20-Bio-B10 Biomechanical Device Design & Human Factors · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2015 — 04-Bio-B10 Analytical Biochemistry. Three hours, closed book, any non-communicating calculator. Six questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All six are solved here, because this set is a study resource rather than an examination script. Every question is essay/descriptive (technique principle, interpretation of an instrument trace or image) rather than numerical, except Question 2(d), which asks for a short exponential-growth calculation from PCR cycle theory.
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
[Figure not reproduced: FL1-H fluorescence histograms from Question 5(a). See the official exam paper or the cited reference text.]
FL1-H reports fluorescence intensity in the channel used to detect GFP. Before treatment, essentially the whole population sits in the FL1-H-negative gate (98.0% FL1-H−, only 2.03% FL1-H+) — this small positive tail is simply the population's baseline autofluorescence/background and defines where the positive gate is drawn. After transfection, the FL1-H+ fraction rises to 13.1% (FL1-H− falls to 86.9%). Since the gate boundary itself already captures ~2% of cells as a false-positive background, the background-corrected estimate of true GFP-expressing cells is roughly 13.1% − 2.0% ≈ 11% of the population — about a six-fold increase in the fluorescent fraction relative to background. This confirms the transfection worked (a clear, reproducible shift into the GFP-positive gate under the strong constitutive promoter, exactly as expected), but the overall efficiency achieved — roughly one cell in eight to ten — is only moderate: many standard lipid-mediated transfection protocols in easily-transfectable mammalian cell lines can reach 30–70% efficiency, so this result, while a real positive signal, suggests the protocol (cell type, DNA:reagent ratio, or time post-transfection) still has room for optimization if a higher yield of GFP-expressing cells were required for a downstream application.
[Figure not reproduced: Side-scatter histograms from Question 5(b). See the official exam paper or the cited reference text.]
Side scatter (SSC) is laser light that is scattered at a wide angle — conventionally collected at roughly 90° (orthogonal) to the path of the incident laser beam — as each cell passes through the flow cell; it is measured by a photodetector positioned to the side of the flow path, physically separate from the forward-scatter detector that sits close to the beam's original axis. Side scatter arises from light refracting and reflecting off internal interfaces within the cell — the nuclear membrane, granules, vesicles, mitochondria, and other internal membranous structures — so its intensity correlates with the cell's internal complexity or granularity rather than with its overall size (which is instead what forward scatter, collected along the beam axis, principally reports). A cell with a highly folded internal membrane system or dense granules (e.g. a granulocyte, or a cell that has taken up particulate/vesicular material) scatters more light to the side than a cell with a smooth, homogeneous cytoplasm. The modest shift seen here (SSC-H+ rising from 2.05% to 16.4% after treatment) is consistent with the transfection procedure itself — uptake of the lipoplex/DNA-carrier complex into endosomal vesicles — measurably increasing the internal granularity of a subset of treated cells, on top of whatever the plasmid's own expression product might contribute.
