Theme 4: DNA Replication and Mitosis - Module 2: Replication Flashcards
what is the macromolecule that determines the characteristics of the cell?
DNA
what was proposed about the base pairing in DNA
should allow for a mechanism by which the genetic information in DNA could be copied
what did Watson and Crick propose about DNA?
that DNA consists of a pair of template chains which are complementary to each other - prior to replication, hydrogen bonds are broken between complementary strands
what does breaking the hydrogen bonds of complementary strands in DNA allow?
unwinding and separation
what does it mean to say a strand is complementary to another?
each strand contains information necessary to reconstruct the other
what do Watson and Crick believe happens when a cell copies its DNA?
each strand serves as a template for ordering of new nucleotides (accruing to base-pairing rules) into a new complementary strand
what is the end result?
replication of DNA begins with one parent DNA dole helix, the end result would be two new helices
what would be the relation between the new helices and the original?
two new helices, with the new double stranded DNA being an exact copy of the “parental” molecule
what did this model of DNA replication predicted by Watson and Crick propose about when a DNA double helix replicates?
that each of the two daughter DNA molecules would have one old stand from the parental molecule and one newly made strand
what is this model of DNA replication known as?
semiconservative model of DNA replication
what was the conservative fashion DNA replication hypothesis?
two complementary parental strands coming back together after replication
what was the dispersive DNA replication hypothesis?
all four strands combining into a mixture of old and new DNA strands
who’s research conclusively demonstrated that DNA replicates in a semiconservative manner?
Matthew Meselson and Franklin Stahl
what did Matthew Meselson and Franklin Stahl do their research on and in what medium was it done?
- E.coli bacterial cells
- medium containing the nucleotide precursors with radioactively labelled heavy isotope of nitrogen (15N)
what were the bacterial cells transferred into?
medium containing 14N, the lighter isotope
after this point every new strand of replicated DNA would be built containing which isotope?
14N rather than the 15N
what did Meselson and Stahl do throughout their experiment?
extracted DNA samples from the growing bacteria
what did they do with DNA that was extracted from each sample?
centrifuged each sample through a solution that separates the DNA based on differing densities of the radioactively labeled molecules
what does incorporation of the 15N isotope do?
makes the DNA “heavier” than that with the 14N isotope so it ends up near the bottom of the tube
what did Meselsen and Stahl identify about the DNA from the bacteria that had been growing in the media containing the 15N isotope
it had only one distinct band
what was observed after the transfer and then one round of replication in the medium containing the 14N isotope?
DNA also appeared as a single band but with lower density
where was the DNA in the 14N isotope placed in the centrifugation gradient with respect to the 15N isotope?
due to its lower density it was positioned higher than the original 15N band in the centrifugation gradient
what must this band contain?
some hybrid of the 14N and 15N DNA
based on these results why did Meselsen and Stahl reject the conservative model of DNA replication?
no individual distinct band correlated with the “heavier” 15N DNA
what did Meselsen and Stahl do in other to test the other two models?
allowed the bacteria to grow and divide for many generations in the 14N- containing media after transfer from the 15N-containing media
what did they find after many rounds of bacterial replication?
that the extracted DNA could be separated into 2 distinct bands
what were these two distinct bands?
one in the position that contained only 14N and another in the position where the hybrid DNA containing the 15N and 14N would be expected
what were these results consistent with?
the fact that with DNA replication each new double helix is mad dup of one old strand and a new strand
what did this evidence support?
that DNA is replicated in a semiconservative fashion in the cell
what results provide further support for the semiconservative model of DNA replication?
- labelled individual nucleotides with fluorescent labels
- allowed researchers to examine replication of eukaryotic DNA in media containing fluorescent nucleotides and follow entire strands of eukaryotic DNA under the microscope
- found cells could contain hybrid and fully labeled nucleotides
- leads to observed faintly and darkly fluorescing strands of labeled DNA even within one chromosome
is the mechanism of relocation similar in prokaryotes and eukaryotes?
yes
in what phase does DNA replication begin? at what regions along the DNA does replication begin?
- S-phase
- at specific regions along DNA called origins of replication
although prokaryotes and eukaryotes both replicate DNA in a semiconservative manner what leads to a difference in the ways they initiate replication?
organization of their genomes
where does prokaryotic replication begin?
begins at a single origin of replication
what happens after this single origin of replication?
replication continues around the circular chromosome from this one initiation site
what is the process of DNA replication similar to?
transcription
during DNA replication which direction is the template strand copied in? and what direction does is produce its daughter strand?
3’ end to the 5’ end
produces daughter strand that elongates in a 5’ to 3’ direction
during DNA replication what does each incoming complementary nucleotide engage in with the nucleotide template strand? and interacts with what?
hydrogen bond
- interacts with the 3’ hydroxyl of the existing polymer that is forming the new daughter strand
what bond forms between the growing daughter strand and the new incoming nucleotide? what does this allow?
- phosphodiester bond
- allows it to become part of the DNA molecule backbone on the daughter strand ( in the process produces a pyrophosphate)
do the two strands of complementary DNA run in a parallel or antiparalllel fashion?
antiparallel
what do both strands serve as?
simultaneous templates from which DNA can be replicated
what does the unwinding of the DNA double helix result in?
the separation of the parental strands
where does the separation of the parental strands occur?
at regions call replication forks within the origins of replication
what does the initiation of replication require?
a short stretch of an RNA molecule or a primer (usually 5-10 nucleotides long) be synthesized and base pair with the template DNA strands
why is the primer required?
since the enzymatic machinery that elongates a new daughter stand can only do so from an existing piece of DNA or RNA
as elongation continues the polymerization of each newly replicated daughter strand is catalyzed by the DNA polymers enzyme in what direction?
5’ to 3’ direction
what does this enzyme do?
synthesizes a replicated DNA strand from the primers that anneal to the template strand
are there important differences in the manner in which each parent strand is replicated?
yes