From Wikipedia: A human being is a multicellular eukaryote consisting of an estimated 100 trillion cells...Even though humans are multicellular animals, many of the basic life processes of human cells are basically the same as in simple unicellular eukaryotes such as yeast and even prokaryotes. A human being is initially just one cell, a zygote, but when that cell divides, it forms an embryo. The early embryo consists of totipotent stem cells, which go on to differentiate. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver, or muscle cells. During differentiation, certain genes are turned on, or become activated, while other genes are switched off, or inactivated. This process is intricately regulated. As a result, a differentiated cell will develop specific structures and perform certain functions. Differentiation can involve changes in numerous aspects of cell physiology; size, shape, polarity, metabolic activity, responsiveness to signals, and gene expression profiles can all change during differentiation. Cancer occurs because of changes to the genome of cancerous cells, leading to deregulation of cellular control mechanisms.

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