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2.  The genetic material must meet 4 criteria
1. Information
2. Transmission
3. Replication
4. Variation
IDENTIFICATION OF DNA AS THE
GENETIC MATERIAL

3. • By the early 1900’s it was known that the
chromosomes carry the genetic (hereditary)
information
• Chromosomes consist of DNA (deoxyribonucleic
acid) and proteins

4. 1909 1911 1950194419291865
Gregor Mendel:
Introduces the concept of heredity

5. 1909 1911 1950194419291865
Wilhelm Johannsen:
Coins the term “Gene

6. 1909 1911 1950194419291865
Thomas Hunt Morgan:
Discovers that genes are responsible for inheritance

7.  Two strains of Streptococcus pneumoniae
 S  Smooth
○ Secrete a polysaccharide capsule
○ Produce smooth colonies on solid media
○ virulent
 R  Rough
○ Unable to secrete a capsule
○ Produce colonies with a rough appearance
○ avirulent
 Two coat types
 II
 III
 Four possible phenotypes
 IIS or IIIS or IIR or IIIR
1928 - Frederick Griffith’s Transformation Experiments

8. Griffith’s Experiments
9-6
 Rare mutations convert strains
 S into R strain & vice versa
 But mutations do not change coat types
 II into III or vice versa

9. 9-8

10. 9-9
 Something from the dead type IIIS transformed
type IIR into type IIIS
 Called this process transformation
 The unknown substance was termed the
transforming principle
Griffith’s Conclusions

11. 1909 1911 1950194419291865
Phoebus Levene:
Discovers that DNA is made up of nucleotides, phosphates,
sugars and 4 bases

12. 1909 1911 1950194419291865
Oswald Avery:
Shows that DNA can transform the property of cells
However, this idea was not
universally accepted

13.  Griffith’s transforming principle was the
genetic material
 Transformation assay to identify actual
biomolecule
 Major constituents - DNA, RNA, proteins,
carbohydrates, & lipids
 Made cell extracts from type IIIS cells
containing each of these macromolecules
1944 - Avery, MacLeod & McCarty
Identify the Genetic Material

14.  Mixed each extract with type IIR cells to test for transformation
 Only extract containing purified DNA transformed type IIR to type IIIS
 Verify that DNA, not RNA or protein, is the genetic material
Avery’s Experiments

15. 1909 1911 1950194419291865
Erwin Chargaff:
Shows that: A + G = T + C = 50%

16. • Used data of Erwin Chargaff, 1940’s and
early 50's
• Chargaff’s Rule: His data showed that in each
species, the percent of A equals the percent of
T, and the percent of G equals the percent of
C.
• Watson was shown this picture by Wilkins in
early 1953.
From the picture it was possible to calculate:
1) the distance between bases (3.4A)
2) the length of the period (34A)
3) the rise of the helix (36 degrees)

17. Franklin’s
Work
In 1951 Rosalind Franklin discovers the Two
Forms of DNA through her X-ray diffraction.
A – Dry Form B – Wet Form

18. X-Ray
Crystallography

19. 1952- Hershey & Chase Confirm DNA is
Genetic Material Using Phage T2

20. Life cycle of
phage T2

21. Phage Attached to E. coli
Cell

22. The Hershey and Chase experiment:
 Used radioisotopes to distinguish DNA from
proteins
 32P labels DNA specifically
 35S labels protein specifically
 Infect non-radioactive E. coli with radioactively-
labeled phages
 Remove phage coats from cells
 Is 32P or 35S inside bacteria?

23. Hershey & Chase Experiment
S

24. Hershey & Chase Experiment

25. The Data
9-18

26.  In 1956, A. Gierer and G. Schramm isolated RNA
from the tobacco mosaic virus (TMV)
 Purified TMV RNA caused the same lesions as
intact TMV viruses
 Therefore, the viral genome is composed of RNA
 Since that time, many RNA viruses have been
found
RNA is Genetic Material in Some Viruses

27. DNA Structure, Functions
and Properties

28. DNA
•DNA - a polymer of deoxyribo
nucleotides
•found in chromosomes, mitochondria and
chloroplasts
• carries the genetic information
28

29. Components of a nucleotide
Base
Sugar
Phosphate

30. Nucleotide
Nucleoside
Base
Phosphate
Sugar
X=H: DNA
X=OH: RNA

31. Basic structure of
pyrimidine and purine

32. Pyrimidines
32

33. Purines

34. Nomenclature of Nucleic Acid Components
Base Nucleoside Nucleotide Nucleic
acid
Purines
Adenine Adenosine Adenylate RNA
Deoxyadenosine Deoxyadenylate DNA
Guanine Guanosine Guanylate RNA
Deoxy guanosine Deoxyguanylate DNA
Pyrimidines
Cytosine Cytidine Cytidylate RNA
Deoxycytidine Deoxycytidylate DNA
Thymine Thymidine Thymidylate DNA
(deoxythymidine) (deoxythymidylate)
Uracil Uridine Uridylate RNA

35. 5-Fluorouracil 6-Mercaptopurine
Anticancer agents
Azidothymidine Dideoxyinosine
Antiretroviral agents
Nucleoside and base analogs can be
used as anti-cancer and anti-virus drugs

36. The primary structure of
DNA is the sequence
5’ end
3’ end
5’
3’
Phosphodiester
linkage

37. Traditionally, a DNA sequence is
drawn from 5’ to 3’ end.
A shorthand notation for this
sequence is ACGTA

38. The secondary structure of
DNA is the double helix

39. The secondary structure of
DNA
Two anti-parallel polynucleotide
chains wound around the same axis.
Sugar-phosphate chains wrap
around the periphery.
Bases (A, T, C and G) occupy the
core, forming complementary A · T
and G · C Watson-Crick base pairs.

40.  hydrogen bonding;
 base stacking
The DNA double helix is held together
mainly by- Hydrogen bonds

41. Hydrogen bond
a chemical bond in which a hydrogen atom of
one molecule is attracted to an electronegative
atom, especially a nitrogen, oxygen, or fluorine
atom, usually of another molecule.

42. Two hydrogen bonds between A:T pairs
Three hydrogen bonds between C: G paired

43. Base Stacking
The bases in DNA are
planar and have a
tendency to "stack".
Major stacking forces:
hydrophobic interaction
van der Waals forces.

44. Variations in form of DNA
 Most of the DNA - B-DNA or B-form
DNA.
 In certain condition, different forms
of DNAs are found
 A-DNA,Z-DNA,C-DNA,D-DNA,E-DNA.
 This deviation in forms are based on
their structural diversity.

45. Variations in form of DNA
 B-DNA: Most common ,originally deduced from
X-ray diffraction of sodium salt of DNA fibres
at 92% relative humidity.
 A-DNA: Originally identified by X-ray diffraction
of analysis of DNA fibres at 75% relative
humidity
 Z-DNA: Left handed double helical structure
winds to the left in a zig- zag pattern .

46.  C-DNA: Formed at 66% relative humidity
and in presence of Li+ and Mg2+ ions.
 D-DNA: Rare variant with 8 base pairs per
helical turn ,form in structure devoid of
guanine .
 E- DNA: Extended or eccentric DNA.

47. 47

48. B-DNA
 Described by James D. Watson & Francis crick.
 Commonly found in DNA.
 DNA molecule consists of 2 helical
polynucleotide chains coiled around common
axis.
 2 helices are wound in such a way so as to
produce 2 interchain spacing or groove –
 Major/wide groove(width 12A°,depth 8.5A°)
 Minor /narrow groove(width 6A°,depth 7.5A°)
 These grooves provide surface with which
proteins,chemicals,drugs can interact.

50.  2 helical wind along the molecules
 2 chains run in opposite direction, they are
antiparallel, the plane of bases are
perpendicular to helix axis.
 Right handed twisting
 Uniform diameter (20A°)
 Complementary base pairing
 Base pair per turn is 10.4
 Rise per base pair is 3.4A°

51. A-DNA
 A-DNA is one of the possible double
helical structure which DNA can adopt
along with other two biologically active
helix structure(B-DNA,Z-DNA).
 Right handed double helix .
 Short and fatty compared to B-DNA.
 Occur only in dehydrated sample of DNA
,Such those used in crystallographic
experiments.

52.  A-DNA was originally identified by X-ray
diffraction analysis of DNA fibres at 75%
relative humidity.
 The grooves are not as deep in B-DNA.
 The bases are more tilted
 The base pairs per turn is 11.
 Rise per base pair is 2.3A°.

54. Z-DNA
 Left handed double helix structure winds
to left in zig-zag manner.(DNA
backbone were in zig-zag manner)so
they are termed as Z-DNA.
 Discovered by Rich, Nordheim &Wang in
1984.
 It has antiparallel strands as B-DNA.
 It is long and thin as compared to B-
DNA.

55.  adjacent sugar have alternating orientation
(against B-DNA which has same orientation).
 Purines: syn confirmation (bases & sugar are
near & on same side)
 pyramidines: anti (bases & sugar are distant,
on opposite sides)
 Only one deep helical grooves.
 There are 12 base pairs per turn with axial
rise 3.8A° & angle of twist 60°.

57. C-DNA
 C-DNA formed at 66% relative humidity
(low)in presence of Li+ or Mg2+.
 Right handed ,with axial rise of 3.32A°
per base pair .
 9.33 base pairs per turn.
 Helical pitch 3.32A°×9.33°A=30.97A°.
 Base pair rotation=38.58°.
 Has diameter of19A°,smaller than that
of A-&B-DNA.
 The tilt of base is 7.8°

59. D-DNA
 Extremely rare variant with only 8base
pairs per helical turn .
 This forms of DNA found in some DNA
molecules devoid of guanine.
 Axial rise of 3.03A°per base pairs .Tilt of
16.7° from axis of helix.
 Actually 2 different forms of D-DNA
1. D(A):Takes part in D-A-B transition.
2. D(B):Associated with D-B change of
confirmation.
 2 DNA structure have same helical
parameters.

60. Structure devoid of guanine

61. E-DNA
 Cytosine methylation of or bromination
of DNA sequence d(GGCGCC)2 is to
induce a novel extended &eccentric
double helix, which we call E-DNA.
 E-DNA has a long helical axis rise and
base perpendicular to the helical axis.
 Deep major groove and shallow minor
groove.
 E-DNA allowed to crystallize for a
period time longer, the methylated
sequence forms standard A-DNA.

62.  E-DNA is the intermediate in the transition
toA-DNA.
 E-DNA is the intermediate in the
crystallographic pathway from B-DNA to
A-DNA.

63. CHARACTRISTICS A-DNA B-DNA C-DNA Z-DNA
CONDITIONS 75% RELATIVE
HUMIDITY;NA+,K
+,Cs IONS
92% R.H:;LOW
ION STRENGTH
60%R.H;Li/Mg
IONS
VERY HIGH SALT CONC.
SHAPE BROADEST INTERMEDIATE NARROW NARROWEST
HELIX SENSE RIGHT RIGHT RIGHT LEFT
HELIX DIAMETER 25.5A° 20.7A° 19.0A° 18.4A°
RISE PER BASE
PAIR(H)
2.3A° 3.4A° 3.32A° 3.8A°
BASE PAIR PER
TURN(N)
11 10.4 9.33 12
HELIX
PITCH(H×N)
25.5A° 35.36A° 30.97A° 45.60A°
ROTATION PER
BASE PAIR
+32.72° +34.61° +38.58° -60°
BASE PAIR TILT 19° 1° 7.8° 9°
GLYCOSIDIC
BOND
ANTI ANTI _ ANTI FOR C,T. SYN FOR A,G.
MAJOR GROOVE NARROW &VERY
DEEP
WIDE & QUITE
DEEP
_ NO
MINOR GROOVE VERY BROAD &
SHALLOW
NARROW & QUITE
DEEP
_ VERY NARROW & DEEP

64. Does DNA fit the requirements of a hereditary material?
Structure
REQUIREMENT DNA Component
Has biologically useful
information to make protein
Genetic code: 3 bases code
for 1 amino acid(protein)
Must reproduce faithfully
and transmit to offspring
Complementary bases are
faithful: found in germ cells
Must be stable within a living
organism
Backbone is strong covalent :
hydrogen bonds
Must be capable of
incorporating stable changes
Bases can change through
known mechanisms

65. In eukaryotic cells,
DNA is folded into chromatin

66. DNA Tertiary Structure
•DNA DOUBLE HELICAL STRUCTURE COILS ROUND
HISTONES.
•DNA BOUND TO HISTONES FORMS
NUCLEOSOMES (10nm FIBRES)
•NUCLEOSOMES CONTAIN 146 NUCLEOTIDES

67. Nucleosomes
any of the repeating globular subunits of
chromatin that consist of a complex
of DNA and histone

68. Structure of nucleosome core
68Biochemistry for Medics

69. Compaction of DNA in a eukaryotic chromosome
69Biochemistry for Medics

70. Supercoil = coil over coil
70Biochemistry for Medics

71. DNA melting and annealing
71Biochemistry for Medics

72. Reversible denaturation and annealing of DNA
72Biochemistry for Medics

73. Melting point (tm) of DNA
The temperature at the mid-point of the transition
73Biochemistry for Medics

74. The tm of DNA depends on:
 size of DNA
 pH
 ionic strength
 Content of G·C base pairs
74Biochemistry for Medics

75. 75Biochemistry for Medics
Functions of DNA and
summary of structure
DNA consists of four bases—A, G, C, and T—that are held in
linear array by phosphodiester bonds through the 3' and 5'
positions of adjacent deoxyribose moieties.
DNA is organized into two strands by the pairing of bases A to T
and G to C on complementary strands. These strands form a
double helix around a central axis.
The 3 x 109 base pairs of DNA in humans are organized into the
haploid complement of 23 chromosomes.
DNA provides a template for its own replication and thus
maintenance of the genotype and for the transcription of the
roughly 30,000 human genes into a variety of RNA molecules.