Question

what is the polymerase chain reaction (pcr)?
pcr amplifies dna within an organism to increase the expression of that dna for the purpose of studying its effects.
pcr modifies dna within organisms, often species of agricultural importance, to increase the occurrence of desirable traits.
pcr selects variations of genes within dna, called alleles, to increase the fitness of organisms by artificial selection.
pcr produces many copies of a dna sequence from a small sample using temperature cycling, primers, free nucleotides, and polymerase enzymes.
pcr determines the base pairs in a dna sequence using temperature cycling, primers, fluorescent dye labels, and polymerase enzymes.

Explanation:

PCR is a technique that uses temperature - cycling, primers, free nucleotides, and polymerase enzymes to make multiple copies of a DNA sequence from a small sample. It does not modify DNA within organisms, select alleles, determine base - pairs (that's sequencing), or amplify DNA within an organism for expression study purposes.

Answer:

PCR produces many copies of a DNA sequence from a small sample using temperature cycling, primers, free nucleotides, and polymerase enzymes.