Biologically speaking, “human development begins at fertilization,” when a woman and a man each combine 23 of their own chromosomes through the union of their reproductive cells. The DNA in the 46 chromosomes of the resulting embryo (zygote stage), then only one cell in size, already contains some 3 billion base pairs of digital data, the genetic blueprint for the entire human body. The DNA in that single cell contains so much information that if it were represented in printed words, simply listing the first letter of each base would require over 1.5 million pages of text. By the time the zygote develops into the adult body, it will consist of 100 trillion cells.
Moore KL, Persaud TVN, Torchia MG. 2020. The developing human: clinically oriented embryology. 11th ed. Elsevier, E-book. p. 56.
Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. 2000. Molecular cell biology. 4th ed. New York: W.H. Freeman. pp. 12, 456.
Guyton AC, Hall JE. 2000. Textbook of medical physiology. 10th ed. Philadelphia: W.B. Saunders. pp. 2, 34.