Biotechnology: Class 11 — Chapter 12: Tools and Techniques

• Magnification: The ratio of the size of the retinal image with the microscope to that without the microscope. For a single lens, magnification ($M$) is calculated as:
  $$M = \frac{f}{f-d}$$
  where $f$ is the focal length of the lens and $d$ is the distance of the object from the lens.
• Compound Microscope Magnification: The product of the magnifying power of the objective lens ($M_o$) and the eyepiece ($M_e$):
  $$M = M_o \times M_e$$
• Resolving Power: The ability of an optical instrument to form separate images of two objects situated very close to each other, measured by the minimum distance between the two points.

A compound light microscope utilizes a dual-lens system to illuminate and magnify specimens. Light from a source (mirror or bulb) is focused by a condenser onto the specimen. The objective lens (typically 4x, 10x, 40x, or 100x) forms a real, magnified image, which is further magnified by the eyepiece (10x or 15x) to form a virtual image for the observer.

• Differential Centrifugation: Separates particles based on differences in their sedimentation rate. Successive runs at increasing centrifugal forces pellet progressively smaller components (e.g., nuclei, then mitochondria, then microsomes).
• Density-Gradient Centrifugation: Particles travel through a continuous or discontinuous density gradient (e.g., sucrose or CsCl) until they reach a zone matching their own buoyant density (isopycnic point).
• Ultracentrifugation: Performed at extremely high speeds (typically $\ge 100,000 \times g$) under vacuum to prevent frictional heating, allowing the separation of macromolecules like proteins and nucleic acids.

Used primarily for separating nucleic acids (DNA/RNA). Agarose forms a porous polysaccharide matrix held together by hydrogen bonds.

• Migration Direction: DNA is negatively charged due to its phosphate backbone and migrates toward the positive electrode (anode).
• Size Separation: Shorter DNA fragments travel faster through the pores than longer fragments.
• DNA Ladder: A mixture of DNA fragments of known sizes run alongside samples to determine absolute molecular weights.
• Visualization: Stained with ethidium bromide (EtBr), which intercalates between bases and fluoresces orange-red under a UV trans-illuminator.

Used for high-resolution separation of proteins and small nucleic acids.

• SDS-PAGE: Proteins are denatured and coated with sodium dodecyl sulfate (SDS), an anionic detergent that imparts a uniform negative charge proportional to mass, allowing separation strictly by molecular weight.
• Gel Matrix: Formed by polymerization of acrylamide monomer and N,N'-methylene-bis-acrylamide (BIS) cross-linker. Pore size is controlled by adjusting total monomer concentration.
• Protein Visualization: Stained using Coomassie Brilliant Blue R-250, which binds non-specifically to proteins, appearing as blue bands.

Small, highly mobile charged molecules added to samples to monitor the progress of electrophoresis. Common dyes include bromophenol blue, xylene cyanol, and Orange G.

• Quantification of serum antibodies in viral diagnostics (e.g., HIV, hepatitis).
• Detection of potential food allergens in the food processing industry.
• Epidemiological tracking of disease outbreaks (e.g., bird flu, cholera).

The fundamental law governing quantitative spectrophotometric analysis. It states that absorbance ($A$) is directly proportional to the concentration ($c$) of the absorbing solute and the path length ($l$) of the sample:

Where $\varepsilon$ is the molar extinction coefficient. Transmittance ($T$) is the ratio of transmitted light intensity ($I$) to incident light intensity ($I_0$):

• Colorimetry: Measures light absorption in the visible spectrum using colored filters to select broad bandwidths of light. Simple, portable, and inexpensive.
• UV-Visible Spectrophotometry: Uses a prism or diffraction grating monochromator to isolate highly specific, narrow wavelengths of light across both ultraviolet (200-400 nm) and visible (400-800 nm) ranges.

The gold-standard enzymatic sequencing method based on the selective incorporation of chain-terminating dideoxynucleotides.

• Mechanism: Uses dideoxynucleotides (ddNTPs), which lack the 3'-OH group required for phosphodiester bond formation. Incorporation of a ddNTP terminates chain elongation.
• Reaction Setup: Four separate reactions (or one multiplexed reaction) containing template DNA, primer, DNA polymerase, normal dNTPs, and a low concentration of one specific ddNTP (ddATP, ddTTP, ddGTP, or ddCTP).
• Resolution: The resulting fragments of varying lengths are separated by high-resolution polyacrylamide gel electrophoresis (PAGE) or capillary electrophoresis.

Based on base-specific chemical cleavage of end-labeled DNA fragments. Uses hazardous chemicals (e.g., dimethyl sulphate for purines, hydrazine for pyrimidines) followed by piperidine treatment to cleave the backbone.

• Automated Sequencing: Utilizes four different fluorescently labeled ddNTPs in a single tube reaction. Fragments are separated by capillary electrophoresis and read in real-time by a laser detector.
• Next-Generation Sequencing (NGS): High-throughput, massively parallel sequencing platforms (e.g., Illumina, Roche/454, SOLiD) capable of sequencing millions of fragments simultaneously, enabling whole-genome sequencing.