DNA is recognized through aDNA conservation mechanism, and mutations through
cytosine deamination can be repaired to their original shape (Fig. 2.18) (Lesk
1969).
First, uracil-DNA glycosidic bond hydrolyzes the glycosidic bond between
deoxyribose and the uracil formed through the cytosine deamination. The DNA
structure remains unimpaired, but one base (uracil) is removed; the resulting empty
space is capped the AP-site (apurinic or apyrimidinic site). The AP-site is then
recognized by AP endonuclease, and a nick is made in the structure near the absent
base site. DNA polymerase I makes a nick in remaining deoxyribose phosphate that
does not possess bases, and cytosine is introduce to pair with guanine in the
complementary strand. The gap is finally filled by the enzyme DNA ligase, and the
DNA is restored to its original shape (Fig. 2.19).
Unlike uracil, thymine possesses a methyl group at its C 5 site, while the DNA
repair enzyme uracil-DNA–glycosylase (UDG) removes only the uracil, and not the
thymine, from DNA. The DNA repair mechanism thus recognizes the methyl
group-possessing thymine as a normal DNA base and the methyl group-lacking
uracil as an attachment to DNA, which allows it to prevent the GC-to-AU mutations
that take place due to cytosine’s natural deamination. In other words, the reason that
DNA possesses thymine as a normal base and does not use uracil appears to be that
this increases the fidelity of genetic information transmission. Unlike DNA, RNA
does not undergo repair, and uses the methyl group-lacking uracil as a normal base.
Fig. 2.18 Repair of a GU base pair. The uracil group from DNA is removed and its place filled
by cytosine
Fig. 2.19 Repair of a GU base pair. The DNA uracil group is eliminated and its space filled with
cytosine
46
2 Introduction to Molecular Biology
cytosine deamination can be repaired to their original shape (Fig. 2.18) (Lesk
1969).
First, uracil-DNA glycosidic bond hydrolyzes the glycosidic bond between
deoxyribose and the uracil formed through the cytosine deamination. The DNA
structure remains unimpaired, but one base (uracil) is removed; the resulting empty
space is capped the AP-site (apurinic or apyrimidinic site). The AP-site is then
recognized by AP endonuclease, and a nick is made in the structure near the absent
base site. DNA polymerase I makes a nick in remaining deoxyribose phosphate that
does not possess bases, and cytosine is introduce to pair with guanine in the
complementary strand. The gap is finally filled by the enzyme DNA ligase, and the
DNA is restored to its original shape (Fig. 2.19).
Unlike uracil, thymine possesses a methyl group at its C 5 site, while the DNA
repair enzyme uracil-DNA–glycosylase (UDG) removes only the uracil, and not the
thymine, from DNA. The DNA repair mechanism thus recognizes the methyl
group-possessing thymine as a normal DNA base and the methyl group-lacking
uracil as an attachment to DNA, which allows it to prevent the GC-to-AU mutations
that take place due to cytosine’s natural deamination. In other words, the reason that
DNA possesses thymine as a normal base and does not use uracil appears to be that
this increases the fidelity of genetic information transmission. Unlike DNA, RNA
does not undergo repair, and uses the methyl group-lacking uracil as a normal base.
Fig. 2.18 Repair of a GU base pair. The uracil group from DNA is removed and its place filled
by cytosine
Fig. 2.19 Repair of a GU base pair. The DNA uracil group is eliminated and its space filled with
cytosine
46
2 Introduction to Molecular Biology