DNA Sequencing and PCR

3:25

Learn all about DNA sequencing and PCR in just a few minutes! Jessica Pamment, professional lecturer at DePaul University, explains how polymerase chain reaction, a technique for rapidly copying DNA, is used in bioinformatics.

DNA Sequencing

Using techniques that exploit the complementary base pairing rule of nucleic acids, the nucleotide sequence of an entire genome can be determined.

DNA sequencing is the process by which the precise order of nucleotides within DNA is determined. Nucleotide bases pair according to specific rules: adenine (A) always pairs with thymine (T) or uracil (U), and cytosine (C) always pairs with guanine (G); this is called complementary base pairing. By using the complementary base pairing rule of nucleic acids, scientists have sequenced an organism's entire set of DNA, or genome.

To learn how to sequence an organism's genome, scientists began with a virus. They first determined the exact nucleotide sequence of the virus by cutting up its DNA into pieces, and then finding the sequence of each piece. They used the fact that adenine will always pair with thymine and guanine with cytosine to determine nucleotide sequences. Techniques often involve the use of modified nucleotides that can be detected during or after binding. By recording the order of the nucleotides that are bound to the unknown DNA strand, researchers can deduce the nucleotide sequence of the original piece of DNA. Shortly after sequencing the first virus in 1977, a yeast became the first eukaryotic (an organism with a nucleus and whose DNA is bound together by proteins) genome to be sequenced. Initially, the process of sequencing was labor-intensive because much of the work of scientists had to be done by hand and thus was time consuming. However, many advances in sequencing techniques have emerged, including the use of machines to sequence thousands of pieces of DNA at the same time. As the process becomes faster and cheaper, large quantities of genetic information are being collected and made available to researchers. Keeping track and making sense of vast quantities of sequence data is one practical application of the field of bioinformatics, which uses computers and mathematics to store and analyze biological data. These data have countless uses, including finding genetic markers for diseases, determining patterns that might not otherwise be visible or clear, and testing for genetic diseases to improve and individualize medical treatments.

Polymerase Chain Reaction (PCR)

Polymerase chain reaction (PCR) is a technique for rapidly copying any piece of DNA using actions similar to natural DNA replication.

Many advances in DNA technology would have been impossible without DNA amplification, the process by which many copies of a piece of DNA are made in the laboratory. One successful technique for rapidly reproducing a section of DNA is called polymerase chain reaction (PCR). DNA is separated into two strands. Each strand is exposed to a DNA primer (anchor and starting point for DNA polymerase to add free nucleotides to a growing strand of DNA) and DNA polymerase (an enzyme that assists in adding free nucleotides to a growing strand of DNA). This allows the DNA strands to replicate using actions similar to those in DNA replication. The primer acts as an anchor to specify the location on a strand that is used as a template for starting to build a new strand of DNA. After one cycle, the original template strand has been copied, resulting in four single strands of DNA. The cycle is repeated, but this time, each of the four single strands of DNA acts as the template strand. With each cycle, the number of copies of DNA grows exponentially. Billions of copies can be made from a single piece of DNA in a few hours. Once the DNA has been amplified in this manner, many different tests can be run on the copies rather than having just one sample available to conduct just one test. For example, this technique allows a single hair follicle or a drop of blood to provide enough DNA evidence to test for genetic markers, diagnose genetic disorders, and detect bacteria or viruses.

Polymerase Chain Reaction (PCR) is a laboratory technique used to make many copies of a section of DNA. In PCR, many DNA copies can be quickly made by taking advantage of complementary base pairing, in which nucleotide bases pair with a specific partner (adenine with thymine, and cytosine with guanine). At the end of each cycle, each newly formed strand becomes a template strand for the next cycle.

<Introduction to Genetic Engineering >Human Genome Project and DNA Databases