DNA replication Flashcards
where in the cell cycle is DNA replicated?
interphase- specifically the S phase
why must DNA be replicated before mitosis occurs?
to ensure that all the daughter cells have the genetic information to produce enzymes and other proteins they need
what is the universally accepted model for how DNA is replicated?
semi-conservative replication
what does semi-conservative mean?
a process in which DNA is copied so that each new DNA molecule contains one original strand and one new strand
what are the 4 requirements for semi-conservative replication of DNA to take place?
-the four types of nucleotide must be present
-both strands of the DNA molecule act as a template for attachment of these nucleotides
-enzyme DNA polymerase
-a source of chemical energy is required to drive the process (ATP)
what is the first step of DNA replication and what is involved in this?
DNA unwinds,
enzyme DNA helicase moves along the DNA double helix,
breaking hydrogen bonds between complementary base pairs on the two strands,
separating the two strands (unzips)
what does the unzipping of the DNA double helix allow for?
both strands can be used as a template in semi-conservative replication as both strands have exposed bases on them
what comes in towards the exposed bases on the template strands? what forms?
free-floating DNA nucleotides which are attracted to their complementary bases, A with T and C with G
hydrogen bonds form between exposed bases and free-floating DNA nucleotides
where does DNA polymerase attach and what does it do?
to the 3’ end of the template strand
and DNA polymerase moves along the template strand 3’ to 5’
DNA polymerase catalyses the condensation reactions between DNA nucleotides in the newly synthesised strand
- forming PHOSPHODIESTER bonds.
why does DNA polymerase attach to the 3’ end of the template strand?
DNA polymerase has a specific tertiary structure
it has an active site that is complementary to only the 3’ end of the template strand
since 3’ end and 5’ end are differently shaped
what did the Meselson and Stahl experiment use?
heavy N15 isotope (used to allow physical separation of daughter DNA molecules as DNA strands made with N15 will be heavier and more dense than DNA strands made with usual N14)
also used bacteria E.coli
how was knowledge about growing bacteria used in the Meselson and Stahl experiment?
bacteria are grown on agar jelly that has nutrients within it.
the substance bacteria is grown in is called the culture medium
this culture medium requires a source of nitrogen
nitrogen is required for the production of DNA as nitrogen is a component in the base of a nucleotide
bacteria can be moved from one culture medium to another
what did Meselson and Stahl grow bacteria in?
a N15 containing medium for many generations so at the beginning essentially all the DNA was labelled with N15 and so had high density
what were the bacterial cells grown in the N15 medium moved to? what could then be done to cells?
new medium that ONLY contained N14
at intervals samples of cells were removed and DNA extracted
what happened to the DNA extracted from the cells?
centrifuged it in a special solution
lighter DNA collected near the top of the centrifuge tube and vice versa
what were the results of the Meselson and Stahl experiment after the first generation?
all of the DNA had a density that was exactly intermediate between the densities of N15 DNA and N14 DNA
this indicated that replicated molecules contained equal amounts of each N isotope
what were the results after the second generation?
half of the DNA had the same intermediate density as after first
other half had same density as N14 DNA
this distribution is the results predicted from the semi-conservative replication of the Watson and Crick structure of DNA
