Breathless: The Scientific Race to Defeat a Deadly Virus

by David Quammen

Scientists have cloned cats, dogs, deer, and other animals. Using microsurgical equipment, you extract the nucleus from one cell of a leopard in captivity at SGNP. This is the leopard you seek to clone—to replicate as closely as possible in a new animal, its genetic twin. It might be a male or a female. Call it Donor 1. You also remove the nucleus from an egg cell, an oocyte, taken from another leopard, necessarily a female (because only females produce eggs). Call her Donor 2. You insert your chosen nucleus into that egg cell, then you activate the thing to begin dividing. When it has developed to the point of comprising a couple hundred cells, those cells will be contained inside a spheroid mass, with a protective layer, called a blastocyst. This is your preimplantation embryo. The nuclear genome in each of those cells is identical to the genome of your clone target, Donor 1. And the cytoplasm of each cell, the gelatinous fluid surrounding the nucleus, contains mitochondrial DNA, a subsidiary genome, inherited only from that female who gave you her egg cell, Donor 2. You surgically implant the embryo in the uterus of a foster mother. For convenience, you might choose a smaller and less fearsome cat for this motherhood role—for instance, a Bornean clouded leopard, a gracefully spotted feline about the size of a border collie. If you are lucky and adept, your Bornean clouded leopard carries the embryo to term and gives birth to a true leopard, the genetic twin—in its nucle...