PCR

Overview of PCR (Polymerase Chain Reaction)

  • Definition: PCR is a technique used to amplify a specific segment of DNA, creating millions of copies of a particular gene.

  • Key Components:

    • Taq Polymerase: An enzyme used to synthesize new DNA strands.

    • dNTPs: Deoxyribonucleoside triphosphates, the building blocks of DNA.

    • Primers: Short DNA sequences that provide a starting point for DNA synthesis.

Steps of PCR

  1. Denaturation:

    • Heating the double-stranded DNA to separate it into two single strands by breaking hydrogen bonds.

    • Example: The circular plasmid DNA is heated until it becomes linear single-stranded DNA.

  2. Annealing:

    • Cool down the reaction to allow primers to bind (anneal) to their complementary sequences on the template strands.

    • Forward Primer: 5' to 3' direction, complementary to the template strand.

    • Reverse Primer: Opposite strand, also 5' to 3' direction but complementary to the coding strand.

  3. Extension:

    • DNA polymerase synthesizes the new DNA strand by adding dNTPs complementary to the template strand in the 5' to 3' direction.

    • The forward primer synthesizes the coding strand while the reverse primer synthesizes the template strand.

DNA Strand Specific Terms

  • Coding Strand: Also known as the sense strand or non-template strand; contains the same sequence as the mRNA (except for U/T differences).

  • Template Strand: Also known as the antisense strand; the strand used by DNA polymerase to synthesize the new strand.

Example of PCR Process

  • Starting DNA: A plasmid with a gene of interest (e.g., segment GGG-CAT-CCC).

  • After one cycle of PCR:

    • Generates two linear DNA molecules each containing the gene of interest but flanked by unwanted plasmid sequences.

Subsequent Cycles of PCR

  • Each cycle involves denaturation, annealing, and extension, leading to exponential amplification of the target sequence.

    • Continued rounds reduce the unwanted sequences, focusing on just the gene of interest.

  • Cycle Progression:

    • After multiple rounds (25-30), there will be millions of copies of the target DNA, mostly free from plasmid sequences.

Understanding the Results

  • Final Products: Molecules generated in the process are identified by their structure: some will have only the amplified gene sequence without any surrounding plasmid sequences.

  • Critical for applications such as cloning, sequencing, and gene expression analysis.