Hogg S. Essential microbiology
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438 GLOSSARY
Nonsense mutation a mutation that results in a ‘stop’ codon being inserted into
the mRNA at the point where it occurs, and the premature termination of translation.
Nosocomial infection an infection that is acquired in hospital or other healthcare
setting.
Nuclear membrane the double membrane surrounding the nucleus of a eucaryotic
cell.
Nucleocapsid the genome of a virus and its surrounding protein coat.
Nucleoid another name for the bacterial chromosome. The site of most of a
procaryotic cell’s DNA.
Nucleolus a discrete region of the eucaryotic nucleus, where ribosomes are
assembled.
Nucleotide the building block of nucleic acids, comprising a pentose sugar, a
nitrogenous base and one or more phosphate groups.
Nucleus the central, membrane-bound structure in eucaryotic cells that contains
the genetic material. Also, the region of an atom that contains the protons and
neutrons.
Obligate anaerobe an organism that is incapable of growth in the presence of
oxygen.
Okazaki fragments discontinuous fragments of single-stranded DNA synthesised
complementary to the lagging strand during DNA replication.
Oncogene a gene associated with the conversion of a cell to a cancerous form.
Oogamy the fusion of a small, motile sperm cell and a larger, immobile egg cell.
Operon a group of related genes under the control of a single operator sequence.
Organotroph an organism that uses organic molecules as a source of electrons.
Outer membrane the outermost part of the Gram-negative cell wall, comprising
phospholipids and lipopolysacchride.
Oxidation a chemical reaction in which an electron is lost.
Oxygenic photosynthesis a form of photosynthesis in which oxygen is produced;
found in algae, cyanobacteria (blue greens) and also green plants.
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GLOSSARY 439
Parasitism an association between two species from which one partner derives
some or all of its nutritional requirements by living either in or on the other (the host),
which usually suffers harm as a result.
Pasteurisation a mild heating regime used to destroy pathogens and spoilage
organisms present in food and drink, especially milk.
Pathogen an organism with the potential to cause disease.
Pellicle a semi-rigid structure composed of protein strips found surrounding the cell
of certain unicellular protozoans and algae.
Pentose a five-carbon sugar.
Pentose phosphate pathway a secondary pathway for the oxidation of glucose,
resulting in the production of pentoses that serve as precursors for nucleotides.
Peptide bond the bond formed between the amino group of one amino acid and
the carboxyl group of another.
Peptidoglycan a polymer comprising alternate units of N-acetylmuramic acid and
N-acetylglucosamine that forms the major constituent of bacterial cell walls.
Phagemid a hybrid cloning vector, comprising elements of plasmid and phage.
Phagocytosis the ingestion and digestion of particulate matter by a cell, a process
unique to eucaryotes.
Phenol coefficient a measure of the efficacy of a disinfectant against a given
organism, compared to that of phenol.
Phenotype the observable characteristics of an organism.
Phospholipid an important constituent of all membranes, comprising a
triacylglycerol in which one fatty acid is replaced by a phosphate group.
Phosphorylation the addition of a phosphate group.
Photophosphorylation the synthesis of ATP using light energy.
Photoreactivation a DNA repair mechanism involving the light-dependent enzyme
DNA photolyase.
Photosynthesis a process by which light energy is trapped by chlorophyll and
converted to ATP, which is used to drive the synthesis of carbohydrate by reducing CO
2
.
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440 GLOSSARY
Phototroph an organism that is able to use light as its source of energy.
Phylogenetic relating to the evolutionary relationship between organisms.
Phytoplankton the collective term used to describe the unicellular photosynthesisers,
including cyanobacteria, dinoflagellates, diatoms and single-celled algae.
Pilus (pl. pili) a short, hair-like appendage found on surface of procaryotes and
assisting with attachment. Specialised sex pili (q.v.) are involved in bacterial
conjugation.
Plankton the floating microscopic organisms of aquatic systems.
Plasma membrane the membrane that surrounds a cell.
Plasmid a small, self-replicating loop of extrachromosomal DNA, found in bacteria
and some yeasts. Specially engineered forms are used as vectors in gene cloning.
Plasmodium a mass of protoplasm containing several nuclei and bounded by a
cytoplasmic membrane.
Plasmogamy the fusion of the cytoplasmic content of two cells.
Plasmolysis the shrinkage of the plasma membrane away from the cell wall, due to
osmotic loss of water from the
cell.
Point mutation a mutation that involves the substitution of one nucleotide by
another.
Polar having unequal charge distribution, caused by unequal sharing of atoms.
Polymerase chain reaction (PCR) a technique that selectively replicates a specific
DNA sequence by means of in vitro enzymatic amplification.
Polypeptide a chain of many amino acids joined together by peptide bonds.
Polyribosome (polysome) a chain of ribosomes attached to the same molecule of
mRNA.
Polysaccharide a carbohydrate polymer of monosaccharide units.
Primary producer an autotroph that obtains energy from the sun or chemical
sources.
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GLOSSARY 441
Primer a short sequence of single-stranded DNA or RNA required by DNA
polymerase as a starting point for chain extension.
Prion a self-replicating protein responsible for a range of neurodegenerative
disorders in humans and mammals
Procaryote an organism lacking a true nucleus and membrane-bound organelles.
Profundal zone the deepest part of a body of water, where the sun is unable to
penetrate
Promoter a sequence upstream of a gene, where RNA polymerase binds to initiate
transcription.
Prophage the DNA of a temperate phage that has integrated into the host
genome. It remains inactive whilst in this form.
Prostheca a stalked structure formed by an extension of the cell wall and plasma
membrane of certain bacteria.
Prosthetic group a non-polypeptide component of a protein, such as a metal ion or
a carbohydrate.
Protease an enzyme that digests proteins.
Protista a eucaryotic kingdom, comprising mostly unicellular organisms.
Protocooperation a form of mutualistic relationship that is not obligatory for either
partner.
Proton a positively-charged subatomic particle.
Protoplast a cell that has had its cell wall removed.
Prototroph the normal, nonmutant form of an organism.
Protozoa a group of single celled eucaryotes with certain animal-like characteristics.
Pseudomurein a modified form of peptidoglycan found in some archaean cell walls.
Pseudopodium a projection of the plasma membrane of the amoebas that causes
the cell to change shape and allows movement.
Psychrophile an organism that grows optimally at low temperatures (<15
◦
).
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442 GLOSSARY
Psychrotroph an organism that is able to tolerate low temperatures, but grows better
at more moderate values.
Reading frame the way in which a sequence of nucleotides is read in triplets,
depending on the starting point.
Real image an image that can be projected onto a flat surface such as a screen.
Recombinant DNA DNA that comprises material from more than one source.
Recombination any process that results in new combinations of genes.
Redox potential (E
o
) the tendency of a compound to lose or gain electrons.
Reduction a chemical reaction in which an electron is gained.
Refractive index the ratio between the velocity of light as it passes through a
substance and its velocity in a vacuum.
Regulatory gene a gene whose protein product has an effect in controlling the
expression of other genes.
Replacement vector a cloning vectors based on phage λ, in which a central section
of nonessential DNA is replaced by the insert DNA.
Replication fork a Y-shaped structure formed by the separating strands of DNA
during replication.
Repressor protein a protein that prevents transcription of a gene by binding to its
operator.
Resolution the capacity of an optical instrument to discern detail.
Restriction endonuclease an enzyme of microbial origin that cleaves DNA at a
specific nucleotide sequence.
Reverse transcriptase an enzyme found in retroviruses that can synthesise DNA from
an RNA template.
Rhizosphere the region around the surface of a plant’s root system.
Ribosomal RNA (rRNA) a form of RNA that forms part of the structure of ribosomes.
Ribosome an organelle made up of protein and RNA, found in both procaryotes
and eucaryotes. The site of protein synthesis.
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GLOSSARY 443
rRNA see Ribosomal RNA
Saprobe an organism that feeds on dead and decaying organic materials.
Previously termed saprophyte.
Saturated fatty acid a fatty acid that has only single covalent bonds between
adjacent carbon atoms. (c.f. unsaturated)
Secondary metabolite a substance produced by a microorganism after the phase
of active growth has ceased.
Selectable marker a gene that allows cells containing it to be identified by the
expression of a recognisable characteristic.
Selective medium a medium that favours the growth of a particular organism or
group of organisms, often by suppressing the growth of others.
Semi-conservative replication the process of DNA replication by which each
strand acts as a template for the synthesis of a new complementary strand. Each
resultant double stranded molecule thus comprises one original strand and one new
one.
Septate separated by septa or cross-walls.
Sex pilus a narrow extension of the bacterial cell, through which genetic material is
transferred during conjugation.
Shuttle vector a cloning vector that can replicate in both bacterial and yeast host
cells.
Silent mutation a mutation that has no effect on the amino acid encoded by the
triplet.
Single-cell protein (SCP) bacteria and yeast grown in bulk for use as a protein-rich
food supplement.
Slime layer a diffuse and loosely attached polysaccharide layer surrounding the cell
of certain procaryotes.
Specialised transduction the transfer of a limited selection of genes due to imprecise
excision of a prophage in a lysogenic infection cycle.
Sporangiophore a specialised aerial hypha that bears the sporangia.
Sporangium a structure inside which spores develop.
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444 GLOSSARY
Spore a resistant, non-motile reproductive cell.
Sporophyte the diploid, spore-forming stage in a life cycle with alternation of
generations.
Sporozoite a motile infective stage of members of the Sporozoa that gives rise to an
asexual stage within the new host.
Sterilisation the process by which all microorganisms present on or in an object are
destroyed or removed.
Steroid a member of a group of lipids based on a four-ring structure.
Stop codon use definition below.
Substrate level phosphorylation the synthesis of ATP by the direct transfer of a
phosphate group from a phosphorylated organic compound to ADP.
Symbiosis a close physical association between two species from which special
benefits may accrue for one or both parties.
The term is sometimes used specifically to describe such a relationship in which both
parties derive benefit (see mutualism).
Tautomerism the ability of a molecule such as a nucleotide base to exist in two
alternative forms.
Taxonomy the science of naming and classifying living (and once-living) organisms.
Temperate phage a bacteriophage with a lysogenic replication cycle.
Terminator a sequence of DNA that indicates transcription should stop.
stop codons one of the three triplet sequences (also called nonsense codons) that
indicate translation should stop.
Thallus a simple vegetative plant body showing no differentiation into root, stem
and leaf.
Therapeutic index a measure of the selective toxicity of a chemotherapeutic agent.
Thylakoid a photosynthetic membrane found in chloroplasts or free in the cytoplasm
(in cyanobacteria). It contains photosynthetic pigments and components of the
electron transport chain.
Total cell count a method that enumerates all cells, living and dead.
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GLOSSARY 445
Trace element an element required in minute amounts for growth.
Transamination a reaction involving the transfer of an amino group from one
molecule to another.
Transcription the process by which single-stranded mRNA is synthesised from a
complementary DNA template.
Transduction the bacteriophage-mediated transfer of genetic material between
bacteria.
Transformation the uptake of naked DNA from the environment and its integration
into the host genome.
Transposable elements a sequence of DNA that is able to relocate to another
position on the genome.
Transition a mutation in which a purine replaces a purine or a pyrimidine replaces a
pyrimidine.
Translation the process by which the message encoded in mRNA is converted into a
sequence of amino acids.
Transfer RNA (tRNA) a form of RNA that carries specific amino acids to the site of
protein synthesis.
Transversion a mutation in which a purine replaces a pyrimidine, or a pyrimidine
replaces a purine.
Triacylglycerol a lipid formed by the joining of three fatty acids to a molecule of
glycerol.
Tricarboxylic acid cycle a series of reactions that oxidise acetate to CO
2
,
generating reducing power in the form of NADH and FADH
2
for use in the electron
transport chain. Also known as citric acid cycle or Krebs cycle.
Triose a three-carbon sugar.
tRNA see Transfer RNA
Tyndallisation a form of intermittent steam sterilisation.
Undefined or complex medium a medium whose precise chemical composition is
not known.
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446 GLOSSARY
Unsaturated fatty acid a fatty acid that contains one or more double bonds
between adjacent carbon atoms.
Vaccination inoculation with a vaccine to provide protective immunity.
Vaccine a preparation of dead or inactivated living pathogens or their products
used to provide protective immunity.
Vector a self-replicating DNA molecule used in gene cloning. The sequence to be
cloned is inserted into the vector, and replicated along with it.
Viable cell count a method that enumerates only those cells capable of
reproducing to form a visible colony.
Virtual image an image that has no physical existence in space, and cannot be
projected onto a screen.
Virion a complete, intact viral particle
Viroid a plant pathogen that comprises only ssRNA and does not code for protein
product.
Virulent phage a bacteriophage with a lytic replication cycle.
Virus a submicroscopic, noncellular parasite, comprising protein and RNA or DNA.
Wildtype the normal, nonmutant form of an organism or gene.
Wobble the degree of flexibility allowed in the third base in a codon when pairing
with tRNA. The wobble hypothesis explains how a single tRNA can pair with more than
one codon.
Yeast artificial chromosome (YAC) a cloning vector able to accommodate inserts of
several hundred kb in size.
Z value the increase in temperature required to reduce the D value (q.v.) by a
factor of ten.
Zoonosis a disease normally found in animals, but transmissible to humans under
certain circumstances.
Zoospore a flagellated, asexual spore.
Zygospore a thick-walled resistant diploid structure formed by members of the
Zygomycota.
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Appendix
Solution to Numerical Questions
Question 2.5: 1000 (10 × 10 × 10)
Question 2.15: 61 cytosine residues; 286 hydrogen bonds (61 × 3 for GC and 52 × 2
for AT)
447