Genetic Engineering and its Applications: A Comprehensive Overview
1. Evolution of the Gene Concept
Initially, the gene was identified as the fundamental unit of heredity, representing DNA fragments containing information for a specific trait. Later, the one gene-one enzyme theory was revised, proposing that a gene provides instructions for synthesizing an enzyme involved in a metabolic pathway. This led to the understanding of the gene as the structural and functional unit of the chromosome. While the gene is the structural unit, it’s important to note that the nucleotide is not. Ultimately, a gene is defined as a segment of DNA or RNA (in viruses) containing the information necessary for the synthesis of a polypeptide chain or a string of RNA in viruses and prokaryotes. In these organisms, genes do not overlap, meaning a nucleotide sequence is not part of more than one gene.
2. DNA in Eukaryotes
Eukaryotic DNA can be categorized into three types:
- Highly Repetitive DNA: Constitutes about 10% of the DNA and consists of short sequences repeated numerous times. It’s found in centromeres, telomeres, and non-transcribed regions. Its origin could be viral or related to transposons.
- Moderately Repetitive DNA: Makes up approximately 20% of the DNA and comprises longer sequences with 10 to 1000 repetitions. These sequences are not directly involved in the synthesis of histones or ribosomal RNA.
- Single-Copy or Unique DNA: Represents about 70% of the DNA and is responsible for mRNA transcription. Each gene typically codes for one enzyme, although some sequences may be repeated with minor variations. Different tissues have proteins that adapt to changes, while others are non-functional (pseudogenes).
3. Genetic Engineering
Genetic engineering involves transferring DNA from one organism to another. The recipient organism’s DNA is referred to as the receptor DNA, while the introduced DNA is called foreign or passenger DNA. This transfer is facilitated by vectors like plasmids and viruses, resulting in recombinant DNA.
A) Gene Insertion
Gene insertion is achieved using a vector. DNA molecules from both the source and recipient are cut with the same restriction enzyme, creating complementary symmetrical ends. This allows the passenger DNA to be inserted into the recipient DNA. However, inserting eukaryotic DNA (containing introns and exons) into prokaryotic DNA is not directly possible. Mature mRNA is isolated from eukaryotic cells, and using viral reverse transcriptase, a DNA sequence without introns is obtained. This DNA can then be inserted into prokaryotic cells.
B) Plasmids and Viruses
Plasmids and viruses serve as carriers for genes. Plasmids are small, circular DNA molecules found in the cytoplasm of bacteria. They are not integrated into the bacterial chromosome and can replicate independently. When passenger DNA is inserted into a plasmid, bacterial lysis releases the plasmid into the surrounding medium. Increasing the permeability of other bacteria through techniques like heat shock allows the plasmid to enter these bacteria, a process known as transformation. Plant parasitic bacteria with the Ti plasmid can integrate a passenger gene into the plasmid’s T-DNA segment. This T-DNA can then be integrated into the chromosome of any plant cell.
Viruses are parasites that infect cells to utilize their resources for replication. They consist of nucleic acid enclosed in a protein capsid. When a virus infects a bacterium, it undergoes a lytic cycle. The virus uses the bacterium’s resources to replicate its DNA and synthesize new capsids. These capsids then encapsulate the viral DNA, forming new viruses. Occasionally, a fragment of bacterial DNA gets incorporated into a viral capsid. When this virus infects another bacterium, it introduces the DNA from the previous bacterium, which then gets replicated along with the viral DNA. This process is known as transduction.
4. Genetic Engineering and Human Disease Therapy
Gene therapy can be either somatic (introducing genes into somatic cells) or germline (introducing genes into gametes). By introducing genes into bacteria, we can synthesize substances like insulin, human growth hormone, and interferon. Attempts have been made to introduce genes into human cells to treat diseases like thalassemia, a genetic disorder characterized by inadequate hemoglobin production. The goal is to modify the gene in bone marrow cells responsible for red blood cell formation. These modified cells would then produce red blood cells with normal hemoglobin. While unsuccessful so far, research continues to explore gene therapy for thalassemia and other genetic diseases like Severe Combined Immunodeficiency (SCID).
5. Genetic Engineering in Agriculture and Animal Production
Genetic engineering has led to the development of transgenic plants and animals. Examples include frost-resistant corn and tomatoes with genes that enable them to fix atmospheric nitrogen. In animal production, genetic engineering has been particularly successful in fish farming. Transgenic fish exhibit faster growth, larger size, and increased resistance to cold temperatures.
6. Cancer as a Genetic Disease
Cancer is characterized by the uncontrolled proliferation of abnormal cells, leading to the formation of tumors. Some tumor cells can escape into the bloodstream and colonize other parts of the body, a process called metastasis. Oncogenes, derived from proto-oncogenes, can transform normal cells into cancerous cells. Additionally, the presence of antigens can also contribute to cell transformation. Oncogenic viruses can disrupt the cell cycle and cause cancer in humans, although not through transduction. Chemicals and radiation are other factors that can contribute to cancer development. These agents don’t directly cause cancer but can promote the formation and accumulation of proteins that disrupt the cell cycle. Monoclonal antibodies are used to target specific cancer cells, and some anti-cancer drugs work by interfering with DNA repair mechanisms.
7. The Human Genome Project
The Human Genome Project aims to determine the complete sequence of nucleotides in human DNA. This knowledge can help identify genetic disorders and develop gene therapies. It also provides valuable insights into human evolution. While nearly 90% of the human genome has been sequenced, further research is needed to understand the location, function, and distribution of genes within the genome.