6.3.4(The polymerase chain reaction)

Describe what PCR relies on

• DNA is antiparallel

• Each strand has a 3’ end and 5’ end

• DNA grows only from the 3 end

• Base pairs pair up according to complementary base pair rulingDe

Describe the differences between PCR and DNA replication

• Only short sequences, of up to 10,000 base pairs, of DNA can be replicated not all chromosomes

• It requires the addition of a primer molecule make the process

• A cycle of heating and cooling is needed to separate the DNA strands, bind primers to the strands and for the DNA strands to be replicated

Describe the process of PCR

• The DNA sample is mixed with DNA nucleotides, primers and Taq polymerase

• The mixture is then heated to 95C to break the H bonds between complementary nucleotide base pairs and denature the DNA strand into 2 single strands of DNA

• the mixture is the cooled to 65C where primers can anneal to end of each strand of DNA at the beginning of the gene

• The Taq polymerase enzyme can now bind to the end with primers.

• The solution will what up to 72C and free nucleotides can now bind to the complementary bases on the DNA strand

• Taq polymerase forms phosphodiester bonds between the nucleotides to form a chain

Explain why Taq polymerase is used instead of DNA polymerase

• Its because Taq polymerase is temperature resistant and so will not denature at high temperatures compared to DNA polymerase

Describe some applications of PCR

• Tissue typing - finding a match for an organ donor

• Forensic science - Linking criminals to the scene of a crime

• Genetic screening of cells from a biospy

• Molecular palaeontology - using fossils to provide insights into evolution

• Identifying viral infections - to enable early treatment with anti-viral therapy

Describe the advantages of PCR

• Produces lots of DNA is short space of time

• Does not require living cells

  • Fast process - Millions of copies of DNA cloned in a few hours

Describe the disadvantages of PCR

• Required a very pure sample as any conatminant DNA will also be amplified

• Cannot produce mRNA and protein

• It can be expensive if you to produce a lot of DNA

• Can only replicate a relatively small DNA fragment