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236 Biopolymers

link together nucleoside triphosphates as shown in Fig. 18,
ensuring that deoxyribonucleotides are incorporated with
bases in the correct sequence for pairing with bases in
the template strand. DNA polymerases cannot begin new
strands, but rather lengthen strands that begin with a short
stretch of RNA “primer” of base sequence complemen-
tary to a stretch of the DNA template strand. Each new

DNA chain grows in the 5 to 3 direction, and, particu-
larly for the new strand complementary to the upper left
strand in VIII, is synthesized in segments. Later the RNA
primers are removed and replaced with DNA segments,
and the pieces of DNA making up one new strand are
joined together by another enzyme, a ligase. The net re-
sult is that two new DNA molecules are formed, each
identical in sequence to the one original molecule, as in
VIII.
FIGURE 20 Tertiary structure of a t-RNA. The four short double Although almost all DNA of a complex organism is
helical segments are shown shaded. [Adapted with permission
located in cell nuclei, most protein synthesis takes place
from Rich, A., and RajBhandary, U. L. (1976) Annu. Rev. Biochem.
outside the nuclei. A messenger molecule, m-RNA, is nec-
45, 805–860. Copyright 1976 Annual Reviews Inc.]
essary to carry the coded instructions from the DNA to the
sites for protein biosynthesis. Since it is the base sequence
one protein is made in a cell at a particular time. The m-, of the DNA which carries these instructions, the base se-
r-, and t-RNA, in contrast, are all directly involved in the quence of the m-RNA must be closely related to that of the
process whereby amino acids become joined together in DNA. This is ensured by a process known as transcription,
the correct sequences. whereby the m-RNA is synthesized in much the same way
In higher organisms, most DNA is found in structures as a new DNA strand by using a DNA chain as template.
called chromosomes in the cell nuclei. The DNA functions A section of DNA specifying one trait of an organism
as genetic material and is passed on from parent to off- is a gene, and each gene codes for one protein or a group
spring. As the offspring develop from single cells to mul- of related proteins. When an m-RNA molecule is formed,
ticelled organisms, by division of the original cells, each a segment of DNA must be unwound and one of the DNA
new cell usually contains DNA identical to that of the first chains is used as a template to specify the base sequence
cell. This is accomplished by a process called replication, of the new RNA. An enzyme, RNA polymerase, links to-
where the two strands of an original DNA double helix gether ribonucleoside triphosphates with the elimination
separate and two new strands are synthesized, each with of pyrophosphate as in Fig. 18a, and the new chain grows
sequence complementary to one of the original strands, as in the 5 to 3 direction. Nucleotides are incorporated into

in VIII. the chain so that the bases pair with those in the tem-
The process is a complex one, for the original helix plate. This ensures that the base sequence of the m-RNA
must be unwound and its two strands separated. Each is complementary to that of the template DNA strand, as in
“old” strand is used as a pattern or template for synthe- IX. Uracil is incorporated into RNA to pair with adenine,
sis of a new strand. Enzymes, called DNA polymerases, instead of the thymine in a DNA strand:

5′ end A C G A T G T A C 3′ end A C G A T G T A C old strand
3′ end T G C T A C A T G 5′ end T G C T A C A T G new strand
original molecule of DNA

A C G A T G T A C new strand
T G C T A C A T G old strand
VIII