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Animation: Copying DNA through PCR

by Pearson
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The polymerase chain reaction is a powerful method for obtaining millions of copies of a DNA fragment from just one original copy. Here, the reaction begins with this mass of DNA, which represents the DNA from a cell. The goal is to amplify a specific region that is called the target DNA. The reaction mixture also contains Taq polymerase. Taq is of a type of heat-stable DNA polymerases. It is derived from a species of bacteria living in hot springs. The mixture also contains short, single-stranded DNA molecules called primers, as well as plenty of dNTPs, which are not shown at this scale. dNTPs are loose nucleotides similar to the ones found within living cells. Let's examine the basic PCR cycle, which is typically repeated about 35 times. The mixture is first heated to nearly boiling--95 degress Celsius. The heat breaks the hydrogen bonds between the two DNA strands, allowing the strands to separate in solution. Despite this intense heat, the Taq polymerase remains active. The temperature is then dropped to around 60 degrees Celsius. At this relatively low temperature, the primers can form hydrogen bonds with the DNA. The primers are critical in amplifying the correct target sequence. Two types are created so that each is complimentary in sequence to one of the two ends of the target DNA. In order to make the primers, the sequences at the ends of the target DNA must be known. In the next step, the temperature is increased to 72 degrees Celsius, which is the optimal temperature at which Taq polymerase functions. Starting at the primer on each end, the target sequence will be copied by the Taq polymerase which will produce the complementary strand of DNA by adding new nucleotides (the dNTPs). Each new dNTP that joins the growing strand is complementary to the nucleotide in the opposite strand. At the end of this cycle, there are two DNA copies instead of the original one copy. In cycle 2 the process repeats. After cycle 2, there are four double-stranded copies. With each cycle, the number of target DNA molecules doubles. Notice that more and more of the fragments consist of just the target DNA. A typical reaction is programmed to run approximately 35 cycles. A machine, called a thermal cycler, performs the temperature changes.
The polymerase chain reaction is a powerful method for obtaining millions of copies of a DNA fragment from just one original copy. Here, the reaction begins with this mass of DNA, which represents the DNA from a cell. The goal is to amplify a specific region that is called the target DNA. The reaction mixture also contains Taq polymerase. Taq is of a type of heat-stable DNA polymerases. It is derived from a species of bacteria living in hot springs. The mixture also contains short, single-stranded DNA molecules called primers, as well as plenty of dNTPs, which are not shown at this scale. dNTPs are loose nucleotides similar to the ones found within living cells. Let's examine the basic PCR cycle, which is typically repeated about 35 times. The mixture is first heated to nearly boiling--95 degress Celsius. The heat breaks the hydrogen bonds between the two DNA strands, allowing the strands to separate in solution. Despite this intense heat, the Taq polymerase remains active. The temperature is then dropped to around 60 degrees Celsius. At this relatively low temperature, the primers can form hydrogen bonds with the DNA. The primers are critical in amplifying the correct target sequence. Two types are created so that each is complimentary in sequence to one of the two ends of the target DNA. In order to make the primers, the sequences at the ends of the target DNA must be known. In the next step, the temperature is increased to 72 degrees Celsius, which is the optimal temperature at which Taq polymerase functions. Starting at the primer on each end, the target sequence will be copied by the Taq polymerase which will produce the complementary strand of DNA by adding new nucleotides (the dNTPs). Each new dNTP that joins the growing strand is complementary to the nucleotide in the opposite strand. At the end of this cycle, there are two DNA copies instead of the original one copy. In cycle 2 the process repeats. After cycle 2, there are four double-stranded copies. With each cycle, the number of target DNA molecules doubles. Notice that more and more of the fragments consist of just the target DNA. A typical reaction is programmed to run approximately 35 cycles. A machine, called a thermal cycler, performs the temperature changes.