Biology Quote by Siddhartha Mukherjee

““It is conceivable that an interplay of genes and epigenes coordinates human embryogenesis. Let us return, yet again, to Morgan's problem: the creation of a multicellular organism from a one-celled embryo. Seconds after fertilization, a quickening begins in the embryo. Proteins reach into the nucleus of the cell and start flicking genetic switches on and off. A dormant spaceship comes to life. Genes are activated and repressed, and these genes, in turn, encode yet other proteins that activate and repress other genes. A single cell divides to form two, then four, and eight cells. An entire layer of cells forms, then hollows out into the outer skin of a ball. Genes that coordinate metabolism, motility, cell fate, and identity fire "on." The boiler room warms us. The lights flicker on in the corridors. The intercom crackles alive. Now a second code stirs to life to ensure that gene expression is locked into place in each cell, enabling each cell to acquire and fix an identity. Chemical marks are selectively added to certain genes and erased from others, modulating the expression of the genes in that cell alone. Methyl groups are inserted and erased, and histones are modified to repress or activate genes. The embryo unfurls step by step. Primordial segements appear, and cells take their positions along various parts of the embryo. New genes are activated that command subroutines to grow limbs and organs, and more chemical marks are appended on the genomes of individual cells. Cells are added to create organs and structures-forelegs, hind legs, muscles, kidneys, bones, eyes. Some cells die a programmed death. Genes that maintain function, metabolism, and repair are turned on. An organism emerges from a cell.””

— Siddhartha Mukherjee