Download M220 Lecture 17 - Napa Valley College

M220 Lecture 17
Classification (continued)
Genetic and Molecular Approaches
1. Conjugation (mating) experiments-do they have the ability to produce conjugal pairs. This
would be indicative of having the same species or strain.
2. DNA base composition experiments-compare the percentage of G (guanine) plus C (cytosine) in
one species to the percentage of G plus C in another. It is calculated using the following
formula: G + C/G +C+A+T X 100. This GC ratio within a species should be a fixed property. The
idea is to use this percentage to determine species relatedness. One conclusion that can be
drawn is that dissimilarity in GC percentages amongst organisms implies that the organisms are
dissimilar. Simply stated, dissimilarity = dissimilarity. However, similarities in GC percentages
amongst organisms do not imply that the organisms are the same. Simply stated, similarity ≠
similarity. Finally, organisms known to be related by other criteria such as biochemical and
morphological characteristics have been shown to have DNA base compositions which are
similar or nearly identical.
3. Nucleic acid hybridization-if organisms are related their DNA base sequences will be nearly the
same. If organisms are not related, their DNA base sequences will be different. DNA from two
test organisms is subjected to heat. This will cause the strands from the double stranded DNA
to separate. Cooling will allow the strands to come back together (reanneal). During this
process complimentary bases will match up. A hybrid is created when the DNA strand from one
organism joins with the DNA strand from the other test organism. Complete hybridization
indicates that the two test organisms are identical. Partial hybridization indicates that they are
related. No hybridization indicates that the organisms are unrelated. Use UV absorbance
properties to determine how well the complimentary bases in DNA are paired. Single stranded
DNA absorbs UV more readily than double stranded DNA. The strands in a hybrid when exposed
to heat will separate. The separate strands will now absorb UV light. Hybrids that are readily
separated by heat are less closely related than those that require more heat for separation. To
compare extent of hybridization and relatedness, expose hybrids to heat and check to see which
hybrid absorbs UV light sooner. The earlier one observes UV light absorbance in a hybrid, the
less the hybrid bases are matched. Stable hybrids (more related) require more heat for
separation. Increased UV absorbance will therefore be seen at higher temperatures with
hybrids from these closely related species as they are more stable thermally.
4. DNA (gene) product
a. Compare amino acid sequences shared on identical proteins. Use amino acid sequencers.
b. Instead of analyzing single proteins, compare several proteins from the cell cytoplasm. Use
a technique known as polyacrylamide gel electrophoresis to separate out the proteins from
a sample. Proteins are placed in wells in a gel and an electric field is applied. The molecules
will move through the matrix at different rates depending upon differences in mass and
charge. After the movement is completed the different proteins appear as separate bands
upon the gel. Compare the band patterns between similar proteins to determine if any
differences exist between the original samples. Note that gel electrophoresis of nucleic
acids involves the use of an agarose gel rather than a polyacrylamide gel.
Final thoughts for Lecture Unit II-A bacterial cell has approximately 2000 to 3000 genes. At any given
time some of those genes are in use undergoing transcription and making mRNA. It is fair to say that the
number of different mRNA’s that are actually produced in a cell would be less than the number of genes
in the genotype. With respect to tRNAs, only 61 are functioning to deliver amino acids. Lastly, a cell
typically makes two different types of rRNA that are ultimately used to manufacture ribosomes.
End of Lecture Unit II
Beginning of Lecture Unit III-Fungi, Viruses and Parasitology
Fungi
Fungi are eukaryotic heterotrophs (use organic compounds as a carbon source) that are very often
parasitic (feed upon live cells) and/or saprophytic (feed upon dead and decaying cells). There are over
100,000 species of fungi. They are a plentiful form of life. In their vegetative form they are non-motile.
Fungi are organisms that do not undergo photosynthesis. They can grow in a temperature range of 0 to
62 degrees C. Optimum temperature is 20 to 30 degrees C. They generally prefer a slightly acidic pH,
and survive in high osmotic environments (can survive in 50% sugar). They need less nitrogen than
bacteria and can survive on such substances as paint and shoe leather. Fungi can metabolize complex
carbohydrates that even bacteria cannot use. They generally grow more slowly than bacteria. To isolate
fungal organisms from bacteria, add antibiotics. This will suppress the bacteria and allow yeasts or
molds to grow.
The fungi consist of the yeasts, which are typically unicellular, non-filamentous, spherical or oval forms.
Yeasts are facultative anaerobes. When O2 is present they will undergo aerobic respiration. When O2 is
unavailable they will undergo fermentation and produce products such as ethanol and CO2. Some can
be dimorphic appearing as cellular forms in certain circumstances and filamentous forms at other times.
The fungi also consist of the molds, which are typically multicellular, filamentous forms. Molds are
mostly aerobic with respect to their metabolism and form a fuzzy, powdery growth seen on plant and
animal tissue. Some molds are deemed to be dimorphic. Finally, the fleshy fungi consist of the
mushrooms that are multicellular, filamentous forms that produce a thick fleshy reproductive structure.
To best describe the primitive body of a mold the word thallus is used. The thallus is a “plant-like”
structure that is not differentiated into stems, roots and leaves. The characteristic unit of a mold is the
hypha (plural is hyphae). Hyphae are non-motile, multinucleated, branching tubes. A mass of hyphae
is called a mycelium. Hyphae can be used to characterize the molds. Structurally hyphae can be nonseptate (coenocytic) where cross walls between cells do not exist or septate, where cross walls between
cells are present. When septate, the cells of hyphae can have just one nucleus (uninucleate) in some
species, or possess more than one nucleus (multinuclear) in others. Functionally, hyphae can be
vegetative or reproductive. Reproductive hyphae can produce either asexual or sexual spores.