Page 380 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Animals 343

The total genetic information for each cell, called the genome, exists in the coded two-stranded
DNA. This genetic information is expressed or processed either through duplication of the DNA so
it can be transferred during cell division to a daughter cell or it can be transferred to manufactured
RNA that in turn transfers the information to proteins that carry out the activities of the cell.
Duplication of double-stranded DNA is self-directed. The DNA, along with accessory proteins,
directs the replication or construction of two complementary strands forming a new, exact repli-
cate of the original DNA template. As each base site on the DNA becomes available through the
unraveling of the double-stranded helix, a new nucleotide is brought into the process held in place
by hydrogen bonding and van der Waals forces so that the bases are complementary. It is then cova-
lently bonded through the action of an enzyme called DNA polymerase. After duplication, each
DNA contains one DNA strand from the original double-stranded helix and one newly formed DNA
strand. This is called semiconservative replication and increases the chance that if an error occurs,
that the original base sequence will be retained.
How is DNA suitable as a carrier of genetic information? While we do not entirely understand
several features are present in DNA. First, because of the double-stranded nature and mode of repli-
cation, retention is enhanced. Second, DNA is particularly stable within both cellular and extracel-
lular environments, including a good stability to hydrolysis within an aqueous environment. Plant
and animal DNA have survived thousands of years. Using polymerase chain reactions (PCR) we can
reconstruct DNA segments allowing comparisons to modern DNA.
Transcription is the term used to describe the transfer of information from the DNA to RNA.
The genome is quite large, on the order of a millimeter in length if unraveled, but within it exists
coding regions called genes. Transcription is similar to DNA replication except ribonucleotides
are the building units instead of deoxyribonucleotides; the base thymine is replaced by uracil; the
DNA:RNA duplex unravels releasing the DNA to again form its double-stranded helix and the single-
stranded RNA; and the enzyme linking the ribonucleotides together is called RNA polymerase.
Many viruses and retroviruses have genomes that are single-stranded RNA instead of DNA.
These include the AIDS virus and some retroviruses that cause cancer. Here, an enzyme called
reverse transcriptase converts the RNA genome of the virus into the DNA of the host cell genome
thus infecting the host.
The transcription of the DNA gives three kinds of RNA—ribosomal, messenger, and transfer.
The most abundant RNA is rRNA. Most rRNA is large and is found in combination with proteins
in the ribonucleoprotein complexes called ribosomes. Ribosomes are subcellular sites for protein
synthesis.
Transfer RNA is the smallest of the RNAs being less than 100 nucleotides long. tRNA combines
with an amino acid incorporating it into a growing protein. There is at least one tRNA for each of
the 20 amino acids used in protein synthesis. mRNA is varied in size but each carries the message
found in a single gene or group of genes. The sequence of bases in mRNA is complementary to the
sequence of DNA bases. mRNA is unstable and short-lived so that its message for protein synthesis
must be rapidly decoded. The message is decoded by the ribosomes that make several copies of the
protein from each mRNA.
The ultimate purpose of DNA expression is protein synthesis. mRNA serves as the intermediate
carrier of the DNA genetic information for protein synthesis. The DNA message is carried in the
form of base sequences that are transferred to RNA also in terms of base sequences and fi nally these
are transferred into amino acid sequences through a translation process based on the genetic code.
This process of information from the RNA to the protein is called translation.
A set of coding rules are in action as in the translation process. Briefly, these are as follows.

First, a set of three adjacent nucleotides compose the code for each amino acid. A single amino
acid can have several triplet codes or codons. Since there are four different nucleotides (or four
3
different bases) in DNA and RNA there exists 4 , or 64 trinucleotide combinations. For instance,
using U as a symbol for uracil, present in RNA, the triplet or code or codon UUU is specifi c for
phenylalanine.

9/14/2010 3:41:17 PM
K10478.indb 343 9/14/2010 3:41:17 PM
K10478.indb 343