Biology Reference
In-Depth Information
The DNA double helix is held together through hydrogen bonding of A
to T and C to G, as is shown in Figure 12-2. Because these hydrogen
bonds are weak, it is possible to separate the two strands of a DNA
molecule into two single-stranded DNA molecules by manipulating the
pH, the salt concentration, or the temperature of the DNA solution. The
converse is also true: A cDNA molecule can bind to a complementary
DNA sequence under the appropriate conditions.
Suppose that we wish to examine the differences in gene expression
between normal liver cells and cancerous liver cells. We begin the
process of making cDNA by isolating total RNA from the cells of
interest. We disrupt the cell membrane and inactivate and remove all of
the proteins that may damage or contaminate the RNA. Total RNA
includes ribosomal RNA (rRNA), transfer RNA (tRNA), and small
nuclear RNA (snRNA), as well as mRNA. Because mRNA is a small
5
9
O
H
O
P
O
N
H
O
O
N
N
3
9
H
2
C
H
N
O
N
OH
O
H
N
N
N
O
CH
3
H
O
CH
2
O
P
O
O
H
H
O
O
N
N
N
O
P
O
H
2
C
H
N
O
N
O
O
N
N
OH
O
CH
2
3
9
O
O
P
O
O
5
9
FIGURE 12-2.
Hydrogen bonding between nitrogenous bases. Note the two bonds (represented by dotted lines)
between A and T and the three bonds between G and C.
