Tools for the gene technologist

Genetic engineering, also known as recombinant DNA technology or genetic modification (GM), is the deliberate manipulation of the genetic constitution of an organism. This involves transferring a gene or genes and other sequences from one species to another, often an unrelated species, so that the recipient organism can express a new gene product or characteristic. Unlike selective breeding, which involves whole sets of genes, genetic engineering typically focuses on transferring a single gene.

The techniques and tools used in genetic engineering are crucial for its application. The "tool kit" for a genetic engineer includes enzymes, vectors, and easily identifiable marker genes.

Key Tools and Techniques in Genetic Engineering:

• Enzymes

  • Restriction Endonucleases (Restriction Enzymes): These enzymes, originally derived from bacteria, recognize and cut DNA molecules at specific nucleotide sequences called restriction sites. They often make a staggered cut, leaving short tails of unpaired bases known as "sticky ends". These sticky ends can form hydrogen bonds with complementary sequences on other DNA fragments cut with the same enzyme, facilitating gene insertion. Restriction enzymes are essentially "molecular scissors" used to cut DNA at precise places.

  • DNA Ligase: This enzyme joins together DNA molecules. It forms phosphodiester bonds to seal the sugar–phosphate backbone of the DNA. This process is called ligation.

  • Reverse Transcriptase: Found in retroviruses, this enzyme synthesizes a DNA strand (called complementary DNA, cDNA) from an RNA template. This method is advantageous because mRNA from eukaryotic cells lacks non-coding regions (introns).

  • DNA Polymerase: This enzyme synthesizes new DNA strands, guided by a template strand of nucleic acid. It is essential for DNA replication and PCR.

  • Cas9: A nuclease enzyme used in gene editing, specifically the CRISPR-Cas9 system. It is guided by a guide RNA (gRNA) to target and cut specific DNA sequences with high precision, allowing for insertion, deletion, or replacement of genetic material.

• Vectors

  • Definition: A vector is a means of delivering genes into a host cell.

  • Plasmids: Small, circular, double-stranded DNA molecules naturally found in bacteria. They often carry genes for antibiotic resistance or other marker genes. They can be cut with restriction enzymes and have new genes inserted, forming recombinant DNA.

  • Viruses: Such as retroviruses, adenoviruses, or bacteriophages, can be used as vectors because they naturally infect cells and insert their genetic material. They need to be genetically engineered to be safe and prevent self-replication.

  • Liposomes: Small spheres of phospholipid that can encapsulate DNA and deliver it to cells by fusing with the cell membrane.

  • Naked DNA: Direct uptake of DNA by host cells, often after being made "competent" by treatments like calcium chloride or electroporation.

  • Micro-injection: Directly injecting DNA into the nucleus of a cell using a very fine pipette, especially for fertilised animal egg cells or when few cells are available.

  • Gene Gun: Firing microscopic gold particles coated with DNA at cells, useful for plant cells with strong cell walls.

  • Agrobacterium tumefaciens: A bacterium that naturally infects plants and can be used to transfer modified plasmids into plant cells, leading to tumor formation and gene expression in the plant.

• Marker Genes: Genes inserted into vectors alongside the desired gene to help identify cells that have successfully taken up the recombinant DNA (transformed cells).

  • Historically, antibiotic resistance genes were used. However, concerns about the accidental transfer of antibiotic resistance to pathogenic bacteria have led to preference for alternative markers.

  • Fluorescent proteins like Green Fluorescent Protein (GFP), originally from jellyfish, are now commonly used. Transformed cells containing the GFP gene will fluoresce under UV light.

  • Other enzymes, like β-glucuronidase (GUS), can also be used as markers, producing colored or fluorescent products when incubated with specific substrates.

• Amplification and Analysis Techniques

  • Polymerase Chain Reaction (PCR): An automated in vitro method to make millions of copies of a specific DNA fragment in a few hours. It involves stages of heating to denature DNA, cooling to allow primers to bind (annealing), and heating for DNA polymerase (often heat-stable Taq polymerase) to synthesize new complementary strands.

  • DNA Probes: Short strands of labeled DNA (radioactive or fluorescent) with a specific base sequence complementary to a target allele or DNA fragment. They are used to locate specific alleles or sequences.

  • DNA Microarrays (Gene Chips): Glass slides with microscopic spots of different DNA probes attached, used to screen for many different genes simultaneously or to assess gene expression.

  • Gene Machine: Technology that allows DNA fragments to be synthesized from scratch without a pre-existing DNA template, based on a desired sequence.

  • Gel Electrophoresis: A technique used to separate DNA fragments (or proteins) according to their size and charge. DNA fragments are negatively charged and move towards the positive electrode, with shorter fragments moving faster and further.

  • Bioinformatics: The collection, processing, and analysis of biological information and data using computer software. Databases store vast amounts of genomic, proteomic, and other biological data.

  • Genetic Fingerprinting (DNA Profiling): A technique used to identify individuals or determine genetic relatedness by analyzing unique patterns of variable number tandem repeats (VNTRs) or short tandem repeats (STRs) in DNA. It involves PCR to amplify DNA, followed by gel electrophoresis to separate fragments, and probes for visualization.