Tools and Techniques — NCERT Solutions

Principle of Agarose Gel Electrophoresis:

Definition: Agarose gel electrophoresis is a technique used to separate, identify, and purify DNA (or RNA) fragments based on their size and charge by passing them through an agarose gel matrix under the influence of an electric field.

Principle:

1. Charge on DNA: DNA molecules are negatively charged due to the phosphate groups in their backbone. When placed in an electric field, DNA migrates from the negative electrode (cathode) towards the positive electrode (anode).

2. Molecular sieve effect: The agarose gel (a polysaccharide derived from seaweed) forms a porous matrix when solidified. This matrix acts as a molecular sieve.

3. Size-based separation:

• Smaller DNA fragments experience less resistance from the gel pores and migrate faster, travelling a greater distance from the well towards the positive electrode.
• Larger DNA fragments experience more resistance and migrate slower, remaining closer to the well.

$$\text{Rate of migration} \propto \frac{1}{\text{Size of DNA fragment}}$$

4. Visualisation: After electrophoresis, the gel is stained with ethidium bromide (EtBr), which intercalates between DNA base pairs and fluoresces under UV light, making the DNA bands visible as bright orange bands.

5. Size determination: By running a DNA ladder (standard molecular weight marker) alongside the samples, the size of unknown DNA fragments can be estimated by comparing their migration distance with that of the ladder bands.

Factors affecting migration:

• Size of DNA fragment
• Concentration of agarose (pore size)
• Applied voltage
• Buffer composition and ionic strength
• Conformation of DNA (linear, circular, supercoiled)

Conclusion: Agarose gel electrophoresis is a simple, rapid, and widely used technique in molecular biology for DNA analysis, restriction mapping, PCR product analysis, and Southern blotting.

Tracking Dye:

The commonly used tracking dye in electrophoresis is Bromophenol Blue.

• It is a small, negatively charged dye molecule that migrates through the gel faster than most DNA or protein molecules.
• It is visible as a blue band and indicates the migration front of the electrophoresis run.
• Another tracking dye used is Xylene cyanol FF, which migrates more slowly than bromophenol blue and is used to track larger fragments.

What happens if tracking dye is not added?

If the tracking dye is forgotten during electrophoresis, the following consequences occur:

1. No visible migration front: There will be no visible band to monitor the progress of electrophoresis. The researcher cannot determine how far the electrophoresis has proceeded.

2. Risk of over-running the gel: Without the tracking dye, the DNA/protein bands may run off the end of the gel (migrate out of the gel completely) before the researcher stops the run, resulting in loss of sample.

3. Wasted experiment: The entire electrophoresis run may be wasted because the researcher cannot judge the appropriate time to stop the run.

4. No harm to separation itself: The absence of tracking dye does not affect the actual separation of DNA or proteins — the molecules will still separate based on size. However, the run cannot be monitored visually.

Conclusion: The tracking dye is essential for monitoring the progress of electrophoresis and preventing over-running of the gel. Without it, the experiment may fail due to loss of sample.

Given:

• Gel A: 4% acrylamide (low concentration)
• Gel B: 12% acrylamide (high concentration)

Concept: In polyacrylamide gel electrophoresis (PAGE), the concentration of acrylamide determines the pore size of the gel:

• Higher acrylamide concentration → smaller pores → harder gel → more friction
• Lower acrylamide concentration → larger pores → softer gel → less friction

(a) Which gel is harder: A or B?

Gel B (12% acrylamide) is harder.

Higher acrylamide concentration leads to greater cross-linking between acrylamide chains, resulting in a denser, harder gel with smaller pores.

(b) Which gel offers greater friction to the proteins: A or B?

Gel B (12% acrylamide) offers greater friction.

The smaller pore size in Gel B creates more resistance/friction to the movement of protein molecules through the gel matrix.

(c) Which gel will be used to separate a mixture containing low molecular weight proteins?

Gel B (12% acrylamide) will be used.

Reason: Low molecular weight (small) proteins can easily pass through large pores (as in Gel A) without being separated. A gel with smaller pores (Gel B, 12%) provides the necessary friction and resistance to effectively separate small proteins based on their size differences.

(d) Which gel will be used to separate a mixture containing both low and high molecular weight proteins?

Gel A (4% acrylamide) will be used (or a gradient gel, but between the two given options, Gel A is more appropriate).

Reason: A gel with larger pores (Gel A, 4%) allows both large and small proteins to enter and migrate through the gel. High molecular weight proteins cannot enter a 12% gel (Gel B) as the pores are too small, and they would be trapped near the well. Gel A with larger pores accommodates a wider range of protein sizes, allowing separation of both low and high molecular weight proteins.

Note: In practice, a gradient gel (e.g., 4–20% acrylamide gradient) is ideal for separating a wide range of molecular weights, but among the two given options, Gel A is preferred.

What is a Chromatogram?

A chromatogram is the visual record or output produced by a chromatography experiment. It represents the separation pattern of the components of a mixture after they have been separated by a chromatographic technique.

• In paper/thin-layer chromatography (TLC): The chromatogram is the physical paper or plate showing the separated spots of different components at different positions (heights/distances from the origin).
• In column/HPLC/GC chromatography: The chromatogram is a graph plotting detector response (signal intensity) on the Y-axis against time (retention time) or distance on the X-axis. Each peak represents a different component.

Key terms:

• $R_f$ value (Retardation factor): Used in paper/TLC chromatography.

$$R_f = \frac{\text{Distance travelled by solute}}{\text{Distance travelled by solvent front}}$$

• Retention time: Time taken by a component to travel through the column and reach the detector (used in GC/HPLC).

Well-Labeled Diagram of a Chromatogram (Paper/TLC type) of a mixture containing three solutes:

        |
        |  Solvent front ←─────────────────────────────
        |
        |         ● Spot C (most mobile, highest Rf)
        |
        |    ● Spot B (intermediate mobility)
        |
        | ● Spot A (least mobile, lowest Rf)
        |
        |─────────────────── Origin (baseline)
        |
        └──────────────────────────────────────────
             Chromatography paper/TLC plate

Labels on the diagram:

1. Origin/Baseline — where the mixture is initially spotted
2. Solvent front — the farthest point reached by the mobile phase
3. Spot A — component with lowest $R_f$ (least soluble in mobile phase / most attracted to stationary phase)
4. Spot B — component with intermediate $R_f$
5. Spot C — component with highest $R_f$ (most soluble in mobile phase / least attracted to stationary phase)
6. Distance travelled by solvent — from origin to solvent front
7. Distance travelled by each solute — from origin to centre of each spot

$R_f$ values:
$$R_f(A) < R_f(B) < R_f(C)$$

Each component is separated because it has a unique affinity for the stationary phase and mobile phase, resulting in different migration distances.

Fluorescence In Situ Hybridisation (FISH)

Principle of FISH:

FISH is based on the principle of complementary base pairing (hybridisation) between a labeled nucleic acid probe and its target DNA/RNA sequence within intact cells, tissues, or chromosomes.

Steps involved:

1. Probe preparation: A single-stranded DNA or RNA probe, complementary to the target sequence, is labeled with a fluorescent dye (fluorophore).
2. Denaturation: The chromosomal DNA in the specimen is denatured (double helix unwound into single strands) using heat or chemicals.
3. Hybridisation: The fluorescent probe is added and allowed to hybridise (bind) to its complementary target sequence on the chromosome under controlled temperature and salt conditions.
4. Washing: Unbound/non-specifically bound probes are washed away.
5. Visualisation: The specimen is examined under a fluorescence microscope. The location where the probe has hybridised appears as a bright fluorescent signal, indicating the position of the target gene/sequence on the chromosome.

$$\text{Probe (fluorescent)} + \text{Target DNA (denatured)} \xrightarrow{\text{hybridisation}} \text{Fluorescent signal at target location}$$

Application of FISH in Chromosome Painting:

Chromosome painting (also called spectral karyotyping, SKY) is an advanced application of FISH in which entire chromosomes or large chromosomal regions are labeled with fluorescent probes.

How it works:

• A cocktail of multiple fluorescent probes, each specific to different regions of a particular chromosome, is prepared.
• Each chromosome (or chromosomal region) is labeled with a unique combination of fluorescent dyes, giving each chromosome a distinct colour.
• After hybridisation, each chromosome appears in a different colour when viewed under a fluorescence microscope with appropriate filters.
• Computer software analyses the fluorescence signals and assigns a unique pseudo-colour to each chromosome pair.

Result: All 23 pairs of human chromosomes can be simultaneously visualised, each in a distinct colour — like a painted karyotype.

Advantages of Chromosome Painting:

1. Detection of chromosomal abnormalities: Easily identifies translocations (exchange of segments between chromosomes), deletions, inversions, and duplications that may be missed by conventional karyotyping.

2. Identification of marker chromosomes: Unknown or structurally abnormal chromosomes (marker chromosomes) can be identified by their unique colour.

3. Cancer cytogenetics: Detects chromosomal rearrangements associated with cancers (e.g., Philadelphia chromosome in CML).

4. Simultaneous analysis: All chromosomes can be analysed simultaneously in a single experiment, saving time.

5. High sensitivity and specificity: Fluorescent probes provide highly specific and sensitive detection of chromosomal changes.

6. Prenatal diagnosis: Used in prenatal diagnosis to detect chromosomal disorders in foetal cells.

7. Evolutionary studies: Helps in comparative genomics and understanding chromosomal evolution across species.

8. No need for high-resolution banding: Can detect rearrangements even when conventional G-banding fails to resolve them.

Applications of Spectroscopy Techniques:

Spectroscopy involves the study of the interaction between matter and electromagnetic radiation. It has wide-ranging applications in biological, chemical, and medical sciences:

1. UV-Visible Spectroscopy:

• Quantification of biomolecules: Measures the concentration of DNA, RNA, and proteins by measuring absorbance at specific wavelengths (e.g., DNA at 260 nm, proteins at 280 nm).
• Enzyme kinetics: Monitors enzyme activity by measuring the change in absorbance of substrates or products over time.
• Purity assessment: The ratio $A_{260}/A_{280}$ is used to assess the purity of nucleic acid preparations.
• Colorimetry: Measures concentration of coloured compounds in solution.

2. Infrared (IR) Spectroscopy:

• Identification of functional groups: Identifies chemical functional groups (–OH, –NH, –C=O, etc.) in organic molecules.
• Structure elucidation: Helps determine the molecular structure of unknown compounds.
• Quality control: Used in pharmaceutical industry to verify the identity and purity of compounds.

3. Mass Spectrometry (MS):

• Identification of unknown compounds: Determines molecular mass and structural information.
• Proteomics: Identifies and quantifies proteins in complex biological samples.
• Metabolomics: Analyses metabolites in biological fluids.
• Drug discovery: Identifies drug candidates and their metabolites.
• Forensic analysis: Identifies unknown substances in forensic investigations.

4. Nuclear Magnetic Resonance (NMR) Spectroscopy:

• Structure determination: Determines the 3D structure of proteins, nucleic acids, and small molecules in solution.
• Drug development: Studies drug–receptor interactions.
• Metabolic profiling: Analyses metabolites in biological samples.

5. Fluorescence Spectroscopy:

• Protein structure studies: Monitors conformational changes in proteins.
• FISH and flow cytometry: Uses fluorescent probes for cell and chromosome analysis.
• Immunoassays: Used in fluorescence-based ELISA and immunofluorescence.
• Biosensors: Development of fluorescence-based biosensors for detecting analytes.

6. Atomic Absorption Spectroscopy (AAS):

• Trace metal analysis: Detects and quantifies trace metals (e.g., Fe, Cu, Zn) in biological and environmental samples.

7. Raman Spectroscopy:

• Molecular fingerprinting: Identifies molecules based on their unique Raman spectra.
• Medical diagnostics: Used in cancer diagnosis and tissue characterisation.

Summary: Spectroscopy techniques are indispensable tools in biotechnology, biochemistry, medicine, environmental science, and forensic science for identification, quantification, structural analysis, and quality control of substances.

Major Components of a UV-Visible Spectrophotometer:

A UV-visible spectrophotometer measures the absorbance or transmittance of a sample at specific wavelengths in the ultraviolet (200–400 nm) and visible (400–700 nm) range of the electromagnetic spectrum.

1. Light Source:

• Provides the electromagnetic radiation required for the measurement.
• Deuterium lamp: Used for the UV region (200–400 nm).
• Tungsten-halogen lamp: Used for the visible region (400–700 nm).
• Modern instruments automatically switch between lamps as the wavelength changes.

2. Monochromator:

• Selects a specific wavelength (or narrow band of wavelengths) of light from the broad spectrum emitted by the light source.
• Components include:
• Entrance slit: Allows a narrow beam of light to enter.
• Diffraction grating or prism: Disperses the light into its component wavelengths.
• Exit slit: Allows only the selected wavelength to pass through to the sample.
• Ensures that only monochromatic (single wavelength) light reaches the sample.

3. Sample Holder (Cuvette/Cell):

• Holds the liquid sample through which the monochromatic light passes.
• Quartz cuvettes are used for UV measurements (glass absorbs UV light).
• Glass or plastic cuvettes are used for visible light measurements.
• The standard path length of a cuvette is 1 cm.
• A reference cuvette (containing only the solvent/blank) is used to correct for background absorbance.

4. Detector:

• Measures the intensity of light that passes through (transmitted light) or is absorbed by the sample.
• Common detectors:
• Photomultiplier tube (PMT): Highly sensitive; converts photons into electrical current.
• Photodiode array (PDA): Detects multiple wavelengths simultaneously.
• The detector converts the light signal into an electrical signal proportional to the light intensity.

5. Signal Processor and Display (Readout System):

• The electrical signal from the detector is processed by an amplifier and converted into a readable output.
• The output is displayed as:
• Absorbance (A): $A = \log\left(\frac{I_0}{I}\right)$, where $I_0$ = incident light intensity, $I$ = transmitted light intensity.
• Transmittance (T): $T = \frac{I}{I_0} \times 100\%$
• Modern instruments are connected to computers for data analysis and storage.

Working Principle (Beer-Lambert Law):
$$A = \varepsilon \cdot c \cdot l$$
Where:

• $A$ = Absorbance
• $\varepsilon$ = Molar extinction coefficient (L mol$^{-1}$ cm$^{-1}$)
• $c$ = Concentration of the sample (mol L$^{-1}$)
• $l$ = Path length of the cuvette (cm)

This law states that absorbance is directly proportional to the concentration of the absorbing species, which forms the basis of quantitative analysis using a spectrophotometer.

Differences between Sanger's Method and Maxam & Gilbert's Method of DNA Sequencing:

Summary:

• Sanger's method is enzymatic, safer, easily automated, and is the preferred method for routine DNA sequencing.
• Maxam & Gilbert's method is chemical, hazardous, and difficult to automate, and has been largely replaced by Sanger's method for routine sequencing.

Principle of Flow Cytometry:

Definition: Flow cytometry is a laser-based, biophysical technology used for the rapid analysis of the physical and chemical characteristics of cells or particles as they flow in a fluid stream through a laser beam, one cell at a time.

Principle:

1. Cell suspension preparation: Cells or particles are suspended in a sheath fluid (usually isotonic saline). The sheath fluid hydrodynamically focuses the cells into a single-file stream.

2. Laser interrogation: The single-file stream of cells passes through a focused laser beam (commonly argon laser at 488 nm or other wavelengths). Each cell passes through the laser beam individually.

3. Light scattering: When a cell passes through the laser beam, it scatters light in different directions:

• Forward Scatter (FSC): Light scattered in the forward direction (small angles, 0.5°–10°). FSC is proportional to the cell size (volume).
• Side Scatter (SSC): Light scattered at 90° to the laser beam. SSC is proportional to the internal complexity (granularity) of the cell — e.g., nuclear lobularity, cytoplasmic granules.

4. Fluorescence detection: Cells can be labeled with fluorescent dyes or antibodies conjugated to fluorophores that bind to specific cell surface markers or intracellular components. When excited by the laser, these fluorophores emit light at specific wavelengths, which is detected by photomultiplier tubes (PMTs) or photodetectors through appropriate optical filters.

5. Data analysis: The scattered and fluorescent light signals are converted into electrical pulses, digitised, and analysed by a computer. Data is typically displayed as:

• Dot plots (bivariate plots of two parameters)
• Histograms (frequency distribution of one parameter)

$$\text{Cell properties measured: Size (FSC), Granularity (SSC), Fluorescence intensity}$$

Applications:

• Cell counting and sorting (FACS — Fluorescence Activated Cell Sorting)
• Immunophenotyping (identifying cell types by surface markers, e.g., CD4+ T cells)
• Cell cycle analysis (DNA content measurement using propidium iodide)
• Apoptosis detection
• Intracellular cytokine detection
• Chromosome analysis and sorting

Conclusion: Flow cytometry is a powerful, high-throughput technique that can analyse thousands of cells per second, providing multiparametric data on individual cells based on their light scattering and fluorescence properties.