Preparing the terrain for W

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1944
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Preparing the terrain for W C
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Preparing the terrain for W C
Other known facts DNA made of nucleotides Space
between nt 3.4 A Diameter 20A 3-5 bonding of nt
Erwin Chargaff AGCT
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Cavendish
L. Bragg lab

• Francis Crick
• 35 years old
• Physicist
• Grad Student - Biophysics

• James Watson
• 23 years old
• Post Doc
• Mentors Luria
• and Delbruck

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Meets F. Crick at Cavendish in 49 Crick was a
loquacious 35 year old grad student
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What did W C discover?
Symmetrical molecule typical of a helix Two
strands 10 nucleotides/turn 34 A per turn Strands
running in opposite directions, complementary A
T C G Phosphate-sugar backbone outside
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Select best answer based on Chargaffs rule

• AG/TC1
• AT/CG1
• AC/TG1
• CG/TA1

a. Is correct, proportion of purinespyrimidines
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Order the following DNA molecules from higher to
lower Tm
1. 5ATTCTAACTTT GAT 3 3TAAGATTGAAACTA
5 2. 5GCTTT AGACTCTCC 3
3CGAAATCTGAGAGG 5 3. 5CGGTACGGTCCACT3
3GCCATGCCAGGTGA5

• 1, 2, 3
• 1,3, 2
• 2, 1, 3
• 3, 2, 1

d. Is correct, higher the number of CG, higher
the temp to denature DNA
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THE Model

• Double Helix
• Position of Phosphates and Bases
• AT, CG Bonding

Structure of DNA from Watson Crick, 1953.
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DNA Molecule
10 bp/turn
Right handed
3.4 nm
Negative charge
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Genetical Implications

• Crick wanted to add implications to initial paper
• Purine-Pyrimidine pairing
• Any sequence (A,T,C,G)
• . . . sequence of the bases is the code which
  carries the genetical information.

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Genetical Implications

• One chain is complement of other
• . . . suggests how the deoxyribonucleic acid
  molecule might duplicate itself.
• H-bonds broken
• Unwind and separate
• Pair of templates

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Meselson and Stahl

• The Most Beautiful Experiment in Biology
• -John Cairns

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Meselson Stahl

• Meselson at Caltech
• PhD student, X-Ray Crystallography
• Stahl to Caltech (1954)
• Bacterial and Phage Genetics under Delbruck
• Stahl declines position at Berkley

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Models For Replication

• Conservative

• Semi-Conservative

Diagrams from http//www.mun.ca/biochem/courses/
3107/Topics/DNA_replication.html
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Results
Replication is Semi-Conservative!
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Meselson and Stahl grew bacteria for many
generations in heavy isotope N15, then
transferred to N14 for several generations. DNA
extracted from the 1nd generation grown in N14
produced 2 N14/ N15 molecules What will be the
number of DNA molecules and composition extracted
from the 3rd generation in N14?

• Answers
• 8 molecules 4 N14/N15 and 4 N14/N14
• 8 molecules 2 N14/N15 and 6 N14/N14
• 8 molecules 6 N14/N15, 2 N14/N14
• None of the above

b. Is correct, try it
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DNA replication
Replication bubble
Replication forks
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DNA Replication fork
Okasaki fragment
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The following enzymes play key roles in DNA
replication in E. coli DNA pol I, DNA pol II,
helicase, gyrase, primase, ligase. Which or
these are used more frequently in the lagging
than in the leading strand

• All are used at the same frequency in both
  strands
• DNA pol I, ligase, helicase
• DNA pol III, primase, gyrase
• Primase, DNA pol I, ligase

d. Is correct
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DNA replication dilemma With each round of
replication, newly synthesized strands become
shorter
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Telomerase solves the dilemma
AACCCC AA