Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 479

the synthesis of DNA is bidirectional. Two forks start at the origin and move in opposite direc-
tions until they meet again, approximately 1800 from the origin.
To initiate DNA synthesis, an RNA primer is required; RNA polymerase requires no primer
to initiate the chain-building process, while DNA polymerase does. We can speculate on why
this is so. In DNA replication, it is critical that no mistakes be made in the addition of each
nucleotide. The DNA polymerase, Pol III, can proofread, in part due to the enzyme's C3 to C5
exonuclease activity, which can remove mismatches by moving backward. On the other
hand, a mistake in RNA synthesis is not nearly so critical, so RNA polymerase lacks this
proofreading capacity. Once a short stretch of RNA complementary to one of the DNA
strands is made, DNA synthesis begins with Pol III. Next, the RNA portion is degraded by
Pol I and DNA is synthesized in its place. This process is summarized in Fig. 10.4 (and 2.34).
DNA polymerase works only in the C5-to-C3 direction, which means that the next nucle-
otide is always added to the exposed C3-OH group of the chain. Thus, one strand (the leading
strand) can be formed continuously if it is synthesized in the same direction as the replication
fork is moving. The other strand (the lagging strand) must be synthesized discontinuously.
Short pieces of DNA attached to RNA are formed on the lagging strand. These fragments
are called Okazaki fragments. The whole process is summarized in Figs 2.34 and 10.4 . The
enzyme, DNA ligase, which joins the two short pieces of DNA on the continuous strand,
will be very important in our discussions of genetic engineering.
This brief summarizes the essentials of how one DNA molecule is made from another and
thus preserves and propagates the genetic information in the original molecule. Nowwe turn
to examine how this genetic information can be transferred.

1 0.3. TRANSCRIPTION: SENDING THE MESSAG E

Transcription is the process of creating a complementary RNA copy of a sequence of DNA.
Therefore, the primary products of transcription are the three major types of RNAwe intro-
duced in Chapter 2: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA
(rRNA). Their rates of synthesis determine the cell's capacity to make proteins. Figure 10.5
shows the major factors associated with the transcription process, where prokaryotes and
eukaryotes translate differently. In prokaryotes, the transcription has a start and an end loca-
tion on the template DNA. RNA synthesis from DNA is mediated (or read) by the enzyme,
RNA polymerase. To be functional, RNA polymerase must have two major subcomponents:
the core enzyme (consisted of five subunits d two

b 0 , and an

a , one b , one u units) and (binds

with) the s (sigma) factor, as illustrated in Fig. 10.5 . The core enzyme contains the catalytic site,

while the factor is a protein essential to locating the appropriate beginning for the message.

The core enzyme plus the

s

factor
would leave after initialization of the transcription, in other cases the s factor remain bonded
to the core enzyme throughout the transcription process.
You may wonder which one of the two strands of DNA is actually transcribed. It turns out
that either strand can be read. However, only one is read at a time, which is different from
DNA replication. RNA polymerase always reads in the C3 to C5 direction, so the direction
of reading will be opposite on each strand. On one part of the chromosome, one strand of
DNA may serve as the template or sense strand, and on another portion of the chromosome
the other strand may serve as a template. During transcription, a DNA sequence is read by

s factor constitutes the holoenzyme. While in some case the s