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Lecture 2

BIOLOGY 1A03 Lecture Notes - Lecture 2: Telomerase, Ribonucleoprotein, Lincoln Near-Earth Asteroid Research


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Rosa Da Silva
Lecture
2

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Module 2 Replication
UNIT I: TRANSMISSION OF INFORMATION
GENETIC INFORMATION IS COPIED
In 1953, Watson and Crick concluded that there is a copying mechanism for this
genetic material
MODEL FOR SEMI-CONSERVATIVE REPLICATION
In 1954, Watson and Crick proposed that DNA consists of a pair of complementary
template chains
Stated that prior to replication, hydrogen bonds are broken between the strands,
allowing for unwinding and separation
COMPLEMENTARY: each strand contains the information necessary to reconstruct
the other
Believed that when a cell copies DNA, each strand serves as a template for a new
complementary strand
While replication begins with one parent helix, there are two helices in the end
which are exact copies of the parent molecule
SEMICONSERVATIVE MODEL: when a DNA double helix replicates, each of the two
daughter molecules would have one old strand from the parental molecule, and one
newly made strand
UNIT II: EVIDENCE FOR THE SEMICONSERVATIVE MODEL
OTHER DNA REPLICATION HYPOTHESES
CONSEVATIVE: two complementary parental strands somehow fuse back together
following replication
DISPERSIVE: all four strands somehow combine into a mixture of old and new
strands
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SUPPORT FOR SEMICONSERVATIVE REPLICATION
Matthew Meselson and Franklin Stahl demonstrated that DNA replicated in the
previously proposed semiconservative manner
In the late 1950s, E.coli bacterial cells were cultured for generations in mediums
containing nucleotide precursors with radioactively labeled heavy nitrogen
isotopes (15N)
The bacteria was then transferred into a medium containing 14N
From that point, every new DNA strand was built containing 14N rather than 15N
isotopes
DNA samples were extracted from the growing bacteria
Each sample was then centrifuged through a solution which separated DNA based
on differing densities of the radioactively labeled molecules
Any DNA containing the 15N would be heavier and end up towards the bottom of
the tube
DNA from bacteria growing in the 15N isotope medium had only one distinct
band
Following transfer into the 14N medium/one round of replication, DNA also
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appeared as a single band but with lower density, meaning it was positioned
above the 15N DNA in the tube indicating that it is a hybrid 14N/15N DNA
The conservative model of DNA was therefore rejected since no individual distinct
band correlated with the 15N DNA
To test the semiconservative/dispersive model, bacteria was allowed to
grow/divide for many generation in the 14N media after transfer from the
15N media
Extracted DNA could be separated into 2 distinct bands: one in the position
containing only 14N, and another where the hybrid14N/15N hybrid was
expected
RESULTS
Consistent with the fact that each new double helix is made up of one old strand
and one new strand
DNA is therefore replicated in a semiconservative fashion
SEMICONSERVATIVE CHROMOSOME REPLICATION
Researchers are presently able to label individual nucleotides with fluorescent
labels which allows for examination of replication in eukaryotic DNA
Discovered that chromosomes of eukaryotic cells could contain hybrid and fully
labeled nucleotides, indicated by faintly/darkly fluorescing strands of labeled
DNA (even within one chromosome)
Provides further support for semiconservative model
Mechanism of replication is similar in prokaryotes and eukaryotes
UNIT III: REPLICATING DNA
INITIATING REPLICATION
Begins in the S phase along the origins of replication
Initiation varies in prokaryotes and eukaryotes due to differing organizations of
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