Page 54 - The Miracle in the Cell

Sugar phosphate backbone Sugar phosphate backbone
Strand 1 Base pairs joined Base pairs joined
by hydrogen bond- by hydrogen
Strand 2 ing bonding
A mistake which has
occurred in strand 1 An unwanted change that
occurs in the bases
Strand 1
Strand 2
Step 1
The damaged portion in
strand 1 is removed.
Uracil-DNA glycosylase
Strand 1
Strand 2
Step 2 The gap in the defective strand
1 is filled in by DNA poly- The base defi-
merase, according to the infor- cient DNA
mation found in strand 2. strand
Strand 1
Strand 2 AP endonuclease removes
the sugar-phosphate piece
Step 3 DNA ligase fills in the
resulting gap.
Strand 1 Result: the defec-
tive region is
Strand 2 repaired.
Figure 2.11 The help of DNA poly-
DNA control mechanisms identify and merase and DNA ligase
repair any mistakes arising in the DNA as
a result of external factors. These mecha-
nisms are made up of enzymes produced
according to instructions found within the
DNA itself. There may be different types
of repair mechanisms, but basically, the Figure 2.12
damaged nucleotide is repaired according An unwanted event that occurs
to information contained in the opposite from time to time: the incorrect
strand's corresponding nucleotide. This copying of the DNA molecule. As
process shown in the figure above basical- can be seen in the first diagram, a
ly consists of three steps: uracil (U) has been placed instead
1) After being identified by an enzyme of a cytosine (C) in the fourth base
called DNA nuclease, the damaged portion position of the DNA. An enzyme
of the DNA strand is removed. This results called Uracil-DNA glycosylase,
in a gap in the DNA strand. which has the duty of repairing
2) Another enzyme, DNA polymerase, only this type of mistake, removes
adds the necessary nucleotide according to the uracil from the DNA strand. AP
the information contained in the undam- endonuclease removes the sugar
aged strand of DNA. phosphate group from this area.
3) The repair of the DNA is not quite fin- DNA polymerase and DNA ligase
ished. The gap resulting in the sugar phos- then attach the relevant nucleotides
phate backbone from this repair in their place.
mechanism is filled in by an enzyme
called DNA ligase.

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