What if the cell encounters lactose, and it already has glucose?
The bacterial cell actually prefers glucose!
The lac operon is also regulated by an activator
The activator is a protein called CAP
It binds to the CAP-binding site and gives the RNA polymerase more access to the promoter
However, a “low glucose” signal molecule has to bind to CAP before CAP can bind to the DNA

41. Fig. 9.20 Activators and repressors of the lac operon

42. Enhancers
DNA sequences that make the promoters of genes more accessible to many regulatory proteins at the same time
Fig. 9.21
Usually located far away from the gene they regulate
Common in eukaryotes; rare in prokaryotes

43. 9.9 Mutation The genetic material can be altered in two ways
Recombination
Change in the positioning of the genetic material
Mutation
Change in the content of the genetic material
Fig. 9.22
Bithorax mutant

44. 9.9 Mutation
Mutation and recombination provide the raw material for evolution
Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives
The rate of evolution is ultimately limited by the rate at which these alternatives are generated
Mutations in germ-line tissues can be inherited
Mutations in somatic tissues are not inherited
They can be passed from one cell to all its descendants

45. Kinds of Mutation Mutations are caused in one of two ways
Errors in DNA replication
Mispairing of bases by DNA polymerase
Mutagens
Agents that damage DNA

46. Kinds of Mutation
The sequence of DNA can be altered in one of two main ways
Point mutations
Alteration of one or a few bases
Base substitutions, insertion or deletion
Frame-shift mutations
Insertions or deletions that throw off the reading frame

47. Fig. 9.23

48. Kinds of Mutation
The position of genes can be altered in one of two main ways
Transposition
Movement of genes from one part of the genome to another
Occurs in both eukaryotes and prokaryotes
Chromosomal rearrangements
Changes in position and/or number of large segments of chromosomes in eukaryotes

51. Mutation, Smoking and Lung Cancer
Agents that cause cancer are called carcinogens
These are typically mutagens
The hypothesis that chemicals cause cancer was first advanced in the 18th century
Many investigations since then have determined that chemicals can cause cancer in both animals and humans
For example, tars and other chemicals in cigarette smoke can cause cancer of the lung