Cracking the AP Biology Exam
POLYMERASE CHAIN REACTION (PCR)
A few years ago, it would take weeks of tedious experiments to identify and study specific genes. Today, thanks to polymerase chain reaction (PCR), we are able to make billions of identical copies of genes within a few hours. To do PCR, the process of DNA replication that is described on this page is slightly modified. In a small PCR tube, DNA, primers, Taq Polymerase, and lots of DNA nucleotides (A’s, C’s, G’s, and T’s) are mixed together.
In a PCR machine, or thermocycler, the tube is heated, cooled, and warmed many times. Each time the machine is heated, the hydrogen bonds break, separating the double-stranded DNA. As it is cooled, the primers bind to the sequence flanking the region of the DNA we want to copy. When it is warmed, Taq Polymerase binds to the primers on both strands and adds nucleotides on each template strand. After this first cycle is finished, there are two identical double-stranded DNA molecules. When the second cycle is completed, these two double-stranded DNA segments will have been copied into four. The process repeats itself over and over, creating as much DNA as needed. Today, a thermocycler is commonplace in science labs. It is regularly used to study small amounts of DNA from crime scenes, determine the origin of our foods, detect diseases in animals and humans, and to better understand the inner workings of our cells.
Insulin, the protein hormone that lowers blood sugar levels, can now be made for medical purposes by bacteria. Yes, bacteria can be induced to use the universal DNA code to transcribe and translate a human gene! This can be done by transformation.
Genes of interest are first placed into a transformation vector, such as a plasmid. Plasmid vectors are small, circular pieces of DNA that contain genes for antibiotic resistance and restriction sites. Plasmids and the gene of interest are cut with the same restriction enzyme, creating compatible sticky ends. When placed together, the gene is inserted into the plasmid creating recombinant DNA.
The bacteria are then transformed using the recombinant plasmid. In most AP biology classes, this is done by the heat shock method. Because the plasmid contains a gene for antibiotic resistance, transformed bacteria will be able to grow on a medium that contains antibiotics whereas bacteria without the plasmid will die. This allows scientists to cleverly identify transformed bacteria.
This laboratory technique has not only been used to safely mass-produce important proteins used for medicine, like insulin, it has an important role in the study of gene expression.
HUMAN GENOME PROJECT
In 1990, an international, publicly funded consortium of scientists was determined to sequence every chromosome, base by base, in the human genome. The latest DNA sequencing machines and innovative computer programming skills were used in this tedious process. A draft of the sequence was first published in 2001. Today, in addition to the human genome, the genomes of many different species are available online for anyone to study at:
These genomes can be used to study our evolutionary history, genetic diseases, and can be used to make pharmaceuticals.