Biomedical Engineering Reference
In-Depth Information
HO
HO
OH
OH
HO
OH
HO
OH
P
P
P
P
O
O
O
O
O
O
O
O
OH
5
Base
5
Base
3
O
2
O
2
3
1
4
4
1
1
4
1
4
3
2
3
2
5
5
O
HO
O
HO
O
OH
HO
HO
O
P
P
P
P
O
O
O
O
O
O
O
O
OH
5
Base
5
Base
3
2
O
O
2
3
1
4
1
4
1
4
1
4
3
2
3
2
5
5
HO
O
HO
O
HO
O
OH
O
HO
P
P
P
P
O
O
O
O
O
O
O
O
OH
5
Base
5
Base
3
O
2
O
2
3
1
4
1
4
1
4
1
4
2
3
3
2
5
5
OH
OH
OH
OH
OH
-DNA
-DNA
-RNA
-RNA
FIGURE 2.31
Structure of DNA and RNA chains. Only short chain (three-unit) segments are shown. Both
a
-
anomers and
b
-anomers form strands uniformly by phosphodiester bonds formed between the 3rd and 5th carbon
atoms of two sugar units. Since nucleotides and ribonucleotides all have phosphate connected on the 5th carbon, we
say that DNA and RNA strands all run from 5 (th carbon) to 3 (rd carbon of the next sugar unit). However,
b
-strands
and
a
-strands run opposite in directions, i.e. 5
/
3 for
b
strand and 3
)
5 for
a
strand. The two strands (
a
and
b
)
always pair together (with “head” and “tail” opposite each other).
(3)
The two chains are held together by hydrogen bonding between pairs of bases. Adenine is
always paired with thymine (two H bonds); guanine is always paired with cytosine (three H
bonds). This feature is essential to the genetic role of DNA.
(4)
The sequence of bases along a polynucleotide is not restricted in any way, although each
strand must be complementary to the other. The precise sequence of bases carries the
genetic information.
The large number of H bonds formed between base pairs provides molecular stabilization,
as shown in
Fig. 2.33
. Regeneration of DNA from original DNA segments is known as DNA
replication. When DNA segments are replicated, one strand of the new DNA segment comes
directly from the parent DNA, and the other strand is newly synthesized using the parent
DNA segment as a template. Therefore, DNA replication is semiconservative, as depicted
in
Fig. 2.34
. The replication of DNA is discussed in more detail in Chapter 10.
Some cells contain circular DNA segments in cytoplasm called plasmids. Plasmids are
nonchromosomal, autonomous, replicating DNA segments. Plasmids are easily moved in
and out of cells and are often used for genetic engineering. Naturally occurring plasmids
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