-
Parental
t
Parental DNA
Generation
1 Generation
2
Density
analysis
HYBRID
- - - -
Light
- -' -'
Light
-
--
Light
-
-
- -'Hybrid
-
-'
Hybrid
-
--
Hybrid
---
Heavy
--'-'Heavy
"'-
HeavY
.
I
,:
'
Base
pairing provides
the
mechanism for
repl.icating
DNA.
with the original
parent
and contains one
parental
strand and one newly synthesized
strand.
The
structure
of DNA carries the informa-
tion
needed t0
perpetuate
its sequence.
The consequences of
this mode
of
replica-
tion
are illustrated in
i
i+iii'ri: :. 1;:.
The
parental
duplex
is replicated to form two daughter
duplexes, each of
which consists
of one
parental
strand
and one
(newly
synthesized) daughter
strand.
The unit conserved
from
one
generation
to
the next is one of the two
individual
strands
compris-
ing
the
parental
duplex. This behavior
is
called
semiconservative
replication.
Figure l.l2 illustrates a
prediction
of this
model. If the
parental
DNA
carries
a
"heavy"
density
label because the organism has been
grown
in medium containing a suitable
isotope
(such
as
r5N;,
its strands can be distinguished
from
those
that are synthesized when the organ-
ism is transferred
to a medium containing nor-
mal
"light"
isotopes.
The
parental
DNA consists of a duplex of
two heavy strands
(red).
After one
generation
of
growth
in
light medium,
the
duplex DNA
is
"hybrid"
in density-it consists of one
heavy
parental
strand
(red)
and one light daughter
strand
(blue).
After a second
generation,
the
two strands
of each
hybrid
duplex
have sepa-
rated.
Each
gains
a
light
partner,
so
that
now one
half of the duplex
DNA remains
hybrid and
the
other half
is
entirely
light
(both
strands
are blue).
The
individual
strands
of these
duplexes
are
entirely heavy or
entirely
light.
This
pattern
was
confirmed
experimentally
in the
Meselson-Stahl
experiment
of 1958,
which
followed
the
semi-
conservative
replication
of
DNA
through
three
generations
of
growth of. E. coli.When
DNA was
extracted
from bacteria
and
its density
mea-
sured
by centrifugation,
the
DNA
formed bands
corresponding
to
its density-healry
for
parental,
hybrid for the
first
generation, and
half
hybrid
and
half light
in the second
generation.
DNA Strands
Separate
at the
Replication
Fork
r
Reptication
of DNA
is undertaken
by a comptex
of
enzymes that
separate
the
parental
strands
and
synthesize
the daughter
strands.
r
The rep[ication
fork
is the
point
at
which
the
parentaI
strands
are
seParated.
.
The enzymes
that synthesize
DNA are catted
DNA
polymerases;
the
enzymes
that
synthesize
RNA
are
catted RNA
polymerases.
.
Nucleases are
enzymes
that
degrade
nucteic
acids;
they inctude
DNAases
and
RNAases
and can
be
divided
into endonucteases
and
exonucleases.
Replication
requires
the
two strands
of the
parental
duplex
to
separate.
However,
the
dis-
ruption
of structure
is only
transient
and
is
reversed as the
daughter
duplex
is
formed.
Only
1.8
DNA Strands
Separate
at the
Reptication
Fork