Magnifietion:
280 000x
Copying DNA
is copied during interphase prior to
t?ArA
W
mifs5i5 and meiosis.lt is importantthat
the new copies are exactly tike the original
molecules. The structure of DNA provides a
mechanism for accurate copying of the molecule.
The process of making copies of DNA is cailed
DNA replication.
Critical Thinking What would be the
outcome if mitosis occurred before replication
took
Two molecules
place?
of
DNA
from one
,j
P-":J
@
S.p.rrtion of strands When a cell begins to
copy its DNA, the two nucleotide strands of a
DNA molecule first separate at their base pairs
when the hydrogen bonds connecting the base
pairs are broken. As the DNA molecule unzips,
the nucleotides are exposed.
DI
Original DNA
strand
p
P _JDt
D7
c
"t''.
.
"
i
T
- y
@
A
.
iiif
iii
:ii
rli
I li
i,,,r,,.ii
ii'l
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l
OriginalDNA
@ n"r" pairing Free nucleotides
base pair with
exposed nucleotides. lf one nucleotide on a
strand has thymine as a base, the free nucleotide
that pairs with it would be adenine. lf the strand
contains cytosine, a free guanine nucleotide will
pair with it. Thus, each strand builds its comple_
ment by base pairing with free nucleotides.
292 oweeloGENES
@
fonding ofbases The
sugar and phosphate parts
or aolacent nucleotides
bond together to form the
h
backbone of the new strand.
Each originalstrand is now
bonded to a new strand.
.nF4
Free nucleotides
rr
,/
fr
New DNA strand
New DNA
molecule
() $r],s of reptication. rhe process of repticat on prod uces two molecu
lei of o-rrii.' eu.f,
new molecule has one strand
frornthe'oiiginal
i
molecule and one,strana *,.i
r,.i
synthesized from free nucleotiJes
L."ni.*rv
i, if,...ff.