In this chapter, Mukherjee focuses on “the most monumental event in the life cycle of a cell” (96), which is when a cell divides and produces daughter cells. There are two types of cell division. The first is mitosis, which German scientist Walter Fleming discovered in the late 1800s by staining chromosomes with dye and watching them divide under a microscope. Under mitosis, one cell (the mother) divides to produce two new daughter cells. These daughter cells are genetically identical to the mother cell. Each daughter cell has 46 chromosomes.

The second type of cell division is meiosis, which leads to “the birth of new cells, sperms, and eggs for the purpose of reproduction” (97). In humans, cells contain two sets of chromosomes because one comes from each parent (known as diploid). To maintain this state, eggs and sperm must have only a single set of chromosomes (known as haploid), each containing 23 chromosomes, prior to fertilization. During meiosis, each diploid cell divides into four haploid daughter cells. These daughter cells are called gametes. Fertilization between two gametes (egg and sperm) produces a zygote that has 46 chromosomes. The zygote includes a combination of DNA from each gamete.

Cell division has four phases. The first consists of Gap 1 (G1), in which the cell starts to grow larger, makes copies of organelles, and collects the molecules it will need later in the process. It represents the first resting phase. The second phase is synthesis (S), in which the cell synthesizes new DNA in the nucleus. The third phase, which is the second resting phase, is called Gap 2 (G2). In this phase, the cell continues to grow, produces more organelles and proteins, begins to reorganize its internal structure in preparation for dividing, and makes sure that the DNA strand has no mutations or damage. The final phase is mitosis (M), when the cell splits into two daughter cells.

Mukherjee introduces English geneticist Paul Nurse and English biochemist and molecular physiologist Tim Hunt. Both were interested in the cell cycle, particularly what controls it. Through studies on mutant yeast, Nurse found that cyclin-dependent kinase (CDK) enzymes were associated with cell division phases. Hunt, through studies on sea urchins, found that a protein called cyclin was associated with the cell cycle too. They realized that these two proteins “act in concert to regulate the transitions in the phases of cell division” (105).

Next, Mukherjee turns to in vitro fertilization (IVF), a form of cellular therapy that “artificially or medically assist[s] human reproduction” (107). Several different scientists worked on this issue, including Landrum Shettles, Robert Edwards, Patrick Steptoe, and Jean Purdy. IVF was first achieved in 1969, although a baby wasn’t grown from this process until a decade later. The discovery faced immediate backlash from the medical and religious communities and the public. Louise Brown was the first “‘test-tube baby’” (115), although Mukherjee takes issue with this term since test tubes weren’t used during the fertilization process.

Mukherjee focuses on the story of Chinese professor and biophysicist He Jiankui (nickname JK) in this chapter. First, however, he provides some context. Since the late 1960s, researchers were interested in whether they could detect chromosomal abnormalities in the embryo. The technique of preimplantation genetic diagnosis (PGD), invented in the 1990s, turned this interest into a reality. PGD involves testing an embryo for particular genetic disorders before it implants on the uterine wall. Mukherjee calls this process a negative process because it doesn’t entail creating a new embryo from new genes.

Scientists soon began exploring whether a positive process, or “the addition or alteration of a gene” (121), was possible. Scientists could do this with animal embryos, but the technique wasn’t easily adapted to human embryos. The 2011 discovery of gene editing, which enables scientists to change a living organism’s DNA, changed this story.

The most commonly used gene editor is the CRISPR-Cas9 protein. It derives from a naturally occurring bacterial defense system. Mukherjee notes that “Cas9, when combined with a piece of RNA to guide it, can be directed to make a deliberate change in the human genome” (123).

Mukherjee returns to the story of JK, who was interested in editing a gene, specifically CCR5, in a human embryo. CCR5 is a gene for a protein on immune cells that HIV uses to enter and infect the cell. Humans with a natural mutation called delta32 on the CCR5 gene are resistant to HIV. Mukherjee notes that this gene choice didn’t make sense. While the father did have HIV infection, the risk of HIV transmission from the sperm to the egg was zero because the sperm is washed during the IVF process. In addition, disabling CCR5 could have dangerous consequences for the immune system. Whether the couple was informed about the possible risks associated with the gene editing procedure is unclear.

JK conducted the gene editing process—despite potential issues, including that it wasn’t the same as the natural mutation—and implanted edited embryos into the womb. He announced the healthy birth of the first two gene-edited babies at a conference in 2018. Scientists grilled JK during the question-and-answer session on his methods for recruiting participants, whether he obtained informed consent, the potential risks associated with gene editing, and more. Mukherjee describes the session “as among the more surreal moments in medical history” (127). JK was unable to answer most of the questions. He was sent to prison for several years and banned from performing future research on IVF. Despite the backlash that JK faced and the current lack of rules and standards to govern human gene editing, other researchers are diving into this work.

The chapter ends by focusing on how single-celled organisms (e.g., yeast, slime molds, and algae) become multicellular organisms. Several experiments have demonstrated that single-celled organisms are “singularly drawn to form multicellular clusters” (134). The reason for this remains unknown, although it may be tied to size and movement. Regardless, the evolution of multicellularity was intentional, especially given that some cells are programmed to self-sacrifice to help create multicellular aggregates.

Mukherjee describes the process of fertilization. While it’s now known as the correct biological process to create life, people in past centuries held different beliefs. One belief, known as preformation (see Part 1), held that humans appeared in the womb as miniature versions of a person. They simply had to grow. Aristotle was one of the first thinkers to break away from this idea. He argued that humans were sculpted from menstrual blood. Albertus Magnus, a German friar in the mid-1200s, agreed with Aristotle. However, he also identified the formation of distinct organs in the embryo. It took another 500 years for researchers to advance this idea. The notion that organs underwent continual change during fetal development finally won over scientists, demolishing preformationism. This idea has been further honed under cell biological theory, which established that “all the anatomic structures of a developing embryo are formed by dividing cells, creating different structures and performing various functions” (138).

Once fertilization happens, the cell divides. This division continues until it creates billions of new cells. During the embryonic stage, these cells form three cell layers. The first is the ectoderm, or inner layer, which gives rise to the nervous system, skin, hair, teeth, and nails. The second is the mesoderm, or middle layer, which dictates the development of key cell types (e.g., connective tissue, muscle, blood, heart, and bone). The third is the endoderm, or outer layer, which forms the linings of the respiratory and digestive systems and some organs (e.g., the liver, lungs, intestines, and pancreas).

These three layers are followed by the formation of the notochord, a flexible rod similar to cartilage. Mukherjee notes, “The notochord will become the GPS of the developing embryo, determining the position and axis of the internal organs as well as secreting proteins called inducers” (139). By adulthood, the notochord turns into pulp in between human bones.

Experiments on frog embryos in the 1920s helped uncover what drives the phases of development for different organs during embryonic development. At each stage of development, already existing cells secrete proteins and chemicals that tell emerging cells where to go and what to develop into. It’s an extremely complex process with interplay among all the different signals.