Mendel’s laws were predictively successful, and they are still used by geneticists today. However, his theory of genetics was limited in several ways. For one, Mendel’s laws of heredity apply only to sexual reproduction; they don’t apply to asexual reproduction—for example, the reproduction of bacteria and other single-celled organisms that propagate by dividing themselves in two. Secondly, his theory says nothing about the physical mechanisms involved in storing, transmitting, and utilizing genetic information. Thirdly, Mendel’s theory could not explain how new genetic information might be introduced. His laws only describe how already existing dominant and recessive alleles can be combined in various ways to produce different traits—a process known as recombination. This third aspect of Mendel’s theory was a serious problem for evolutionary biology, because it implied that the possible range of genetic variation is limited. Recombination can only explain the emergence of traits corresponding to alleles (or combinations of alleles) that were already present in an organism’s ancestors.
Granted, recombination can produce a remarkable variety of new traits. Consider, for example, the following vegetables: broccoli, cauliflower, cabbage, Brussels sprouts, kale, and collard greens. All of those come from the very same species of plant! The species called Brassica oleracea—known in its native form as “wild cabbage”—has been cultivated for thousands of years and selectively bred to produce the delightful variety of veggies we all hated eating when we were kids. Nevertheless, there are limits to how far selective breeding can go. No combination of dominant and recessive cabbage alleles will yield a baby squirrel, for instance.
For the same reason, mere genetic recombination doesn’t explain how major evolutionary transitions between radically different species could occur. It doesn’t explain how reptiles could evolve into birds, for example, unless all of the genes required for growing feathers are already present in the reptile population! (Perhaps the trait of “having feathers” requires a certain combination of recessive alleles, and reptiles have dominant alleles that prevent the “feathers” trait from showing up until the dominant alleles are eliminated by natural selection. But that sounds rather implausible.)
All of these problems with Mendel’s theory were eventually solved, however, as scientists came to understand how genetic information is stored and transmitted. After a series of discoveries in the first half of the 20th century, scientists had figured out that genetic information is stored in molecules of deoxyribonucleic acid (DNA). Then, in 1953, a team of two scientists—American geneticist James Watson and British biophysicist Francis Crick—successfully determined the molecular structure of DNA (a feat for which they later earned a Nobel Prize). It turned out that DNA is shaped like a double helix. It resembles a twisted ladder, and genetic information is encoded in the “rungs” of the ladder. Each rung is called a base pair, and consists of two of the following four chemical compounds, the names of which are abbreviated with the letters A, T, C, and G: adenine (A), thymine (T), cytosine (C), and guanine (G). These four compounds, called nucleobases, are the carriers of genetic information. Thus, we can think of genetic code as a sequence of the letters A, T, C, and G, analogous to the sequences of ones and zeros used in computer programming.
Each gene or allele of Mendel’s theory corresponds to a segment of DNA. In other words, each gene is a digital code, much like a computer program, that carries instructions for the assembly and operation of other components within a living cell. The mechanisms involved in carrying out these instructions are truly marvelous, but I won’t go into the details here.I’ll say just a little bit more about these mechanisms in chapter 10, in the context of discussing intelligent design theory. For just a taste of the magnificent complexity and efficiency of these processes, see the video below, which explains how molecular machines copy genetic code from DNA, trim off superfluous segments, then transmit the edited information to molecular factories called ribosomes, which follow the instructions to assemble specific types of proteins—the building blocks of living cells.
Each cell in a living organism’s body contains a copy of the same set of DNA molecules. An exception will be noted below. That set of DNA molecules is called the organism’s genome. The human genome consists of 92 DNA molecules, paired together in 46 bundles called chromosomes, which in turn are also grouped in 23 pairs. Each cell in your body contains a copy of that entire genome.
DNA molecules store a mind-boggling amount of digital code. Altogether, the DNA molecules in a single human cell contain over 3 billion base pairs! If you unraveled all the DNA from one cell of your body and stretched it out, it would be 2 to 3 meters long! And your body contains so many cells that if you stretched out the DNA from all your cells, end to end, it would reach from the sun to Pluto and back at least 12 times.perhaps as many as 17 times, according to some estimates
When organisms grow or reproduce, new cells are created, so new copies of the genome are needed. Usually a complete copy of the genome is created for each new cell. In sexual reproduction, however, specialized cells called gametes carry only half of each parent’s genome, and two gametes (one from each parent) fuse together to produce a complete genome for the child. (For humans and animals, the male gamete is called a sperm and the female gamete is called an ovum or egg.) This explains why Mendel’s first law of heredity holds for sexually-reproducing organisms.
In asexual reproduction, in contrast, a complete copy of the parent’s genome is passed to the offspring. For example, bacteria and most other single-celled organisms reproduce through a process called mitosis, in which the entire genome is duplicated. The cell then divides into two new cells, each with one copy of the genome. Mitosis also occurs when multicellular organisms like plants and animals produce new cells to grow, heal wounds, etc.
To create new copies of DNA, an assembly of incredibly complex molecules act as machines that separate the DNA into two strands, splitting it down the middle of each ladder rung as though unzipping a zipper. These molecular machines then reconstruct each half of the DNA into a new DNA molecule by matching each nucleobase with a partner: the nucleobase A always pairs with T, and C always pairs with G. This results in two perfect copies of the original genetic code—provided everything works as expected.
Occasionally, however, something goes wrong. Sometimes the molecular machinery malfunctions. More frequently, perhaps, the original DNA is damaged by radiation or toxic chemicals, so the copying mechanism has to repair the damage by inserting new base pairs, perhaps different from the old. These “mistakes” resulting from malfunctioning machinery or damaged DNA are called mutations.
If a mutation occurs in a single-celled organism, that change in its genetic code will be passed to its descendants as well. Similarly, if mutations occur in the gametes of sexually-reproducing organisms, their offspring may inherit those mutated genes. Randomly-occurring mutations are presumed to be the source of new genetic traits, which may be preserved by natural selection, allowing for greater diversity than would be possible with genetic recombination alone.
According to the theory of evolution, biological life began with just one single-celled organism. The processes of random mutation gradually diversified the traits of that organism’s descendants, over a period of billions of years, until eventually some of its descendants developed traits that allowed them to reproduce sexually; then genetic recombination also played a role in producing new variations. Meanwhile, natural selection eliminated many traits, thereby steering evolution toward the specific varieties of life we see today. But did all that really happen? We’ll consider evidence for the theory on the next page.