An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 82

Viruses are replicating particles that are obligate parasites. Some viruses use DNA to store
genetic information, while others use RNA. Viruses specific to bacteria are called bacterio-
phages or phages. A prion is an infectious agent composed of protein in a misfolded
form, in contrast to all other known infectious agents that contain nucleic acids (either
DNA, RNA, or both). Prion is not able to reproduce but produced by inducing proteins
to misfold.
Stem cells are unspecialized cells that have two defining properties: 1) potency, or
the ability to differentiate into diverse specialized cells (different cell types) including
differentiated stem cells and 2) reproductivity or the ability to self-regenerate (same
cell type).
All cells contain at least one of the three macromolecules: protein, RNA, and DNA. Other
essential components of these cells are constructed from lipids and carbohydrates.
Proteins are polymers of amino acids; typically, 20 different amino acids are used. Each
amino acid has a distinctive side group. The sequence of amino acids determines the
primary structure of the protein. Interactions among the side groups of the amino acids
(hydrogen bonding, disulfide bonds, and hydrophobic or hydrophilic regions) determine
the secondary and tertiary structure of the molecule. Separate polypeptide chains can asso-
ciate to form the final structure, which termed quaternary. The 3D shape of a protein is crit-
ical to its function.
DNA and RNA are polymers of nucleotides. DNA contains the cell's genetic informa-
tion. RNA is involved in transcribing and translating that information into real proteins.
mRNA transcribes the code, tRNA is an adapter molecule that transports a specific
amino acid to the reaction site for protein synthesis, and ribosomal RNA is an essential
component of ribosomes, which are the structures responsible for protein synthesis. In
addition to their roles as monomers for DNA and RNA synthesis, nucleotides play
important roles in cellular energetics. The high-energy phosphate bonds in ATP can store
energy. The hydrolysis of ATP when coupled with otherwise energetically unfavorable
reactions can drive the reaction toward completion. NAD and NAPH are important
carriers of reducing power.
Carbohydrates consist of sugars, and the polymerized products of sugars are called poly-
saccharides. Sugars represent convenient molecules for the rapid oxidation and release of
energy. The polysaccharides play an important structural and stability roles (such as cellu-
lose, hemicelluloses, and lignin) and can be used as a cellular reserve of carbon and energy
(as in glycogen, PHA, starch and fructan).
Lipids and related compounds are critical in the construction of cellular membranes. Some
fats also form reserve sources. A number of growth factors or hormones involve lipid mate-
rials. Phospholipids are the primary components of biological membranes.
The maintenance of cellular integrity requires the selective uptake of nutrients. One class
of nutrients is the macronutrients, which are used in large amounts. Micronutrients and trace
nutrients are used in low concentrations; some of these compounds become toxic if present at
high levels.
In a defined medium, all components added to the medium are identifiable. In a complex
medium, one or more components are not chemically defined (e.g. yeast extract). Complex
media are commonly mixtures obtained from cell culture or natural sources without
(complete) componentwise separation/purification.