Post on 22-Nov-2014
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1953: James Watson and Francis Crick create double helical model for DNA
After Morgan showed genes are located on chromosomes, it sparked the debate of
DNA and protein being the candidates for the genetic material
Much was known about proteins
o Had great heterogeneity and specificity of function
Essential for the hereditary material
Little was known about nucleic acids
o Their chemical and physical properties seemed to uniform to account for the
multitude of inherited traits
Frederick Griffith: 1928 British Medical Officer
o Studied streptococcus pneumoniae- pneumonia causing bacteria
o Had two strains- a pathogenic and a harmless
o When he killed the pathogenic strain with heat and mixed with the harmless
strain , some cells transformed into pathogenic cells
Transformation: a change in genotype and phenotype due to
assimilation of external DNA by a cell
Oswald Avery: 1944 American Bacteriologist
o Built on Griffith’s work and proved that DNA was the responsible agent for
transformation
o Colleagues: Maclyn McCarty and Colin MacLeod
o Experiment: Separate each part of the bacteria and test one by one
o Skepticism arose because not much was known about DNA
Viruses provided additional evidence for DNA as the genetic material
o To reproduce, viruses must infect and take over a cell’s metabolic machinery
o Bacteriophages (phages): Viruses that infect bacteria
1952: Alfred Hershey and Martha Chase
o Showed that DNA was the genetic material of a phage was known as T2
T2 was all DNA and a little protein
Proved that DNA was the genetic material by growing T2 with E. Coli
Two sets of petri dishes were prepped- one with Sulfure-35 to
stain the protein and one with Phosphorous-32 to stain DNA
After letting the T2 and E. Coli mingle, they noticed that the
inside of the E. Coli cells were blue- showing that DNA entered
the cell
Erwin Chargraff: Biochemist
o Analyzed the base composition of the DNA from multiple euakryotes
Noticed that all the four bases (A, C, T, G) were present in eukaryotes,
but in characteristic ratios that varied from eukaryote to eukaryote
Chargraff’s Rule: The amount of Adenine approximately equaled the
amount of Thymine and the amount of Guanine approximately
equaled the amount of Cytosine
Once biologists accepted DNA as the genetic material, they needed to know it’s
structure
o Watson and Crick used X-ray crystallography pictures from Rosalind
Franklin
o Watson was familiar with the types of patterns that helical molecules
produce in X- ray crystallographies
The picture told Watson that the structure was helical and enabled
him to deduce the width of the helix and the spacing of the
nitrogenous bases along it
The width showed that the helix was two stranded
o The crystallography showed that the helix makes one full turn every 3.4
nanometer (nm)
Bases are stacked 0.34 nm apart
10 layers of base pairs in each turn on the helix
o Adenine pairs with Thymine
o Guanine pairs with Cytosine
Purine + Purine= too wide
Pyrimadine + Pyrimadine= too narrow
Purine + Pyrimadine= width consistent with X-ray data
The Watson- Crick model proved Chargraff’s rule
When a cell copies a DNA molecule, each strand serves as a blueprint, demanding
certain nucleotides to make a complementary strand.
o Semiconservative Model: Type of DNA replication in which the replicated
double helix consists of one old strand, derived from the old molecule, and
one newly made strand
Humans have about 6 billion base pairs per cell
o Only takes a few hours to copy all this DNA
o Only a few errors per 1 billion nucleotides
More than a dozen enzymes and proteins participate in DNA replication
DNA replication starts at special sites
o Origins of replication: special sites on DNA where replication starts
o Proteins attach to DNA and begin to unzip the DNA into two strands
o Replication occurs from both top to bottom and bottom to top
simultaneously
o DNA Polymerases: enzymes that catalyze the replication of DNA strands
o Rate of replication is 500 million nucleotides per second
Human replication occurs at 50 million nuecleotides per second
o Nucleoside triphosphates serve as substrates for DNA Polymerase
The two DNA sugar- phosphate backbones run anti-parallel to eachother (parallel
but in opposite
directions)
Picture showing the antiparallel
structure of the DNA Double Helix
Structure
The numbers 3 and 5 refer to the carbons in the the backbone.
Leading Strand: The new continuous complementary DNA strand synthesized
along the template strand in the mandatory 5’3’ direction
Lagging Strand: A discontinuously synthesized DNA strand that elongates in a
direction away from the replication fork
o First synthesized as a series of pigments- known as Okazaki fragments
o Pigments are about 100-200 nucleotides in length
DNA ligase joins the sugar-phosphate backbones of the Okazaki Fragments to create
a single DNA strand
Replication of cellular DNA:
Helicase is an enzyme that untwists the double helix at the replication fork.
Summary of DNA Replication: