DNA Cloning and Recombinant DNA | Molecular Genetics | MCAT Biology Prep

Sanger sequencing, developed by Frederick Sanger in 1977, is a DNA sequencing method that uses chain-terminating fluorescently labeled dideoxynucleotides to create fragments of varying lengths, which are then separated by capillary electrophoresis and detected by fluorescence to determine the DNA sequence; the method involves preparing plasmid DNA containing the target sequence, performing thermal cycling (denaturation at 96°C, annealing at 50°C, extension at 60-65°C) with DNA polymerase, and using automated capillary electrophoresis to read the fluorescently labeled bases at the ends of the fragments.

Ligation joins DNA fragments together using DNA ligase enzyme after restriction digestion creates compatible ends. The ligation reaction produces a mixture of plasmids, some containing the insert and some empty, which are then transformed into host cells for selection. Antibiotic resistance genes in plasmids allow selection of cells that have successfully taken up the plasmid. After transformation, plasmid DNA is extracted from bacterial colonies for verification. Sequencing confirms the plasmid contains the correct genetic sequence, as even single base errors can disrupt gene function.

The Sanger sequencing method determines the base sequence of a DNA strand by using four reaction mixtures, each containing all four DNA bases plus one modified base; during PCR amplification, these modified bases randomly terminate DNA chain elongation at specific positions, creating fragments of varying lengths that end with the same modified base; gel electrophoresis separates these fragments by size, and by analyzing the band patterns from all four reactions, the original DNA sequence can be reconstructed by reading the order of terminating bases from the shortest to longest fragment and then reversing the complementary strand.

Sanger sequencing, also known as chain termination method, determines DNA base sequences by using DNA polymerase to synthesize complementary strands while incorporating stop nucleotides (dideoxynucleotides) that randomly terminate chain elongation at specific bases; the resulting fragments of varying lengths are separated by gel electrophoresis and analyzed to deduce the original DNA template sequence.

Sanger sequencing determines DNA sequences through chain termination. The method requires template DNA, primers, DNA polymerase, all four deoxynucleotides, and one type of dideoxynucleotide per reaction. Dideoxynucleotides terminate chains when incorporated, while deoxynucleotides allow extension. Critical to success is the ratio—dideoxynucleotides must be present at much lower concentrations (typically 1%) than deoxynucleotides. Equimolar concentrations would cause excessive termination, limiting reads to only the first few bases. Four separate reactions are run, each with a different dideoxynucleotide, then separated on polyacrylamide gels to determine the sequence.

A genomic library is created by first digesting an organism's complete genome with restriction enzymes to produce gene-containing fragments, then separating these fragments by size using gel electrophoresis, followed by inserting each fragment into a Lambda phage vector to create recombinant DNA molecules, which are then introduced into bacterial cells for amplification; the library is screened using radioactively labeled DNA probes that hybridize specifically with target genes, allowing researchers to identify and isolate specific genes of interest for further study.

A genomic DNA library is a collection of cloned DNA fragments representing the entire genome of an organism, constructed by fragmenting genomic DNA with restriction enzymes, inserting fragments into cloning vectors (such as plasmids, bacteriophage lambda, or artificial chromosomes), and storing them in host cells for screening and analysis; the screening process involves techniques like colony hybridization to identify specific genes of interest within the library.

A genomic library is created by cutting an organism's total DNA (such as yeast DNA) with restriction enzymes, attaching the fragments to vectors, transforming them into bacteria, and growing colonies on selective media. Each bacterial colony contains a single plasmid carrying one piece of the original genome. For example, when cloning the yeast ARG1 gene, the library contains thousands of colonies, each potentially carrying a different yeast gene fragment. The goal is to identify which colony contains the specific gene of interest.

Genomic libraries contain random fragments of an organism's entire genome, constructed by fragmenting genomic DNA and inserting into cloning vectors. Each bacterial cell in the library contains a different genomic fragment. cDNA libraries are constructed from mRNA using reverse transcription, representing only expressed genes without introns. Bacterial Artificial Chromosomes (BACs) are used for cloning very large DNA fragments (100-300 kb) as single-copy vectors. These libraries enable researchers to screen for specific genes or sequences of interest.

A genomic library is a collection of DNA fragments from an organism's entire genome, created by cutting the genome into smaller manageable pieces using restriction enzymes. The process involves isolating DNA from cells, cutting it with restriction enzymes (chosen carefully to produce appropriately sized fragments), inserting these fragments into plasmid vectors using DNA ligase, transforming bacteria with the recombinant plasmids, and storing thousands of bacterial colonies each containing different genomic sections. Together, these bacterial colonies form a complete representation of the original genome in smaller, more workable pieces.

Recombinant DNA technology enables production of therapeutic proteins including insulin, interferons, and growth hormones. The process involves isolating the gene of interest, cloning it into plasmids, and introducing plasmids into host bacteria for protein production. This technology has revolutionized medicine, agriculture (creating pest-resistant crops), and biotechnology, enabling mass production of proteins that would otherwise be difficult or impossible to obtain.

Recombinant DNA technology involves isolating genes from source organisms, inserting them into plasmids, introducing plasmids into host cells (typically E. coli), growing cells in bioreactors, and purifying the resulting proteins. For insulin production, genes for A and B chains are isolated, inserted into separate plasmids, introduced into bacterial cells, and grown in bioreactors. The proteins are separated, purified, and chemically joined using disulfide bonds to form active insulin. Developed by Genentech and approved in 1983, this method provides a safer alternative to animal-derived insulin. The process demonstrates how genetic engineering can produce therapeutic proteins for medical use, revolutionizing diabetes treatment.

The instructor explains biotechnology applications: insulin is a peptide hormone with A chain (21 amino acids) and B chain (30 amino acids) connected by disulfide bonds. Proinsulin is the precursor containing C peptide, which is cleaved during maturation. Recombinant DNA technology involves: isolation of DNA, cutting with restriction enzymes, ligation into vectors, transformation into host cells, and expression of recombinant proteins. Human insulin is produced using this technology. Bioreactors (stirred tank and airlift types) enable large-scale production. Downstream processing involves purification and recovery of biological products. Industrial microorganisms include Saccharomyces cerevisiae (baking/brewing), Trichoderma polysporum (cyclosporin), and Monascus purpureus (statins).

This segment explains the production of human insulin using recombinant DNA technology. The process involves: (1) isolating the human insulin gene from pancreatic beta cells, (2) cutting the gene using restriction enzymes, (3) inserting the gene into a plasmid vector, (4) introducing the recombinant plasmid into E. coli bacteria, (5) allowing the bacteria to multiply and produce insulin, and (6) purifying the insulin for medical use. The teacher explains that Eli Lilly company successfully produced human insulin in 1982, revolutionizing diabetes treatment.

Recombinant DNA technology involves manipulating genetic material to create new combinations of genes. The process includes: (1) cutting DNA with restriction enzymes, (2) inserting genes into vectors (often viruses), (3) introducing vectors into host cells, (4) cloning host cells to produce multiple copies, and (5) expressing the inserted genes to produce proteins. Applications include producing therapeutic proteins like insulin for diabetes, growth hormone for deficiency disorders, clotting factors for hemophilia, and erythropoietin for anemia. This technology enables the production of synthetic hormones for treating human diseases.

CRISPR-Cas9 enables precise genome modification by reprogramming Cas9 with guide RNAs to target specific genomic positions. Cas9 introduces double-strand breaks that can be repaired by homology-directed repair (using templates to introduce specific mutations) or non-homologous end joining (error-prone repair causing insertions/deletions). In eukaryotic cells, both pathways are efficient. In bacteria, Cas9-induced breaks are highly lethal but can be exploited for selection. Co-transformation with Cas9, guide RNAs, and oligonucleotide templates, combined with λ-red recombinase, increases desired mutation frequencies from ~10^-4 to nearly 100%. Recent advances include CRISPR cassettes combining guide RNA expression and repair templates on single DNA molecules for high-throughput mutagenesis.

The CRISPR-Cas9 system consists of the CRISPR array, leader sequence with strong promoter, Cas9 protein with two nuclease domains, tracrRNA for PAM recognition and pre-crRNA processing, and spacer sequences for target specificity. In interference, Cas9 uses the spacer sequence to recognize matching DNA sequences (with PAM) and cleaves both strands, creating double-stranded breaks. This technology has been adapted for genome editing in eukaryotic cells. When Cas9 creates a double-stranded break, the cell repairs it through non-homologous end joining (introducing insertions/deletions that disrupt genes) or homologous recombination (using a provided DNA template for precise gene replacement). Beyond genome editing, CRISPR-Cas9 has been adapted for gene activation (fusing Cas9 to transcriptional activator domains), gene repression (fusing to repressor domains), base editing (fusing to DNA editors to change individual nucleotides), and epigenetic editing (fusing to epigenetic modifiers).

When introduced into eukaryotic cells (plants or animals), the CRISPR-Cas9 system creates double-stranded breaks in DNA at targeted locations. Unlike bacteria where this leads to viral degradation, eukaryotic cells repair these breaks using their sophisticated machinery. This repair can introduce small changes at the break site or integrate new DNA sequences, making CRISPR-Cas9 a powerful tool for genome engineering in complex organisms.

CRISPR-Cas9 is a revolutionary gene-editing technology enabling site-specific DNA modifications. Unlike traditional GMOs that introduce foreign DNA, CRISPR-Cas9 allows precise editing of existing genes using guide RNA to direct Cas9 enzyme to specific genomic locations. This technology can be applied to both prokaryotic and eukaryotic organisms, and edited genes can be inherited by offspring.

Base editing enables direct conversion of individual nucleotides without double-strand breaks by fusing Cas9 to deaminase enzymes (APOBEC or ADAR), converting C→U or A→G. Prime editing combines Cas9 with reverse transcriptase to enable precise insertions, deletions, and substitutions using prime editing guide RNAs that encode both target sequences and desired edits. CRISPR can also deliver epigenetic modifiers—dCas9 fused to VP64 activators, p300 histone acetyltransferases, or DNA methyltransferases—to regulate gene expression at specific promoters or enhancers without altering DNA sequence.