Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 485

chromosomes, the shortening eliminates some of the nonessential, repeated sequence rather
than the protein-encoding DNA sequence farther away from the chromosome end. Telome-
rase is often activated in cancer cells to enable cancer cells to duplicate their genomes indef-
initely without losing important protein-coding DNA sequence. Activation of telomerase
could be part of the process that allows cancer cells to become immortal.
Now that we have learned transcription, we can now move to examine translation. RNA
molecules are made with a DNA template. The translation of information on mRNA into
proteins occupies a very large fraction of the cells' resources. Like a large automobile plant,
the generation of blueprints and the construction of the manufacturing machinery are worth-
less until the final product is made.

10.4. TRANSLATION: MESSAGE TO PRODUCT

The central dogma of molecular biology describes the passing of genetic code to protein
from DNA as shown in Figs 10.1 and 10.2 . Table 10.2 summarizes the process of transferring
information from DNA template to proteins. From a DNA template, a complementary RNA
strand is made, from which it is translated into protein. Our emphasis now is on the protein
formation.

10.4.1. Genetic Code: Universal Message
The blueprint for any living cell is consisted of 20 amino acids as shown in Table 2.4. There
are four letters for DNA and/or RNA as shown in Fig. 2.21. With the four letters, a three-letter
word is needed to completely expressing the 20 amino acids. Therefore, the genetic code is
made up of three-letter words (codons) with an alphabet of four letters as discussed earlier.
Sixty-four words are possible, but many of these words are redundant (as only 20 individual
amino acids needed expression). Even though such a “language” may appear to be simple, it
is sufficient to serve as a complete blueprint for the expression of any “message” or
“construction” of the reader.
The dictionary for this language is given in Table 10.3 , and an illustration of the relation-
ship of nucleotides in the chromosome and mRNA to the final protein product is given in
Fig. 10.10 . The code is degenerate in that more than one codon can specify a particular amino
acid (for example, UCU, UCC, UCA, and UCG all specify serine). Three codings, UAA, UAG,
and UGA, do not encode normally for any amino acids. As a result, these codons act as stop

TABLE 10.2 The Central Dogma of Molecular Biology for Protein Production

DNA, template RNA Protein

4 nucleotides (A, C, G, T) 4 nucleotides (A, C, G, U) 20 amino acids

0
Transcription
by polymerase

0
Translation
by ribosome

ACCGCTTGACTACAC UGGCGAACUGAUGUG Try-Arg-Thr-Asp-Val

Phosphodiester bond Phosphodiester bond Peptide bond