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Chapter 14

BIOL 1107 Chapter Notes - Chapter 14: Maclyn Mccarty, Enterobacteria Phage T2, Frederick Griffith

Course Code
BIOL 1107
Thomas Abbot

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14.1 Establishing DNA as the Hereditary Molecule
Frederick Griffith (1928)- experimented with Streptococcus pneumoniae (severe form of pneumonia in
mammals), used 2 strains, smooth (s) which is virulent/highly infective and rough (r) which is
S killed mice in a day or 2 but R did not affect mice
S was smooth because of capsule that surrounded cell and gave it a smooth/shiny appearance
Some mice were infected with heat-killed S cells and live R cells, and still died
Dead S cells released some molecules that could genetically change R cells to S cells
(molecule released = transforming principle)
Oswald Avery, Colin MacLeod, Maclyn McCarty (1940s)- performed experiment to identify chemical
nature of this transforming principle, reproduced transformation with bacteria in culture tubes instead of
Used heat to kill S bacteria and then used enzymes to break down the extracted hereditary
material (protein, DNA, RNA)
Destroying protein or RNA had no effect but destroying DNA stopped the transformation
of R to S cells and no smooth colonies were seen
Published results identifying DNA as the transforming principle (most agreed but some
said protein was the genetic material responsible)
Alfred Hershey, Martha Chase (1952)- studied infection of bacterium Escherichia coli by bacteriophage
Bacteriophage (phage): virus (infectious agent made of either DNA or RNA surrounded by
protein coat) that infects bacteria
Phage life cycle begins when phage attaches to surface of bacterium and infects it
Some (like T2) stop cell from producing its own molecules and use cell’s
resources to produce progeny
After 100-200 phages are assembled in cell, viral enzyme breaks cell wall and
releases them, process takes about 30 minutes
T2 phage core only consists of DNA surrounded by proteins, so it had to be one of them that was
entering cell and directing infective cycle
Experimental approach was to label each and follow through phage life cycle
DNA contains phosphorus but not sulfur, so they used radioactive phosphorus as a tag
Proteins contain sulfur but not phosphorus, so they used radioactive sulfur as a tag
Results showed that most of labeled DNA entered cell, affirming that genetic material is DNA,
not protein
Transformation: alteration of a cell’s hereditary type by the uptake of DNA released by the breakdown
of another cell
14.2 DNA Structure
DNA structure was explained by Watson and Crick in 1953
DNA contains 4 nucleotides (deoxyribonucleotides because sugar is deoxyribose)
Each deoxyribonucleotide consists of the 5 carbon sugar deoxyribose
Carbon atoms are numbered with primes from 1’ to 5’, a phosphate group, and one of the
4 nitrogenous bases
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Purines- adenine and guanine, built from pair of fused rings of carbon and
nitrogen atoms
Pyrimidines- thymine and cytosine, built from single carbon-nitrogen ring
Chargaff’s Rules
Erwin Chargaff measured amounts of nitrogenous bases in DNA and discovered definite ratios
amount of adenine = amount of thymine
amount of guanine = amount of cytosine
Ratio of guanine + cytosine : adenine + thymine was species-specific
Sugar-phosphate backbone: structure in a polynucleotide chain that is formed when deoxyribose sugars
(DNA) or ribose sugars (RNA) are linked by phosphate groups in an alternating sugar-phosphate-sugar-
phosphate pattern
Each phosphate group is a bridge between the 3’ carbon of one sugar and the 5’ carbon of the
Entire linkage, including bridging phosphate group, is called a phosphodiester bond
Polynucleotide chain of DNA has polarity, directionality
5’ end: phosphate group is bound to 5’ carbon of a deoxyribose sugar
3’ end: hydroxyl group is bonded to 3’ carbon of a deoxyribose sugar
Watson and Crick analyzed research data of others, especially Maurice H. F. Wilkins and Rosalind
Franklin, who were using X-ray diffraction to study structure
X-ray diffraction: X-ray beam is directed at molecule in the form of a regular solid, ideally crystal form
Within crystal, regularly arranged atoms bend and reflect the X-rays into smaller beams that exit
the crystal at definite angles determined by atoms in crystal
If X-ray film is placed behind crystal, exiting beams produce a pattern of exposed spots
Patterns obtained by Wilkins and Franklin indicated that DNA molecules within fiber were cylindrical
and about 2 nm in diameter
X-shaped distribution indicated helical structure
Watson and Crick constructed scale models of the 4 DNA nucleotides and fitted them together till the
arrangement satisfied both Wilkins’ and Franklin’s data and Chargaff’s analysis
Trials led to double-helix model
Double-helix model
DNA is double-stranded, with 2 polynucleotide chains wound around each other in a right-
handed double helix with a 2nm diameter
Sugar-phosphate backbones are on outside of helix with base pairs on the inside
The strands are antiparallel (have opposite polarity), run in opposite directions
Connecting the 2 backbones are base pairs (rungs of spiral ladder) and a purine-pyrimidine pair
is exactly wide enough to fill the space (A-T and G-C)
Complementary base pairing
Base pairs are stabilized by hydrogen bonds
Base pairs lie in flat planes almost perpendicular to long axis of DNA molecule
Each base pair occupies 0.34 nm of length along long axis, 10 base pairs are stacked in a
full turn of DNA, double helix takes up 3.4 nm along length of molecule
DNA has major and minor grooves, within which base pairs are accessible
Hereditary material is coded into DNA by the particular sequence of the nucleotides
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