Tools and Techniques

Correct Option: (b) visualise the DNA molecules

Justification: Ethidium bromide (EtBr) is a fluorescent intercalating dye that inserts itself between the stacked base pairs of DNA. When exposed to UV light, it fluoresces bright orange, making the DNA bands visible on the agarose gel. It does not separate DNA, provide charge, or track electrophoresis progression (that is done by tracking dyes like bromophenol blue).

Correct Matching:

| Column I | Column II |
|---|---|
| (a) Separation of ionic solutes | Ion-exchange chromatography (IEC) |
| (b) Separation of biomolecules with different binding specificities | Affinity chromatography (AFC) |
| (c) Separation of volatile components | Gas chromatography (GC) |

Explanation:
- Ion-exchange chromatography (IEC): Separates molecules based on their net charge. Ionic solutes bind to oppositely charged groups on the stationary phase and are eluted by changing salt concentration or pH.
- Affinity chromatography (AFC): Based on specific, reversible biological interactions (e.g., enzyme–substrate, antigen–antibody). It separates biomolecules with different binding specificities.
- Gas chromatography (GC): Used to separate volatile components (gases or substances that can be vaporised) based on their differential partitioning between a mobile gas phase and a stationary phase.

Correct Option: (d) All of the above

Justification: Mass spectrometry (MS) is a versatile analytical technique that:
- Identifies unknown compounds by determining their molecular mass and fragmentation pattern.
- Elucidates the structure of molecules by analysing the mass-to-charge (m/z) ratio of fragment ions.
- Quantifies compounds by measuring the intensity of ion signals, which is proportional to the amount of the compound present.
Hence, all three functions are performed by mass spectrometry.

Correct Matching:

| Type | Antigen | Antibody | Procedure |
|---|---|---|---|
| (i) | Bound to surface | Labeled primary antibody used | Direct ELISA |
| (ii) | Bound to surface | Unlabeled primary + labeled secondary antibody used | Indirect ELISA |
| (iii) | Captured (sandwiched) between two antibodies | Detection antibody (labeled) used | Sandwich ELISA |

Explanation:
- Direct ELISA: The antigen is bound (coated) to the surface of the microplate well. A single enzyme-labeled primary antibody is used to detect the antigen directly.
- Indirect ELISA: The antigen is bound to the surface. An unlabeled primary antibody binds the antigen, and then a labeled secondary antibody (directed against the primary antibody) is used for detection. This amplifies the signal.
- Sandwich ELISA: The antigen is captured between two antibodies — a capture antibody (bound to the plate) and a detection antibody (labeled). This is highly specific and sensitive.

*Note: The table in the question as printed contains some mismatches; the corrected and standard descriptions are provided above.*

Correct Option: (a) I and III

Explanation:

Given: In agarose gel electrophoresis, DNA (negatively charged due to phosphate backbone) migrates from the negative electrode (cathode) towards the positive electrode (anode) under an electric field.

Principle of size-based separation:
- The agarose gel acts as a molecular sieve.
- Longer (larger) DNA fragments experience greater friction/resistance from the gel matrix and therefore migrate slowly, remaining close to the well (loading end). → Statement I is correct.
- Smaller DNA fragments experience less resistance and migrate faster, moving closer to the positive end of the gel. → Statement III is correct.

Statements II and IV are incorrect:
- Statement II is wrong because longer fragments do NOT move towards the positive end; they stay near the well.
- Statement IV is wrong because smaller fragments do NOT remain close to the well; they migrate farther.

$$\therefore \text{Correct answer: (a) I and III}$$

Correct Option: (b) Scanning electron microscope

Justification:
- Scanning Electron Microscope (SEM) is specifically designed to image the surface of objects. A focused beam of electrons scans the surface of the specimen, and secondary electrons emitted from the surface are detected to produce a detailed, three-dimensional image of the surface topology with high resolution.
- Transmission Electron Microscope (TEM) provides images of the internal ultrastructure of thin sections, not the surface.
- Phase contrast and fluorescence microscopes are light microscopes used for living/stained cells and do not provide surface-resolved images at the nanometre scale.

$$\therefore \text{SEM is preferred for a resolved image of the surface of an object.}$$

Correct Matching:

| Scientist | Contribution |
|---|---|
| (a) Engvall and Perlman | ELISA |
| (b) Robert Hooke | Microscopy |
| (c) Sanger | DNA sequencing |

Explanation:
- Engvall and Perlman (1971): Developed the ELISA (Enzyme-Linked Immunosorbent Assay) technique, a plate-based assay for detecting and quantifying proteins, antibodies, and hormones.
- Robert Hooke (1665): Pioneered microscopy; he used a compound microscope to observe cork cells and coined the term 'cell' in his book *Micrographia*.
- Frederick Sanger: Developed the chain-termination (dideoxy) method of DNA sequencing, also known as Sanger sequencing, which became the gold standard for DNA sequencing for several decades.

Correct Option: (b) Microarray

Justification: DNA microarray (also called gene chip or DNA chip) technology allows simultaneous analysis of the expression levels of thousands of genes at once. mRNA from a sample is converted to cDNA, labeled with fluorescent dyes, and hybridised to thousands of gene-specific probes spotted on a chip. The fluorescence intensity at each spot indicates the expression level of the corresponding gene. This makes it the most feasible technique for large-scale gene expression quantification.

- Mass spectrometry is used for protein/compound identification and quantification, not gene expression on a large scale.
- FISH identifies specific chromosomal locations, not expression levels of many genes.
- Agarose gel electrophoresis separates DNA/RNA fragments by size, not gene expression profiling.

Given: Different types of microscopy techniques need to be differentiated.

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(a) Scanning Electron Microscopy (SEM) vs. Transmission Electron Microscopy (TEM)

| Feature | SEM | TEM |
|---|---|---|
| Principle | A focused electron beam scans the surface; secondary electrons are detected | Electrons are transmitted through an ultra-thin specimen; image formed by electrons passing through |
| Image type | 3D surface image | 2D internal ultrastructure image |
| Specimen preparation | Specimen coated with a thin metal (e.g., gold) layer | Ultra-thin sections (50–100 nm) required |
| Resolution | ~1–20 nm (lower than TEM) | ~0.1–0.2 nm (very high resolution) |
| Information obtained | Surface morphology and topology | Internal cellular structures (organelles, membranes) |
| Magnification | Up to ~1,00,000× | Up to ~10,00,000× |

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(b) Dark Field Microscopy vs. Bright Field Microscopy

| Feature | Dark Field Microscopy | Bright Field Microscopy |
|---|---|---|
| Principle | Only scattered/diffracted light from the specimen enters the objective; background is dark | Direct transmitted light passes through the specimen; background is bright |
| Background | Dark (black) | Bright (white/light) |
| Specimen appearance | Specimen appears bright/luminous against dark background | Specimen appears dark against bright background |
| Staining required | Not necessary; useful for unstained, transparent specimens | Often required to provide contrast |
| Applications | Viewing live, unstained microorganisms (e.g., spirochetes, flagella) | Routine histological and microbiological observations |
| Contrast | High contrast for transparent objects | Low contrast for unstained transparent specimens |

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(c) Phase Contrast Microscopy vs. Confocal Microscopy

| Feature | Phase Contrast Microscopy | Confocal Microscopy |
|---|---|---|
| Principle | Converts phase differences (due to differences in refractive index) in light passing through the specimen into amplitude (contrast) differences visible to the eye | Uses a laser beam focused on a single plane; a pinhole eliminates out-of-focus light; optical sections are obtained |
| Light source | White light (conventional lamp) | Laser light |
| Specimen | Unstained, living, transparent cells | Fluorescently labeled specimens |
| Image | 2D image with enhanced contrast | High-resolution 2D optical sections that can be reconstructed into 3D images |
| Depth of field | Entire depth of specimen contributes to image (blurring from out-of-focus planes) | Only a single focal plane is imaged (no out-of-focus blur) |
| Applications | Observing living cells, cell division, organelle movement | 3D imaging of cells, co-localisation studies, imaging thick specimens |
| Resolution | Moderate | Very high (sub-micron) |

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.

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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

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(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.

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(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.

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(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.

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(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).

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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)

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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.

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

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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.

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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:

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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.

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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.

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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.

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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.

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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.

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6. Atomic Absorption Spectroscopy (AAS):
- Trace metal analysis: Detects and quantifies trace metals (e.g., Fe, Cu, Zn) in biological and environmental samples.

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

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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.

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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.

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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.

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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.

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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.

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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.

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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:

| Feature | Sanger's Method (Chain Termination / Dideoxy Method) | Maxam & Gilbert's Method (Chemical Cleavage Method) |
|---|---|---|
| Developed by | Frederick Sanger (1977) | Allan Maxam and Walter Gilbert (1977) |
| Principle | Uses dideoxynucleotides (ddNTPs) as chain terminators during in vitro DNA synthesis by DNA polymerase | Uses chemical reagents to cleave DNA at specific bases (A, G, C, T) |
| Basis | Enzymatic — relies on DNA polymerase to synthesise new DNA strands | Chemical — relies on chemical modification and cleavage of DNA |
| Starting material | Single-stranded DNA template + primer | Double-stranded or single-stranded DNA (end-labeled) |
| Key reagents | ddATP, ddTTP, ddGTP, ddCTP (dideoxynucleotides); DNA polymerase; dNTPs | Chemical reagents: dimethyl sulphate (DMS) for G; formic acid for A+G; hydrazine for C+T; piperidine for cleavage |
| Labeling | Radioactive ($^{32}$P) or fluorescent label incorporated into newly synthesised DNA | Radioactive ($^{32}$P) label at the 5' end of the DNA fragment |
| Process | Four separate reactions, each with one ddNTP, produce fragments of varying lengths terminated at specific bases | Four separate chemical reactions cleave DNA at specific bases |
| Safety | Relatively safer (enzymatic reactions) | Involves hazardous chemicals (e.g., hydrazine is toxic and carcinogenic) |
| Automation | Easily automated; forms the basis of automated DNA sequencers | Difficult to automate |
| Sensitivity | High; can sequence longer stretches of DNA | Moderate |
| Current use | Widely used even today; basis of automated and next-generation sequencing | Largely replaced by Sanger's method; mainly used for specific applications (e.g., footprinting) |
| Read length | Up to ~1000 bp per reaction | Similar range but less commonly used for long reads |
| Gel used | Denaturing polyacrylamide gel electrophoresis | Denaturing polyacrylamide gel electrophoresis |

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.

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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)

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

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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

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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.