5c Genetic Modification (Genetic Engineering)¶

Part of 5 Use of Biological Resources.

Genetic modification changes an organism by inserting a gene from another source. The key ideas are the molecular tools, the role of vectors and the practical uses of modified organisms.

Learning Objectives¶

Recombinant DNA Tools¶

Genetic engineering (genetic modification) is the process of modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.

Two key types of enzyme are used:

• Restriction enzymes cut DNA at specific base sequences, leaving 'sticky ends' — short stretches of unpaired, single-stranded DNA at each cut. Different restriction enzymes recognise different sequences.
• Ligase joins pieces of DNA together by forming covalent bonds between them.

To create recombinant DNA: 1. The desired gene is cut from the donor organism's DNA using a restriction enzyme, leaving sticky ends. 2. A plasmid (or viral DNA) acting as the vector is cut with the same restriction enzyme, producing matching sticky ends. 3. The gene and plasmid are mixed together. The sticky ends are complementary and join by base pairing. 4. Ligase seals the joins, incorporating the gene into the plasmid to form recombinant DNA.

Vectors and Gene Transfer¶

Vectors are the vehicles used to carry recombinant DNA into a host cell. Commonly used vectors include: - Plasmids: small circular loops of DNA found in bacterial cells that can be taken up by bacteria and replicated alongside the bacterial chromosome. - Viruses: can be used to inject DNA into host cells.

Once the recombinant DNA enters a suitable host cell (e.g. a bacterium), the host cell replicates, copying the inserted gene each time. The cells can then be grown at large scale and express the desired protein product.

Uses of Genetic Modification¶

Producing human insulin: Before genetic modification, insulin for treating diabetes was extracted from pigs or cattle. Today, it is produced by genetically modified bacteria: 1. The insulin gene is cut from human DNA using restriction enzymes, leaving sticky ends. 2. A bacterial plasmid is cut with the same restriction enzyme. 3. The human insulin gene is inserted into the plasmid; ligase joins the sticky ends. 4. The recombinant plasmid is placed into a bacterial cell (the vector). 5. The bacteria are grown in large fermenters. They reproduce and express the human insulin gene, producing large amounts of human insulin.

Genetically modified crops: Plant cells can also be genetically engineered to express genes from other organisms. Current applications include: - Resistance to specific insects (reducing the need for pesticides). - Resistance to herbicides (allowing herbicide application that kills weeds but not the crop). - Improved yield or nutritional content.

Transgenic organisms contain genes transferred from a different species. Transgenic bacteria produce human insulin; transgenic plants may have pest-resistance genes from other species.

Common Confusions¶

• Vector vs gene: The gene is the DNA segment being transferred. The vector (plasmid or virus) is the carrier that introduces the gene into the host cell.
• Restriction enzyme vs ligase: Restriction enzymes cut DNA; ligase joins it. Both are needed to create recombinant DNA.
• Selective breeding vs genetic modification: Selective breeding works within a species using existing alleles; genetic modification can introduce genes from entirely different species.

Key Terms¶

• Genetic modification (genetic engineering): altering the genome of an organism by inserting a gene from another organism.
• Restriction enzyme: an enzyme that cuts DNA at a specific base sequence, leaving sticky ends.
• Sticky ends: short, single-stranded overhanging sequences of DNA left after cutting with a restriction enzyme; they allow complementary matching with other sticky ends.
• Ligase: an enzyme that joins DNA fragments by forming bonds between nucleotides.
• Recombinant DNA: DNA formed by combining genetic material from two or more different sources.
• Vector: a carrier (plasmid or virus) used to introduce recombinant DNA into a host cell.
• Plasmid: a small circular loop of DNA in bacterial cells that can be used as a vector in genetic engineering.
• Transgenic: describing an organism that has had genetic material from another species inserted into its genome.

Connected Pages¶

• 3b Inheritance
• 5b Selective Breeding
• 5d Cloning
• IGCSE Biology Key Terms Glossary