Chapter 3: Immortal Coils

Chapter 3, titled "Immortal Coils," asserts the fundamental concept that all living organisms, encompassing animals, plants, viruses, and bacteria, function as temporary survival machines built for the preservation and propagation of replicators, specifically the DNA molecules known as genes. DNA, or deoxyribonucleic acid, is described as an "immortal coil"—an elegant double helix composed of long chains of nucleotide building blocks abbreviated as A, T, C, and G. This genetic material acts as a set of intricate architectural plans for constructing the body, residing in the nucleus of nearly every cell. The two primary roles of DNA are self-replication, which occurs faithfully during normal cell division (mitosis), and the indirect supervision of protein manufacture, translating the four-letter nucleotide code into the amino-acid alphabet used to build protein molecules that constitute the body’s physical fabric and chemical control mechanisms. Genes are highly interdependent and operate cooperatively within a gene complex to control embryonic development, and importantly, characteristics acquired during an individual's lifetime cannot be genetically inherited. A crucial mechanism influencing gene stability is sexual reproduction (meiosis), which involves the intricate shuffling of chromosomes. Humans possess 46 chromosomes in 23 pairs, with each pair carrying alternative instructions (alleles) for the same genetic slot, defining concepts like recessive and dominant genes. During the production of sex cells (sperms or eggs), a vital process called crossing-over occurs, where matching sections of paternal and maternal chromosomes physically exchange bits. Because whole chromosomes are broken up and mixed every generation, they are temporary units, while individual bodies are too fleeting to serve as the significant unit of evolution. Therefore, the gene is defined as any segment of chromosomal material small enough to last, in the form of copies, for enough generations to function as the basic, enduring unit of natural selection. Genetic units achieve this necessary longevity primarily by being short enough to avoid being frequently split by crossing-over. New genetic arrangements are usually formed by this shuffling, though rare errors like point mutation (a single-letter change) or inversion (flipping of a chromosome section) can also occur; inversion, in particular, can bring several cooperative genes into a tightly linked group that acts like a single "super-gene," as seen in the example of butterfly mimicry. The chapter then explores the question of why individuals die, utilizing Sir Peter Medawar’s evolutionary theory which posits that senile decay is a consequence of the accumulation of lethal or semi-lethal genes whose detrimental effects appear late in life, after the organism has already reproduced, allowing them to slip through the sieve of natural selection. Finally, the apparent paradox that sexual reproduction is inefficient for the individual—as only half of their genes are passed on to offspring—is resolved by viewing sexuality as a trait favored by the selfish gene itself, as recombination aids in distributing successful genetic units throughout the larger gene pool.