Chapter 14, Problem 11STP

Summary Introduction

To explain:

The three steps that take place in a polymerase chain reaction (PCR).

Introduction:

Polymerase chain reaction is a technique used to make millions of copies of a specific region of DNA fragment. It can replicate DNA outside the living organisms. The technique is highly sensitive. It can detect even a single DNA molecule in a sample. PCR is an important tool for molecular biologists as it helps in the diagnosis of infectious diseases.

Answer to Problem 11STP

The three steps of PCR are:

1. The DNA fragment, DNA polymerase, four nucleotides and two DNA primers are heated in a tube. The primers are used as starting points for DNA synthesis. 2. The two strands of DNA fragment separate due to heating and on cooling the primers bind to each strand of template DNA. 3. DNA polymerase then incorporates the correct nucleotides between the two primers to form new DNA molecules.

Explanation of Solution

The three steps of PCR are:

1. The DNA fragment, DNA polymerase, four nucleotides and two short single-stranded pieces of DNA called primers are placed in a tube. The tube is heated and PCR starts. The primers are complementary to the ends of DNA fragment that will be copied and are used as starting points for DNA synthesis. 2. The two template strands of DNA fragment gets separated due to heating. On cooling, the primers bind to each strand of template DNA. An automated machine called thermocycler is used to cycle the tube containing all the components through various hot and cool temperatures. 3. Each primer is made to bind to one strand of DNA fragment. DNA polymerase then incorporates the correct nucleotides between the two primers as in DNA replication. The process of heating, cooling and nucleotide incorporation is repeated 20 to 40 times resulting in millions of copies of original fragment.