WEBVTT

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The zygote's 46 chromosomes

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represent the unique
first edition

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of a new individual's
complete genetic blueprint.

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This master plan resides

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in tightly coiled
molecules called DNA.

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They contain the instructions
for the development

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of the entire body.

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DNA molecules
resemble a twisted ladder

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known as a double helix.

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The rungs of the ladder are
made up of paired molecules,

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or bases, called guanine,

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cytosine, adenine, and thymine.

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Guanine pairs
only with cytosine,

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and adenine with thymine.

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Each human cell contains
approximately 3 billion