What Happens During Genetic Modification? - Part 1

All living organisms are made up of countless, minute cells. These cells contain a nucleus that is made up of Deoxyribose Nucleic Acid (DNA) located on chromosomes. All of the information that forms an organism and keeps it functioning is found in this DNA. The DNA can be further broken down into genes, which are sections of DNA that determine particular characteristics. The instructions from these genes ensure that specific proteins are formed, thus allowing the structure of the organism to be created.

http://www.biotechnologyonline.gov.au/popups/img_cell.html

In order for the gene that carries a certain characteristic to be found within the DNA, a single-stranded piece of DNA, known as a gene probe is used to search for the unique base sequence of the gene. Through designing a particular base sequence for the probe, which corresponds to that of the gene, the section of DNA can be located. The base sequence can consist of any arrangement of adenine (A), cytosine (C), guanine (G) and thymine (T), with A complimenting T and C complimenting G. For example if the gene consisted of the base sequence, AGGCTC, then a gene probe would need to be designed with the base sequence, TCCGAG, so that complimentary pairs would match up.

http://www.biotechnologyonline.gov.au/popups/img_helix.html

Before testing the DNA, the double-stranded DNA molecules must be unzipped through a certain treatment. Once the DNA is unzipped into single strands, the gene probe can attach itself to the specific section of DNA. This probe is also easily detected, as it is fluorescent.

http://tle.westone.wa.gov.au/content/file/969144ed-0d3b-fa04-2e88-8b23de2a630c/1/human_bio_science_3b.zip/content/005_dna/images/pic043.gif

You may be wondering how DNA testing relates to genetic modification. Well, once the gene is located, its chromosome and position on the chromosome can be determined; therefore the gene can be cut from the DNA of the organism and pasted into a different organism or into a new position on the same organism. This process is referred to as ‘gene splicing’. To cut sections of DNA, a group of chemicals, known as restriction enzymes are used. These restriction enzymes were originally located within bacteria, with their role being to act as defence against foreign DNA. They are a defence mechanism that inactivates the foreign DNA by cutting it up, thus they are very efficient in slicing sections of DNA when used in genetic modification.

 Specific restriction enzymes cut different sections of DNA. For example, the enzyme EcoRI (taken from the human bacteria, E.coli) recognises the sequence GAATTC and cuts the DNA between the guanine (G) and adenine (A). Furthermore, restriction enzymes can be used on any organism, as all organisms are chemically identical (i.e. their DNA contains the same bases, just different arrangements).

http://employees.csbsju.edu/HJAKUBOWSKI/classes/SrSemMedEthics/Human%20Genome%20Project/restriction.gif

DNA can be cut with straight, ‘blunt’ ends or ‘sticky’ ends, which is where one strand of DNA is cut slighter longer than the other. Generally, sticky ends are used as the overhanging strand of DNA allows a different strand of DNA to be easily matched to it.

http://explorebio.wikispaces.com/file/view/sticky_ends.jpg/204841238/sticky_ends.jpg

Once the section of DNA has been cut, it is pasted into a new chromosome or plasmid (small circular DNA molecule found in bacteria) so that it can be transferred to another organism. The same restriction enzymes are used to cut the plasmid or chromosome using the sticky ends method, so that it has the same base sequence as the end of the section of DNA. The overhanging stands of DNA are then bound using the enzyme, ligase, thus pasting it together.

This newly pasted section of DNA is then cloned so that it can be inserted into a variety of different organisms or used multiple times in order to ensure a higher success rate of transforming the cell. The plasmid or chromosome containing the new DNA (referred to as recombinant) is cloned through a process of asexual reproduction and division. In the case of bacteria, the plasmid is introduced into a separate bacterium, where it is spread onto nutrient rich agar plates. Each bacterium then grows into millions of cells that are identical to the recombinant plasmid. This results in many more genes being available for insertion into organisms.

http://www.bio.davidson.edu/courses/Molbio/MolStudents/spring99/alex/PLASMID.gif

When the gene is to be inserted into a new cell, another process called transformation is to be undertaken. This is due to the fact that the DNA must be taken out of the nucleus of its current cell and inserted into the nucleus of the new cell. The DNA must pass through the cell membrane and in some cases, the cell wall, then finally the nuclear membrane. The process is different depending on whether the organism is an animal or a plant and this will be explained further in the next two blog posts.

Eat Safe!

Dr Georga Leisemann (Feeding Your Family)