Page 194 - An Introduction to Microelectromechanical Systems Engineering

DNA Analysis 173

Sugar-phosphate backbone Base pair Weak hydrogen bond

(a)
Hydrogen dissociates
in solution Phosphate
Section to amplify Sugar
HO P O Base
5’GTCATGCAGGTCGACTCTG3’
O
3’CAGTACGTCCAGCTGAGAC5’ 5’
HC O Cytosine
2
H H
Denature at 95ºC H H
O 3’ H
HO P O
3’CAGTACGTCCAGCTGAGAC5’ 5’GTCATGCAGGTCGACT CTG 3’
O
HC 5’ O Thymine
Add primers to select starting sections 2
H H
H H
3’CAGTACGTCCAGCTGAGAC 5’ 5’GTCATGCAGGTCGACTCTG 3’ 3’
O H
5’CATG 3’ 3’ GAGA5’
HO P O
O
Add DNA polymerase enzyme and dNTPs; 5’
2
Incubate at 60ºC HC O Guanine
H H
3’CAGTACGTCCAGCTGAGAC 5’ 5’GTCATGCAGGTCGACTCTG 3’ H H
3’
5’CATGGCAGGT CCAGCTGAGA5’ O H
One cycle complete
3’CAGTACGTCCAGCTGAGAC 5’ 5’GTCATGCAGGTCGACTCTG 3’
5’CATGCAGGTCGACTCTG GAGTACGTCCAGCTGAGA 5’
Repeat
(b)
Figure 6.3 Illustration of (a) the twisted double-helix structure of DNA; and (b) the polymerase
chain reaction (PCR). Denaturing of the starting DNA template at 95ºC yields two strands, each
containing all of the necessary information to form a complementary replica. The addition of
primers defines the starting point for replication. At 60ºC, the DNA polymerase enzyme catalyzes
the reconstruction of the complementary DNA strand from an ample supply of nucleotides
(dNTPs). The reconstruction always proceeds in the 5’→3’ direction. The cycle ends with copies of
two portions of the helices, in addition to the starting template. The cycle is then repeated. The
exploded view of three nucleotides (CTG) in the denatured template shows their chemical
composition, including the 3’-hydroxyl and 5’-phosphate groups. (After: [6, 7].)

and G, respectively. This is the four-letter alphabet of DNA. The human genome has
23 separate pairs of chromosomes, averaging 130 million base pairs in length, for a
total of about three billion base pairs. Genes that form the template for proteins are
typically 27,000 base pairs long, but only about 1,000 are used; the rest are extra
“filler” bases.
Each nucleotide molecule has two ends, labeled 3’ and 5’, corresponding to the
hydroxyl and phosphate groups attached to the 3’ and 5’ positions of carbon atoms
in the backbone sugar molecule [see Figure 6.3(b)]. In the long DNA chain, the 3’