20.1a History of DNA and Structure Cell Division, Genetics,
Molecular Biology
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DNA Deoxyribonucleic acid (DNA) Found in nucleus of all
organisms (within chromosomes) DNA only molecule capable of
replicating itself Contains instructions that ensure continuity of
life - coded within chemical messages of DNA - regulates the
production of proteins Ability to change due to mutations and new
combinations of genes
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Frederich Miescher 1869 extracted viscous white substance from
bandages of wounded soldiers - slightly acidic, phosphorus &
nitrogen rich - called it nuclein Nuclein composed of acidic
portion (nucleic acid) and alkaline portion (protein) Single
nucleic acid was later shown to be 2 nucleic acids -
deoxyribonucleic acid (DNA) - ribonucleic acid (RNA) DNA material
of heredity: early focus was on proteins
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Joachim Hammerling Acetabularia: green algae, 3 distinct
regions (cap, stalk, foot) Nucleus in foot: cut off cap and new cap
regenerated, cut off foot, no new foot regeneration Suggested
hereditary material located in nucleus
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Frederick Griffith Lab Exercise pg 644 Streptococcus pneumoniae
2 forms - virulent: S-form (coated) - harmless: R-form S-form cells
heated and killed, injected into mice and they lived Heated cells
mixed with R-form cells, killed mice Concluded there must be
something chemical altering the living cells: transformation -
transformed into virulent cells
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Avery, McCarty, MacLeod Lab Exercise pg 645 1944 experiments
with Streptococcus pneumoniae in test tubes Treated heat-killed
virulent bacteria with a protein- destroying enzyme: transformation
still occurred Treated heat-killed virulent bacteria with
DNA-destroying enzyme: transformation DID NOT occur Concluded DNA
was transforming principle - likely source of hereditary
information
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Alfred Hershey & Martha Chase 1952 used bacteriophages
(virus) that infect bacterial host (2 components: DNA and protein
coat) Infects by injecting DNA into it, virus multiplies within and
then bursts out, killing the cell Hershey & Chase concluded
that only the DNA, not protein coat, enters bacteria - tagged viral
proteins with isotope of sulfur (not component of DNA) - tagged
viral DNA with isotope of phosphorus (component of DNA) Allowed
tagged bacteriophage to infect bacterial cell Cells blended to
remove protein coats and centrifuged to isolate virus from
bacteria
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Bacterial cells found to contain isotope of phosphorus, not
isotope of sulfur Isotope of sulfur found in culture medium
Conclusion! DNA was hereditary material
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James Watson & Francis Crick Known that DNA comprised of
chains of nucleotides - consist of 5-carbon cyclic ring:
deoxyribose sugar - one of 4 nitrogenous bases attached to 1 carbon
- phosphate group attached to 5 carbon 4 bases: adenine (A),
guanine (G), thymine (T), cytosine (C) - A & G: purines (double
ring) - C & T: pyrimidines (single ring) Evidence from Edwin
Chargaff: calculated that amount of adenine always equal to amount
of thymine (same for guanine and cytosine). Observed for almost all
species Evidence from Rosalind Franklin: x-ray diffraction,
photograph taken - shows that DNA was a helix, likely
double-stranded
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James Watson & Francis Crick All the evidence compiled,
Watson & Crick created a 3D model Portrayed relationship
between bases as well as bond angles and spacing of atoms -
consistent with observations from other researchers to that point
Won Nobel Prize in 1962 along with Maurice Wilkins (researcher in
charge of Rosalind Franklins work) Rosalind Franklin left out she
died prior to 1962
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DNA Structure 2 strands of nucleotides Each nucleotide
contains: - deoxyribose sugar - phosphate group - nitrogenous base
Covalently bonded into double helix like a twister ladder -
hydrogen bonds keep helix together Base pairs are rungs,
sugar/phosphate backbones are struts Complementary base pairing to
form rungs - A pairs with T - C pairs with G
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DNA Structure
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Opposite strand always have the complementary sequence of bases
5 ATGCCGTTA 3 3 TACGGCAAT 5 Antiparallel: run parallel but in
opposite directions - one strand has 5 carbon & phosphate group
at one end and 3 carbon & hydroxyl group of deoxyribose sugar
at other end - other strand runs opposite 3 to 5 Direction
important to enzymes interacting with DNA - only read or copy DNA
in one direction DNA Structure