Page 381 - Carrahers_Polymer_Chemistry,_Eighth_Edition

344 Carraher’s Polymer Chemistry

Second, the code is nonoverlapping so that every three nucleotides code for an amino acid and
the next three code for a second amino acid and the third set code for a third amino acid, and so on.
Third, the sets of nucleotides are read sequentially without punctuation. Fourth, the code is nearly
universal. Fifth, there are codes for other than amino acids, including stop or terminate UAG, and
start or initiate AUG.
In essence, tRNA has two active sites—one that is specific for a given amino acid and the second

that is specific for a given set of three bases. The tRNA “collects” an appropriate amino acid and
brings it to the growing polypeptide chain inserting it as directed by the mRNA. There is then a
collinear relationship between the nucleotide base sequence of a gene and the amino acid sequence
in the protein.
The amount, presence, or absence of a particular protein is generally controlled by the DNA in
the cell. Protein synthesis can be signaled external to the cell or within the cell. Growth factors and
hormones form part of this secondary messenger service.
The translation and transcription of DNA information is polymer synthesis and behavior, and the
particular governing factors and features that control these reactions are present in the synthesis and
behavior of other macromolecules—synthetic and biological.
For the human genome there exists so-called coding or active regions called exons and noncoding
regions called introns. The average size of an exon is about 120–150 nucleotide units long or coding
for about 40–50 amino acids. Introns vary widely in size from about 50 to more than 20,000 units.
About 5% of the genome is used for coding. It was thought that the other 95% was silent or junk
DNA. We are finding that the introns regions play essential roles. Interestingly introns are absent in

the most basic prokaryotes, only occasionally found in eukaryotes, but common in animals.
10.5 RNA INTERFERENCE

RNA interface (RNAi) is a somewhat newly discovered part of our body’s natural immune system.
RNAi is not another form of RNA, but rather it is a sequence involving protein enzymes that blocks
the action of certain foreign RNA, thus its name RNA interference or simply RNAi. Andrew Fire
and Craig Mello won the 2006 Nobel Prize in Medicine for their work with RNAi.
Following is a general discussion of what RNAi is and does, including some potential applica-
tions from its use.
While DNA is the depositary of our genetic information, it is “held captive” or “protected” by the
nuclear envelope remaining within this envelope. Yet the information held by the double-stranded
DNA is transferred throughout the cell with results felt throughout the body. Briefl y, polymerase
transcribes the information on the DNA into single-stranded mRNA. The mRNA single strands
move from the cell nucleus into the cell cytoplasm through openings in the nuclear envelope called
nuclear pore complexes.
In the cytoplasm, ribosome, another protein enzyme, translates the information on the mRNA
into protein fragments eventually giving entire proteins. This “normal” sequence is depicted in
Figure 10.10.
Viruses are genetic materials enclosed in a protein “coat.” Viruses show a very high specifi city
for a particular host cell, infecting and multiplying only within those cells. Viral genetic material
can be either DNA or RNA but is almost always double stranded. Viral attacks generally result in
a virus infecting a cell by depositing its own genetic material, here RNA into the cell’s cytoplasm.
The purpose of depositing this viral-derived RNA is to have it replicate as rapidly as possible in an
attempt to hijack the cell into producing viral-directed protein. We need to remember that almost all
of the viral-derived RNAs are double stranded, rather than the cells single-stranded RNA.
By accident, it was discovered about a decade ago that the cell had a new weapon in its auto-

immune system. Thought there had been prior hints, the first reasonable evidence that something
was there was the work of Jorgensen, Cluster, English, Que, and Napoli (Plant. Mol. Biol. 31:957
[1996]). Their effort to darken the purple color of petunias, working toward a so-called black color,

9/14/2010 3:41:17 PM
K10478.indb 344 9/14/2010 3:41:17 PM
K10478.indb 344