1. Restriction Enzymes Discovered in bacteria Allow bacteria to protect themselves from viri Specialized proteins (enzymes) that act as nucleases Break bonds between nucleotides (building blocks of nucleic acids, including DNA) Recognize specific, short sequences of nucleotides in DNA Cleave DNA at the recognition sites 1

2. 5 end Nucleoside Nitrogenous base Phosphate group Sugar (pentose) (b) Nucleotide (a) Polynucleotide, or nucleic acid 3 end 3C3C 3C3C 5C5C 5C5C Nitrogenous bases Pyrimidines Cytosine (C) Thymine (T, in DNA)Uracil (U, in RNA) Purines Adenine (A)Guanine (G) Sugars Deoxyribose (in DNA) Ribose (in RNA) (c) Nucleoside components: sugars 2

3. Sugar-phosphate backbones 3' end 5' end Base pair (joined by hydrogen bonding) Old strands New strands Nucleotide about to be added to a new strand 3

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