BIOL 1010 Lecture Notes - Telomere, Isotopes Of Nitrogen, S Phase

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8 Nov 2012
DNA Replication
Lecture 29-30
- DNA replication and the cell cycle
- DNA is reproduced by Semiconservative replication
o Conservation of the original helix
o The Meselson-Stahl Experiment
o Semiconservative replication in Eukaryotes
o Origins, Forks and Units of Replication
- Enzymes involved in DNA synthesis in Bacteria
o DNA polymerase I
o Synthesis of Biologically Active DNA
o DNA polymerase II and III
DNA is hereditary material
- In 1928, Frederick Griffith showed that genetic material is a specific
- Two distinguishable strains of Streptococcus pneumonia (a bacterium that
causes pneumonia in mammals): with smooth colonies (S) and rough
colonies (R)
- Cells of the smooth strain were encapsulated with a polysaccharide coat
and cells of the rough strain were not
- These alternative phenotypes were inherited
Transformation of bacteria
- The S strain of the bacterium, which was protected from a mouse’s
defensive system by a capsule, was pathogenic;
- The R strain, a mutant lacking the capsule, was nonpathogenic;
- Heat-killed S cells were harmless
- A moisture of heat-killed S cells and live R cells caused pneumonia and
- Live S bacteria could be retrieved from the dead mice injected with the
- Griffith concluded that molecules from the dead S cells had genetically
transformed some of the living R bacteria into S bacteria
- In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod discovered
that the transforming agent had to be DNA
- More evidence came from
studies of bacteriophages-
viruses that infect bacteria
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The Hershey-Chase Experiment
- Viral proteins labeled with radioactive sulfur,
remained outside the host cell during infection.
- 32P labeled viral DNA entered the bacterial cell
Additional evidence that DNA is genetic material
- A eukaryotic cells doubles its DNA content prior
to mitosis
- During mitosis, the doubled DNA is equally
divided between two daughter cells
- An organism’s diploid cells have twice the DNA
as its haploid gametes
- In 1947, Erwin Chargaff analyzed the DNA content
of different organisms:
o DNA composition is species-specific
o The amount of ratios of nitrogenous bases
vary from one species to another
- The number of adenine (A) residues approximated
the number of thymine’s (T), same was true for
guanines (G) and cytosine’s (C)
- The A=T and G=C equalities became known alter as
Chargaff’s rules
Rosalind Franklin’s experiments
- DNA is helix with a uniform width of 2 nm
- Purine and pyrimidine bases are stacked .34nm
- The helix makes one full turn every 3.4 nm along it’s
- There are ten layers of nitrogenous base pairs in
each turn of the helix
Watson and Crick- building models to confirm to x-ray
- To be consistent with a 2nm width,
a purine on one strand must pair
(by hydrogen bonding_, with a
pyrimidine on the other
antiparallel helix
The double helix
- In April 1953- new model for DNA
structure, the double helix was published
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