Page 379 - Carrahers_Polymer_Chemistry,_Eighth_Edition

342 Carraher’s Polymer Chemistry

highly folded. Interestingly, there is a similarity between the folded structures of rRNA from many
different sources even though the primary structure, base sequence, is quite varied. Thus, there
appears to be preferred folding patterns for rRNAs.
Messenger RNA is the carrier of messages that are encoded in genes to the sites of protein syn-
thesis in the cell where this message is translated into a polypeptide sequence. Because mRNAs are
transcribed copies of the genetic unit, they are sometimes referred to as being the “DNA-like RNA”.
mRNA is made during transcription, an enzymatic sequence in which a specifi c RNA sequence is
“copied” from a gene site. rRNA and tRNA are also made by transcription of DNA sequences but
unlike mRNA, they are not subsequently translated to form proteins.
Actual reproduction steps involving DNA and RNA often occur in concert with protein where
the protein can act as a clamp or vice holding the various important members involved with the
particular reproduction step in place. Thus, the protein complex acts as an assembly line tunnel or
doughnut with the reactants present within the interior.
There are two types of cells. Prokaryote cells lack a cell nucleus or any other membrane-bound
organelles. Most are single celled. Bacteria and archaea are examples of organisms with prokaryote
cells. Eukaryote cells possess enclosed membranes and form the basis of animals, plants, and fungi.
In prokaryote cells the mRNA can be used immediately after it is produced or it may be bound to
a ribosome. In comparison, in eukaryote cells mRNA is made in the cell nucleus and it is moved
across the nuclear membrane after it is synthesized into the cytoplasm where protein synthesis
occurs.

10.4 FLOW OF BIOLOGICAL INFORMATION
Nucleic acids, proteins, some carbohydrates and hormones are informational molecules. They carry
directions for the control of biological processes. With the exception of hormones, these are mac-
romolecules. In all these interactions, secondary forces such as hydrogen bonding and van der
Waals forces, and ionic bonds, and hydrophobic/hydrophilic character play critical roles. Molecular
recognition is the term used to describe the ability of molecules to recognize and interact (bond)

specifically with other molecules. This molecular recognition is based on a combination of these
interactions just cited and on structure.
Molecular recognition interactions have several common characteristics. First, the forces that are
involved in these interactions are relatively weak and they are noncovalent. They are on the order
of about 1–8 kcal/mol (4–30 kJ/mol) compared to covalent bonds of the order of about 80 kcal/mol
(300 kJ/mol) for a C–C sigma bond. A single secondary bond is generally not capable of holding
molecules together for any length of time. But for macromolecules, there is a cumulative effect so
that the forces are not singular but are multiplied by the number of such interactions that are occur-
ring within the particular domain. Second, these interactions are reversible. Initial contact occurs
as the molecules come into contact with one another often through simple diffusion or movement of
the molecules or segments of the molecules. These initial contacts are often not suffi cient to cause
the needed binding though some transitory interactions to occur. Even so, in some cases the cumu-

lative bonding is sufficient to allow a transient but significant interaction to occur. This complex can

then begin a specific biological process. Eventually, thermal motions and geometrical changes cause

the complex to dissociate. Ready reversibility is an important key that allows a relatively few “sig-
naling” molecules to carry out their mission. Third, bonding between the particular molecular sites

is specific. There must exist a combination of complementary bonding, hydrophobic/hydrophilic
sites, ionic charge, and geometry that allow effective long-term (generally no more than several
seconds) interactions to occur.

In general, the flow of biological information can be mapped as follows:
DNA → RNA → Protein→ Cell structure and function

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