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adenine
One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also refers to a nucleotide having an adenine base component.
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bacteriophages
One of a class of viruses that infects bacteria.
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clone
A genetically identical copy of DNA, a cell, or a multicelled organism.
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cytosine
One of the four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also applies to a nucleotide that contains a cysteine base.
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DNA
Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.
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DNA ligases
Type of enzyme that catalyzes the sealing of short stretches of DNA into a continuous strand during replication; also seals strand breaks.
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DNA polymerases
Type of enzyme that catalyzes the addition of free nucleotides to new DNA strands during replication; also proofreads and corrects mismatches.
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DNA proofreading mechanisms
Any enzyme-mediated process that fixes DNA replication errors or strand breaks.
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DNA replication
Process by which a cell duplicates its DNA molecules before it divides into daughter cells.
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guanine
One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also may refer to a nucleotide that contains a guanine base.
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helicases
Type of enzyme that catalyzes breaking of hydrogen bonds during DNA replication so the two strands of double helix can unwind from each other.
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nucleotides
Small organic compound with a five-carbon sugar, a nitrogen-containing base, and a phosphate group. Functions as coenzymes or monomers of nucleic acids.
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purines
A nucleotide base with a double ring structure; e.g., adenine or guanine.
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pyrimidines
A nucleotide base with a single ring structure; e.g., cytosine, thymine, uracil.
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repair enzymes
Type of enzymes that repairs nucleotide mismatches in a DNA strand.
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semiconservative replication
[Gk. semi�, half, + L. conservare, to keep] Mechanism by which a DNA molecule is duplicated. The double helix unzips along its length, exposed bases of each strand are a template upon which a new strand is assembled, then each conserved strand and its new partner wind up in a double helix. Two double helixes, each with a parental strand and new strand of DNA, result.
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stem cells
Self-perpetuating, undifferentiated animal cell. A portion of its daughter cells becomes specialized; e.g., red blood cells from stem cells in bone marrow.
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telomeres
A cap of repetitive DNA sequence on the end of a chromosome. Each nuclear division, enzymes digest a bit of it; cells stop dividing when only a nubbin remains.
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thymine
One of four nitrogencontaining bases in nucleotide monomers of DNA; also applies to a nucleotide with a thymine base component.
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x-ray diffraction images
Film image of xrays scattered by a crystalline sample; the resulting pattern of streaks and dots can be used to calculate the spacing between the atoms in the crystal lattice.
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