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key terms for chapter 9


substance that is prepared from a killed or weakened disease-causing agents.


able to cause disease


change in genotype caused when cells take up foreign genetic material


virus that infects bacteria

double helix

two strands twisted around each other


subunits that make up DNA


5 carbon sugar

adenine and guanine

classified as purines

thymine and cytosine

classified as pyrimidines

base pairing rule

rule stating that cytosine pairs with guanine and adenine pairs with thymine in DNA, and that adenine pairs with uracil in RNA

complementary base pairs

characteristic of nucleic acids in which the sequence of bases on one strand is paired with the other

dna replication

process of making a copy of DNA

dna helicases

when the double helix is unwound.

replication forks

areas where the double helix separates

dna polymerases

enzyme that catalyzes the formation of the DNA molecule

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