12.2 Visualizing and Characterizing DNA, RNA, and Protein

Nucleic Acid Detection and Analysis

Identifying and analyzing specific sequences of DNA or RNA is foundational to microbial genetics. The techniques in this section let you find a target sequence in a complex mixture, separate fragments by size, and detect genetic variation between organisms or strains.

Detection of DNA Sequences

A nucleic acid probe is a short, single-stranded piece of DNA or RNA designed to be complementary to a specific target sequence. When the probe encounters its target, it hybridizes (binds through complementary base pairing: A-T, G-C).

To actually see where hybridization occurred, probes are tagged with a detectable label:

• Radioactive isotopes (detected by autoradiography)
• Fluorescent dyes (detected by UV light or laser scanning)
• Enzymes (produce a color change in a chemical reaction)

The label lights up wherever the probe has bound, confirming the target sequence is present.

Process of Gel Electrophoresis

Gel electrophoresis separates DNA fragments by size. It works because DNA carries a net negative charge from its phosphate backbone, so it migrates toward the positive electrode when placed in an electric field.

How it works, step by step:

1. DNA samples are loaded into wells at one end of an agarose (for larger fragments) or polyacrylamide (for smaller fragments) gel.
2. An electric current is applied across the gel.
3. DNA fragments migrate through the gel matrix. Smaller fragments move faster because they slip through pores in the gel more easily; larger fragments lag behind.
4. After separation, fragments are visualized with a DNA-binding stain such as ethidium bromide, which fluoresces under UV light.

Common applications:

• Determining the size of DNA fragments (by comparing to a known molecular weight ladder)
• Separating restriction enzyme-digested DNA
• Analyzing PCR products
• Assessing the quality and quantity of a DNA sample

Principles of RFLP Analysis

Restriction fragment length polymorphism (RFLP) analysis detects genetic variation by exploiting the fact that mutations can create or destroy restriction enzyme recognition sites. If two individuals (or two bacterial strains) differ at a restriction site, digesting their DNA with the same enzyme produces fragments of different lengths.

Steps in RFLP analysis:

1. Genomic DNA is digested with a specific restriction enzyme.
2. The resulting fragments are separated by gel electrophoresis.
3. Fragments are transferred to a membrane (this is the Southern blot step).
4. The membrane is hybridized with a labeled probe that binds to the region of interest.
5. The banding pattern is visualized. Different patterns between samples indicate polymorphisms (sequence differences).

Applications of RFLP:

• Genetic mapping and linkage analysis
• Paternity testing and forensic identification
• Diagnosis of genetic disorders
• Strain typing of bacteria and viruses

Blotting Techniques and Gene Expression Analysis

Southern vs. Northern Blotting

Both Southern and northern blotting follow the same general logic: separate nucleic acids by size, transfer them to a membrane, then use a labeled probe to find your sequence of interest. The difference is the target molecule.

Southern blotting detects specific DNA sequences:

1. DNA is digested with restriction enzymes and separated by gel electrophoresis.
2. The gel is treated with alkali to denature the double-stranded DNA into single strands.
3. Fragments are transferred (blotted) onto a nylon or nitrocellulose membrane.
4. The membrane is incubated with a labeled DNA probe that hybridizes to the target sequence.
5. Hybridization is detected (autoradiography, fluorescence, etc.), revealing the target band(s).

Northern blotting detects specific RNA sequences:

1. Total RNA is separated by gel electrophoresis (no restriction digest needed since RNA is already single-stranded fragments of varying size).
2. RNA is transferred to a membrane.
3. The membrane is hybridized with a labeled DNA or RNA probe.
4. Detection reveals the presence and size of the target RNA transcript.

Microarray Analysis for Gene Expression

Microarrays let you monitor the expression of thousands of genes simultaneously on a single chip. This is powerful for answering questions like which genes are turned on when a bacterium encounters an antibiotic?

How microarray analysis works:

1. A glass or silicon chip is spotted with thousands of DNA probes, each corresponding to a known gene.
2. RNA is extracted from the cells or tissue you're studying and reverse-transcribed into cDNA.
3. The cDNA is labeled with fluorescent dyes (often two colors are used to compare two conditions, such as treated vs. untreated).
4. Labeled cDNA is washed over the chip, and complementary sequences hybridize to their matching probes.
5. A scanner measures fluorescence intensity at each spot. Brighter signal = higher expression of that gene.

Microarray data reveals which genes are upregulated or downregulated under specific conditions. Because these experiments generate massive datasets, bioinformatics tools are essential for analysis and interpretation. Microarrays have been widely used in cancer research, developmental biology, and microbial pathogenesis studies.

Protein Analysis and Amplification Techniques

Separation of Protein Variants

Proteins can be separated by different properties depending on the technique used.

SDS-PAGE (sodium dodecyl sulfate–polyacrylamide gel electrophoresis) separates proteins by molecular weight. SDS is a detergent that denatures proteins and coats them with a uniform negative charge, so their migration through the gel depends only on size. Smaller proteins move faster, just like in DNA electrophoresis.

Isoelectric focusing (IEF) separates proteins by their isoelectric point (pI), the pH at which a protein carries no net charge. The gel contains a pH gradient; each protein migrates until it reaches the pH matching its pI, then stops.

Two-dimensional (2D) gel electrophoresis combines both methods for much higher resolution:

1. First dimension: proteins are separated by pI using IEF.
2. Second dimension: the IEF strip is laid across an SDS-PAGE gel, and proteins are separated by molecular weight.

This produces a 2D map of spots, each representing a distinct protein, allowing you to resolve thousands of proteins from a complex mixture.

Protein visualization methods:

• Coomassie blue staining — a general-purpose protein stain
• Silver staining — much more sensitive, detects smaller amounts of protein
• Western blotting — uses antibodies to detect a specific protein (analogous to Southern/northern blotting but for proteins)

PCR and DNA Sequencing Applications

Polymerase chain reaction (PCR) amplifies a specific DNA sequence, generating millions of copies from a tiny starting amount.

PCR steps (repeated for 25–35 cycles):

1. Denaturation (~95°C): Heat separates the double-stranded DNA into single strands.
2. Annealing (~50–65°C): Short DNA primers bind to complementary sequences flanking the target region.
3. Extension (~72°C): A heat-stable DNA polymerase (Taq polymerase, originally isolated from Thermus aquaticus) synthesizes new DNA strands starting from each primer.

Each cycle doubles the amount of target DNA, so amplification is exponential ($2^n$ copies after $n$ cycles).

Applications of PCR:

• Detecting pathogens or specific mutations in clinical samples
• Amplifying low-copy DNA for cloning or further analysis
• Generating template DNA for sequencing

DNA sequencing determines the exact order of nucleotides in a DNA molecule. Two major approaches:

• Sanger sequencing (chain-termination method): The reaction includes normal dNTPs plus fluorescently labeled ddNTPs (dideoxynucleotides), which lack the 3'-OH needed for chain elongation. When a ddNTP is incorporated, synthesis stops. This generates fragments of every possible length, which are separated by capillary electrophoresis to read the sequence one base at a time.
• Next-generation sequencing (NGS): Platforms like Illumina, Ion Torrent, and PacBio perform massively parallel sequencing, reading millions of DNA fragments simultaneously. This dramatically increases throughput and reduces cost compared to Sanger sequencing.

Applications of DNA sequencing:

• Whole-genome sequencing of microorganisms
• Targeted sequencing of specific genes or regulatory regions
• Identifying mutations and genetic variants
• Comparative genomics and evolutionary studies

Advanced Techniques in Molecular Biology

Spectroscopic and Chromatographic Methods

Several analytical techniques help characterize the structure and behavior of biomolecules beyond what gel-based methods can reveal.

• UV-visible spectroscopy measures how a molecule absorbs light at specific wavelengths. For nucleic acids, absorbance at 260 nm is used to quantify DNA/RNA concentration. For proteins, absorbance at 280 nm (due to aromatic amino acids) serves a similar purpose.
• Fluorescence spectroscopy tracks molecules tagged with fluorescent labels, useful for studying molecular interactions and conformational changes.
• Chromatography separates and purifies biomolecules based on physical or chemical properties (size, charge, hydrophobicity, or binding affinity). Common types include size-exclusion, ion-exchange, and affinity chromatography.
• Mass spectrometry identifies molecules by measuring their mass-to-charge ratio. It's widely used for protein identification, quantification, and characterizing post-translational modifications.

Structural Analysis of Biomolecules

X-ray crystallography determines the three-dimensional structure of proteins and nucleic acids at atomic resolution. A purified molecule is crystallized, and X-rays are directed through the crystal. The diffraction pattern is used to calculate the positions of individual atoms.

This technique has been central to understanding enzyme mechanisms, protein-ligand interactions, and rational drug design. Many of the protein structures in databases like the Protein Data Bank (PDB) were solved using X-ray crystallography.