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Lecture 13

BIOB11H3 Lecture Notes - Lecture 13: Chromosome, Giemsa Stain, Pyrophosphate

Biological Sciences
Course Code
Dan Riggs

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Learning objectives
1) Discuss three possible mechanisms by which DNA replication
could occur and elaborate why a semi conservative mechanism
is most likely employed
2) Diagram the sedimentation patterns of a messelson and stahl
experiment at the end of the first, second third and third round
replication after the switch to N15 containing medium
-After one round of replication all molecule sediments are at the
hybrid—with equal N14/N15
-After the second round of replication half is hybrid and the other
half is light
Explain how the processes of DNA replication and
transcription are similar
- The process of replication and transcription both being at a
defined site. The origin vs the transcription site
- Must denature DNA strands so that the template can be read and
new strands made
- Proteins and protein complexes sponsor initiation and elongation
- BOTH the denaturation of DNA at the orgin and the continuous
unwinding of strands during elongation induce torsional stress
Conservative—parental strands together
Semi conservative-parental/daughter strand hybrids
Dispersive- random parental/ daughter strands
Density label: ---Grew cells for many generations in N15 medium and
label DNA with N15, wash cells and switch to normal N14 medium
Upon replication DNA would now have some N15 and N14, thuse
density of DNA would differ. At different time points purified DNA
separated by centrifugation an the N14/N15 rations determined
DNA polymeranse--- only add nucleotides to a 3’ OH group soooo
they must contain a template strand copy and a primer strand at the 3’
where OH can be added
Leading vs lagging strand
Okazaki fragments

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DNA is a double stranded anti parallel structure where the amount of
A=T and G=C
Each strand serves as a template for the formation of another strand
There are three possible replication mechanisms
Conservative—parallel strands together
-two original strands would remain together (after serving as
templates) as would the two newly synthesized strands. As a result
one of the daughter duplexes would contain only parentl DNA while
the other daughter duplex would contain only synthesized
Semi-conservative- parental/daughter strand hybrid
-each daughter contains one strand from the parent
Dispersive—random parental/daughter strands
-parental strands would be broken into fragments and the new strands
would be synthesized in short segements. The old fragments and new
segments would be joined together to form a complete strand.
Therefore the daughter contains strands that were composites of old
and new DNA
The semi-conservative is the most feasible based on time energy and
potential for errors
Meselson and Stahl used desnity labeling and ultracentrigugation to
show that bacterial replication is semi conservative. J.H taylor used
radiolebelleing to show that eukaryotic cells also employ semi
conservative replication
Used E.coli to investigate the mechanisms of replication. Employed
desnity labeling
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