Chapter 1-DNA Structure and Replication
~DNA:
~constitutes the genetic material
~DNA structure: double helix formed of antiparallel strands
~semi-conservative: each copy has one parental and one new strand
~bidirectional
~methods using simplified DNA replication steps:
~PCR and DNA sequencing
~Griffith’s experiment-1928:
~established there is transformation factor that transforms bacteria from non-lethal strain to lethal,
virulent strain
~two types of Pneumoccocus
~virulent type is S type; S type if injected kills mouse
~non-virulent is R type; bacteria killed by mouse
~can heat the S strain to kill bacteria; the heat inactivates the bacteria and mice survive
~mixed heat-killed S type and living R type and could recreate a lethal type of bacteria
~transformation factor transforms R type into S type that comes from heat-killed S type
~the R type in blood acquire a factor from S type that transforms them into S and kills the mice
~Avery, McLeod, and McCarty-1944:
~transformation is disrupted by DNase
~DNA is the transforming factor
~did in vitro so away from mice by plating the cells and looking at morphology of the colonies
~plated bacteria on agar plates, took extracts from heat-killed S bacteria and mixed with live R type
~treated the extracts w/ diff conditions-destroyed lipids, proteins, RNA, and DNA
~when DNase was added, DNA was destroyed, and no bacteria recovered
~when the other components were destroyed, S bacteria were recovered
,~Hershey-Chase experiment:
~confirmed that DNA is heritable genetic info
~bacteriophage T4 DNA is hereditary molecule during infection
~grow bacteriophage in radioactive phosphate; phosphate/P-32 incorporated into DNA
~bacteriophage used to infect E. coli and DNA finds itself into the cell and then can make the proteins
required for further viral assembly
~radioactive sulfur incorporates into proteins; found cells devoid of S-35 and found itself into the empty
phage particles
~DNA has phosphate, protein has sulfur
~DNA structure:
~Chargaff’s found DNA composed of 4 nucleotides
~Chargaff’s rule: in DNA %A=%T and %G=%C; ratio of G/C=1 and A/T=1
~the % of nucleotide can vary b/w organisms
, ~DNA is a right-handed helix; when stands on the short axis see it climbing from left to right going up
and back
~the side facing you moves up and to the right
~exposes nucleotides
~major grooves and minor grooves
~the helicity isn’t symmetrical; there is a short span b/w nucleotides followed by a larger span
~strands are complementary and anti-parallel
~G pairs w/ C with 3 H bonds; A pairs w/ T with 2 H bonds
~5’ end is a phosphate end; 3’ end is a hydroxy end
~DNA structure cont:
~double helix diameter is 20 A or 20*10^-10 m
~one helical turn=34 A
~3.4 A between base pairs
~nucleus diameter: 5 microns or 5*10-6 m
~the average mammalian cell length=30 microns or 30*10-6 m
~mammalian genome: 3*109 base pairs so how does it fit into DNA
, ~Rosalind Franklin:
~DNA is a double helix which is why the X formed in photo
~photograph 51
~nucleotides:
~the building blocks
~hydrogen bonds are non-covalent
~composed of sugar, nitrogenous base, and the phosphate group
~the sugar=pentose called deoxyribose; deoxy because of C2 position and doesn’t have a hydroxyl
~1’ is the linkage to the nitrogenous base; 3’ hydroxyl group
~5’ linkage to phosphate groups; 1 (mono), 2 (di), 3 (tri) phosphate groups
~phosphodiester bonds-5’ to 3’ linkage that connects nitrogenous base and phosphate group
~nitrogenous base: purines and pyrimidines
~purines are two rings and are A and G
~pyrimidine: single 6 atom ring; T and C
~base pairing:
~G pairs with C through 3 H bonds; non-covalent; low energy bond
~A pairs with T through 2 H bonds
~all nucleic acid strands are antiparallel: DNA, RNA duplex, and DNA/RNA hybrids
~purines pair with pyrimidines
~DNA structure cont:
~during replication, there is a triphosphate
~the alpha phosphate covalently links to the 5’ C
~the two phosphates/pyrophosphates are eliminated/hydrolyzed and provide energy
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