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DNA Replication - Coggle Diagram
DNA Replication
DNA is stable + can pass
unchanged from parent cells
to daughter cells
occurs before mitosis/meiosis
when DN is replicated
Semi-Conservative
Replication
enzyme
DNA helicase
'unzips'
2 strands of DNA by breaking
hydrogen bonds between bases
each of original stands become
template
free nucleotides
linked to template
strands in correct sequence following
base pairing rules with bases on
template strand
the nucleotides within each
newly created strand by
enzyme
DNA polymerase
DNA polymerase catalyses the formation
of
phosphodiester bonds
between deoxyribose + phosphate of adjacent nucleotides in the new strand
2 new DNA molecules
formed each containing
1 template strand
DNA + 1 new DNA strand
The Meselson and
Stahl Experiment (1958)
Conservative
parental DNA remain intact and
copied full new DNA molecule
Semi-conservative
New DNA contains only 1 strand
of parental DNA + 1 strand new DNA
Experiment
E-coli bacteria from culture 14N
moved to culture 'heavy isotope' 15N
This means 15N incorporated into
the bases of DNA in ALL bacteria as all
older bacteria (containing 14N) died +
were replaced
Bacteria then
transferred
back to culture 14N
Bacteria analysed at
key stages:
bacteria growing in 14N
(before transfer)
bacteria growing in 15N
(after transfer)
one generation after
transfer back to 14N
two generations after
transfer back to 14N
Density-gradient
centrifugation
used
to separate bacterial DNA
14N accumulate near top of centrifuge tube
15N form zone near bottom of centrifuge tube
Explanation
1st generation
all new bacteria had hybrid DNA
that contained 1 parental
strand (15N) and 1 new strand (14N)
intermediate position
2nd generation
Half contain hybrid
DNA (1-15N + 1-14N)
Other half bacteria only 14N
3rd generation
& subsequent
same pattern
proportionally FEWER bacteria
containing hybrid DNA
MORE bacteria containing
only light 14N DNA
lighter
14N band become
THICKER
intermediate
hybrid band become
THINNER