10.11 Blotting, Sequencing, Microarrays & CRISPR-Cas9

10.11 Blotting, Sequencing, Microarrays & CRISPR-Cas9

3. Blotting Techniques and DNA Sequencing Technologies

Blotting Methodologies: The SNOW DROP Framework

Blotting allows researchers to identify specific macromolecules within complex biological mixtures by combining gel electrophoresis, membrane transfer, and molecular hybridization:

S  outhern  <--->  D  NA
N  orthern  <--->  R  NA
O  -        <--->  O  -
W  estern   <--->  P  rotein

1. Southern Blot (DNA):

   • Method: Genomic DNA is digested with restriction enzymes, resolved by agarose gel electrophoresis, denatured into single strands with alkaline solution (NaOH), and transferred onto a nitrocellulose or nylon membrane. The membrane is hybridized with a labeled (radioactive 32P or fluorescent) complementary single-stranded DNA or RNA probe.
   • Applications: Identifies specific gene sequences, detects gene deletions or amplifications, and evaluates Restriction Fragment Length Polymorphisms (RFLPs) for forensic or diagnostic genotyping.

2. Northern Blot (RNA):

   • Method: Intact cellular RNA is electrophoresed under denaturing conditions (formaldehyde gel) to prevent secondary structure formation, transferred to a membrane, and hybridized with a labeled nucleic acid probe.
   • Applications: Evaluates gene expression levels (abundance of specific mRNA transcripts across tissues), transcript size, and alternative splicing isoforms.

3. Western Blot (Protein):

   • Method: Cellular proteins are denatured and separated by SDS-PAGE, transferred to a PVDF or nitrocellulose membrane, blocked with non-fat milk or bovine serum albumin (BSA) to prevent non-specific binding, and probed with an antigen-specific primary antibody. An enzyme-conjugated secondary antibody (e.g., horseradish peroxidase, HRP) binds the primary antibody, generating chemiluminescence upon substrate addition.
   • Applications: Quantifies target protein expression, confirms antibody reactivity, and detects post-translational modifications (e.g., phosphorylation shifts).

DNA Sequencing Technologies

1. Sanger Dideoxy Chain-Termination Sequencing

Developed by Frederick Sanger, this method relies on the enzymatic incorporation of modified nucleotide terminators:

• Dideoxynucleoside Triphosphates (ddNTPs): ddNTPs (ddATP, ddCTP, ddGTP, ddTTP) lack both the 2'-hydroxyl and the 3'-hydroxyl groups on the pentose sugar. Because DNA polymerases strictly require a free 3'-OH group to attack the α-phosphate of an incoming dNTP, the incorporation of a ddNTP terminates chain extension irreversibly.
• Modern Automated Procedure: A sequencing reaction contains template DNA, an oligonucleotide primer, DNA polymerase, a high concentration of standard dNTPs, and a low concentration of four distinct fluorescently labeled ddNTPs (each tagged with a different color fluorophore). During synthesis, random ddNTP incorporation generates nested fragments terminating at every nucleotide position. Capillary gel electrophoresis resolves fragments differing in size by a single base, and an automated laser detector reads the fluorophores sequentially from smallest to largest (5' → 3' on the synthesized strand).

Normal dNTP:  5'-Phosphate --- [Deoxyribose: 2'-H, 3'-OH]  ===> Chain Extension Continues
Sanger ddNTP: 5'-Phosphate --- [Dideoxyribose: 2'-H, 3'-H] ===> CHAIN TERMINATION (No 3'-OH!)

2. Next-Generation Sequencing (NGS)

High-throughput technologies capable of sequencing billions of bases in parallel at fractional cost compared to Sanger sequencing:

• Illumina Sequencing-by-Synthesis: Genomic DNA is fragmented and ligated to adapters. Fragments are immobilized on a glass flow cell, amplified via bridge PCR into clonal clusters, and sequenced using reversible fluorescent terminators and DNA polymerase. A camera images the fluorescent signal of each newly incorporated base across millions of clusters simultaneously.
• Applications: Whole-genome sequencing (WGS), whole-exome sequencing (WES), RNA sequencing (RNA-seq for transcriptome profiling), and personalized oncology.

4. Microarrays and CRISPR-Cas9 Targeted Genome Editing

DNA Microarrays

DNA microarrays enable parallel quantification of thousands of gene expression transcripts simultaneously:

• Platform: A solid glass slide or silicon chip printed with thousands of microscopic spots, each containing single-stranded DNA oligonucleotides corresponding to a specific gene.
• Experimental Strategy: mRNA is isolated from two comparative biological samples (e.g., malignant tumor tissue vs. healthy control tissue), converted into cDNA via reverse transcriptase, and labeled with different fluorophores (e.g., Cy5 red for tumor cDNA; Cy3 green for normal cDNA). The labeled cDNAs are mixed equimolarly and hybridized competitively to the microarray.
• Signal Interpretation:
  • Yellow spot: Equal hybridization of green and red cDNA (gene expressed at identical levels in both tissues).
  • Red spot: Higher hybridization of tumor cDNA (gene is upregulated / overexpressed in cancer, e.g., oncogene).
  • Green spot: Higher hybridization of normal cDNA (gene is downregulated in cancer, e.g., tumor suppressor).
  • Black / Dark spot: Gene not expressed in either tissue.

CRISPR-Cas9 Targeted Gene Editing

Originally discovered as an adaptive immune defense system in Streptococcus pyogenes against bacteriophage invasion, CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) and Cas9 (CRISPR-associated protein 9) have been engineered into a revolutionary platform for targeted genome editing.

Molecular Machinery:

1. Cas9 Endonuclease: A multi-domain bacterial endonuclease with two catalytic cleavage centers (HNH and RuvC domains) that introduce a blunt double-strand break (DSB) in target DNA.
2. Single Guide RNA (sgRNA): A synthetic engineered RNA chimera combining the crRNA (CRISPR RNA) and tracrRNA (trans-activating crRNA). The sgRNA contains a programmable 20-nucleotide sequence at its 5' end that base-pairs with the complementary genomic DNA target.
3. Protospacer Adjacent Motif (PAM): Cas9 strictly requires the presence of a short conserved motif—most commonly 5'-NGG-3' for S. pyogenes Cas9—immediately adjacent to the 3' end of the target DNA protospacer. Cas9 first interrogates genomic DNA for the PAM sequence; upon PAM recognition, it melts the adjacent duplex and interrogates whether the DNA sequence base-pairs with the sgRNA. If complementary, Cas9 cleaves both DNA strands exactly 3 base pairs upstream of the PAM.

CRISPR-Cas9 Cleavage Geometry:
5' --- [20-nt Target Genomic Sequence] --- [ N G G ] (PAM) --- 3'
3' --- [Complementary DNA Sequence  ] --- [ N C C ] ----- 5'
                    ||  (Cas9 cleaves 3 bp upstream of PAM)
5' --- [Target Sequence DSB]  +  [Cleaved Terminus] --- 3'

Cellular Repair Pathways and Genetic Outcomes: Once Cas9 induces a targeted double-strand break, the host cell repairs the damage via one of two endogenous pathways:

• Non-Homologous End-Joining (NHEJ): In the absence of a repair template, the cell rapidly ligates the broken ends via error-prone NHEJ. NHEJ frequently introduces random insertions or deletions (indels). Indels within a protein-coding exon create frameshift mutations that generate premature stop codons, knocking out gene function (Targeted Gene Knockout).
• Homology-Directed Repair (HDR): If researchers co-introduce an exogenous donor repair DNA template containing desirable sequence modifications flanked by homologous DNA arms, the host cell repairs the break via error-free homologous recombination. HDR allows researchers to correct specific pathogenic mutations, insert point mutations, or knock in entire fluorescent reporter genes (Precision Gene Knock-in).

Summary of Laboratory Molecular Biology Techniques