PCR, Transformation, and DNA Sequencing

PCR, polymerase chain reaction, turns one or a few DNA molecules into billions of copies so there is enough to analyze. Each cycle has three steps. Denature: heat to about 95C separates the double helix into two single strands. Anneal: cool to around 50-65C so short primers, designed to match the sequence flanking the target, bind to each strand. Extend: a heat-stable DNA polymerase (commonly Taq) adds nucleotides to each primer, building a new complementary strand. Because both original strands get copied, the number of target molecules doubles every cycle: starting from 1 copy, after n cycles you have 2ⁿ copies. After 20 cycles that is 2²⁰, roughly a million copies, from a single template.

Transformation is how bacteria pick up foreign DNA, often a plasmid, a small circular piece of DNA separate from the main chromosome. Heat shock or an electric pulse (electroporation) makes the cell membrane temporarily permeable, letting the plasmid slip inside. If that plasmid carries a working promoter and other regulatory sequences, the bacterium treats the foreign gene like one of its own: its RNA polymerase transcribes it and its ribosomes translate it, producing the protein the gene codes for. This is how bacteria are engineered to make products like insulin. Restriction enzymes make this possible by cutting DNA at specific recognition sequences, leaving staggered sticky ends that let a foreign gene be spliced into a plasmid cut with the same enzyme.

Once DNA is amplified and sequences are known or cut into fragments, gel electrophoresis separates fragments by size, smaller ones travel farther through the gel. Comparing the band patterns, a DNA fingerprint, from two samples on the same gel shows whether they share the same fragment sizes, which is the basis for matching crime-scene DNA to a suspect or confirming paternity. This topic ties PCR amplification, cutting and pasting DNA, and reading its output together into one toolkit for manipulating and identifying DNA.