How Cells Work: The Basics

Abstract

Deoxyribonucleic acid—DNA—is perhaps the most famous molecule in the world. If you close your eyes, you can probably picture its iconic double-helix shape, like a rope ladder twisted around its central axis. The “ropes” are long strings of simple sugars known as the sugar-phosphate backbone, connected by “rungs” made from much smaller organic molecules called nucleotides.

What Life Is Made Of

DNA

Deoxyribonucleic acid—DNA—is perhaps the most famous molecule in the world. If you close your eyes, you can probably picture its iconic double-helix shape, like a rope ladder twisted around its central axis. The “ropes” are long strings of simple sugars known as the sugar-phosphate backbone, the “rungs” much smaller organic molecules called nucleotides.

Of course, it’s probably not a good idea to use a strand of DNA as an actual ladder. Issues of scale aside, the “rungs” aren’t quite as solid as they look at first glance. Each one is actually a pair of nucleotides, either adenine and thymine or guanine and cytosine (usually abbreviated as ATGC). And while nucleotides are firmly attached to their parent backbone by powerful phosphodiester bonds, the hydrogen bonds that connect nucleotide to nucleotide are comparatively weak. Apply too much pressure, and the hydrogen bonds start to break, allowing the two strands of DNA to separate into individual ribbons, known as the 3′ and 5′ strands (three prime and five prime).

The two strands are perfect mirror images of one another. The sugars that make up their backbones “point” in different directions, and they have complementary nucleotides. When the 3′ strand has an A, the 5′ strand will have a T, where the 5′ has a G, and the 3′ will have a C. Nucleotides can only pair with their specific partner, meaning that there’s only one way to connect two long strands of DNA. This specificity also means that you only need one strand to reconstruct the entire double-helix, a property that—as we’ll see later—is crucial to DNA’s ability to replicate itself.

And it’s the nucleotides that carry the actual genetic information. If a computer’s code is ultimately binary, DNA is quaternary. Instead of a sequence of 1s and 0s, the code of life is written as a sequence of adenine, thymine, guanine, and cytosine groups. A sequence of three nucleotides is called a codon; it’s these codons that the cell reads when the time comes to build proteins.

Cells also make use of DNA’s cousin, ribonucleic acid (RNA). Chemically, RNA is very similar to DNA—the sugars that make up their backbone are slightly different, and it uses a fifth nucleotide called uracil in place of thymine—but it doesn’t form a double-helix, making it less well-suited to long-term data storage. Instead, individual strands of RNA float freely around the cell, waiting to be used. It’s usually much more short-lived than DNA, although some simple organisms use it as their sole type of genetic material.

Proteins

Proteins are massive biomolecules with a staggering variety of roles in the cell. Some are purely structural; others catalyze chemical reactions. They receive signals, transport smaller molecules across membranes, break down sugars, replicate DNA, and perform a thousand other tasks. If you’re talking about a molecule doing something in a cell, it’s almost certainly going to be a protein.

Just as DNA is a long molecule built using only four different blocks—adenine, guanine, cytosine, and thymine—proteins are long molecules made from simpler building blocks called amino acids. There are a total of 20 different amino acids, each with different chemical properties. Once assembled, these properties quickly drive the protein to start folding into complicated three-dimensional forms through a complex system of interactions that we still don’t fully understand. Even small proteins are typically made up of hundreds or even thousands of amino acids; the largest are made of multiple amino acid chains twisting together into a single structure.

Twenty different amino acids appear in proteins, which are analogous to the four different nucleotides in DNA: every protein can be represented as a string over those 20 letters, just as every DNA molecule can be represented as a string over four letters. Recall that a DNA codon contains three base pairs: generally, each possible codon maps to one of the 20 amino acids.¹

Each amino acid is connected to its neighbors in the linear chain by strong covalent bonds that stem from two atoms “sharing” some of the same electrons. The folding process, however, depends on weaker (and longer-range) connections such as ionic bonds (where oppositely charged atoms attract one another magnetically), hydrogen bonds (where an entire hydrogen atom is shared between two molecules), and van der Waals forces, which are sort of like atomic static electricity. Some amino acids are hydrophobic and try to minimize their exposure to water; others are hydrophilic and happily co-exist with water molecules.

When you take all this into account, you wind up with unique, complicated molecules. The details of a protein’s shape determine what kind of molecules it can interact with, and what kind of chemical reactions it can take part in—and given how complicated those shapes can be, those interactions can be highly specific. Many proteins can only “stick” to a small number of other proteins—this type of interaction ultimately drives most cellular activity. Predicting the shape of a protein based solely on its primary sequence (the sequence of amino acids that defines it) as a complex computational problem, as is predicting if two proteins will interact.

Proteins don’t just interact with one another, either. Some have evolved to detect small signaling molecules, or interact with certain metabolic products. Others attach themselves to highly specific DNA sequences (e.g., “TTAGCTCTA”).

And just to make everything that much more confusing, protein shapes—and, thus, their functions—aren’t static. Most of the time, the very process of binding to its preferred target causes one (or both!) proteins to change shape and behave in new and different ways.

Polymers

A molecule that is composed of two identical subunits is a dimer; three identical subunits compose a trimer; and N identical subunits compose a polymer. An enzyme in which binding sites do not behave independently is an allosteric enzyme; in the example here, the enzyme exhibits cooperative binding.

Lipids and Membranes

Hydrophobic and hydrophilic reactions are also crucial for the third type of important biomolecule: phospholipids. “Lipid” is a broad category of large organic molecules that don’t mix well with water, such as fats and waxes. Phospholipids, however, have a hydrophilic “head,” meaning that one side wants to avoid water and the other wants to embrace it. Dump a bunch of phospholipids into water, and they will naturally assemble into a back-to-back formation with their “heads” pointed out and their “tails” pointed in, forming a very stable two-layer sheet called a phospholipid bilayer. These bilayers are the main component of all cellular membranes—both the plasma membrane that encloses the entire cell, and the individual membranes that encircle smaller subcellular compartments like the nucleus.

Plasma membranes also contain many proteins—pores, receptors, and several other varieties. Because the interior of a membrane is hydrophobic (i.e., doesn’t like water), similarly hydrophobic protein elements can “hide” inside and force the rest of the protein to remain adjacent to the membrane.

Types of Life

Prokaryotes

At the most basic level, all living organisms can be divided into two categories: eukaryotes and prokaryotes, depending on whether or not they keep their genetic material sectioned off from the rest of the cell.

Prokaryotes, at first glance, seem to be a pretty monotonous lot—bacteria, bacteria, and more bacteria. (To be fair there are also archaea, but they look so much like bacteria that it wasn’t until the 1970s that people noticed that they were genetically different.) Structurally speaking, prokaryotes are more or less just bags of proteins and genetic material, where the “bag” is a plasma membrane—which, in many prokaryotic cells, is reinforced by a layer of stiff starchy armor called a cell wall. Numerous proteins are anchored in the plasma membrane, and inside is a convoluted soup of proteins, sugars, nucleic acids, and other biomolecules known as the cytoplasm. Prokaryotic DNA takes the form of a simple loop, which floats somewhere inside the plasma membrane.

If you look a little closer, though, you’ll find an absolutely unbelievable amount of diversity among prokaryotes. These seemingly simple organisms can be found in every nook and cranny of the Earth, from hot springs to ice fields to deep-sea vents, and feed on everything from sunlight to elemental sulfur. In 2006, it was even discovered that some prokaryotes can sense magnetic fields.

Magnetotactic bacteria contain chains of tiny magnetic crystals enclosed in lipid bilayer membrane. The chain acts like a compass needle—it can be used to orient the bacteria relative to the earth’s magnetic field.

Perhaps the most famous and most-studied prokaryote is Escherichia coli (E. coli to its friends), a bacterium normally found in the human intestine.

Eukaryotes

Eukaryotes are probably more familiar, at least in the sense that most living things you’ve ever seen are eukaryotes. As a rule of thumb, anything you can see with your naked eye is a eukaryote. It’s certainly true that all multicelled organisms are eukaryotes, but the category also includes a number of single-celled organisms like yeast and amoebas. Despite seeming like they have more in common with their prokaryotic cousins than, say, an elephant, the structure and biochemistry of single-celled eukaryotes like yeast is far closer to that of the pachyderm than to a bacteria.

For a start, eukaryotic cells are big, much more so than prokaryotes. The famous E. coli bacterium, for instance, is about 2 μm long, but a typical mammalian cell is 10–30 μm long (Fig. 1). You might as well be comparing a hamster to a human, or a human to a 60-foot sperm whale.

Fig. 1

Relative sizes of various biological objects

Eukaryotes also have a much richer inner life. Every cell, regardless of whether or not it’s part of a more complex organism, has its own set of internal organs, conveniently enough known as organelles, each one enclosed in its own little plasma membrane. The nucleus keeps the DNA safe and segregated, mitochondria house all the machinery needed to turn sugars into cellular energy, the endoplasmic reticulum is a vast factory for protein synthesis, to name a few of the most prominent examples, but that’s just scratching the surface—check out Fig. 2 for a (marginally) more comprehensive overview.

Fig. 2

Internal organization of a eukaryotic animal cell

Not only is eukaryotic DNA tucked away in its own compartment, there’s also much, much more of it, stored in a much more complicated way. For a start, there’s more than one loop—instead, we have many separate sections of DNA, each one wound around thousands of proteins called histones. Histones are then packed together to form nucleosomes; nucleosomes are compressed into supercoils, and ultimately thousands of supercoils collect into a single large complex known as a chromosome.

All this wrapping is extraordinarily effective. A single human cell is microscopic, but if you took all of its DNA and stretched it out end-to-end, it would make a strand almost six feet long. A multicellular organism like you has billions of miles worth of DNA in their body.

As another way of emphasizing the relative complexity of eukaryotes, it’s very possible that a eukaryotic cell’s organelles were once completely independent prokaryote-like creatures. According to the theory of endosymbiosis, smaller prokaryotes once took shelter inside the membranes of their larger cousins, providing some sort of service—photosynthesis, say—in exchange for safety and a share of the cell’s resources. Over time, the symbiotic cells became smaller and more specialized, delegating more and more functions and shifting more and more DNA to the host cell until they were little more than internal organs.

The two biggest candidates for this role are mitochondria and chloroplasts, the organelle plants use to turn sunlight into energy. Even today, these organelles retain scraps of their original genomes, with DNA sequences that use completely different codes than those found in the cells’ nucleus.

Multicellular Life, Tissues, and Signaling

The human body—indeed, most any kind of multicellular body—is made up of a staggering variety of different cell types. Red blood cells carry oxygen, muscle cells contract and expand our muscles, nerve cells carry electrical signals across (comparatively) vast distances, and a thousand more. And yet, somehow, they all contain the same exact sequences of DNA.

How does one set of instructions give rise to such a staggering variety of end products? The answer is surprisingly simple—different cells use different parts of the genome. Some genes, such as those related to DNA transcription or metabolic activity, are active in all cells. Other, more specific sequences are only found in certain types of cells.

In addition to differentiating into different types, cells (even bacterial cells) communicate among each other, a process called signaling.

Viruses

In studying biology, prokaryotes are a good start because they are the simplest living things. Eukaryotes, in particular multicellular ones, are relevant because, well, that’s what we are—and as far as we know, they are the most complex living things. But we’re not done surveying types of life yet, because there are also the entities that are only sort of alive, such as viruses.

Viruses don’t have plasma membranes, or cytoplasm, or metabolic processes, or any of the other machinery normally involved in keeping a cell alive. Instead, they infect more complex organisms and hijack their processes—just as an email virus uses existing programs on an infected machine to propagate.

A typical virus, such as the lambda phage, is made up of only two components—a protein coat, and a strand of DNA or RNA with instructions for how to make said coat. In spite of this simplicity, the lambda phage has a rather interesting life cycle.

When it encounters the right type of host cell, the coat binds to the cell’s plasma membrane and injects its payload of genetic material directly into the cytoplasm. Once it’s there, the host cell has no way of telling the difference between its own nucleic acids and those of the invader. The organelles, whose job it is to make proteins, simply “read” the new code and build the new proteins, using the cell’s own resources to do it.

Just to make matters more confusing for the poor cell, the first thing many of these illicitly produced proteins do is splice the virus’s DNA into one of the host’s chromosomes—in the case of the lambda phage, the protein that does this is called the lambda integrase. Once the integrase has done its dirty work, the cell continues to grow and reproduce, and as it does so, it passes the viral sequences on to its daughters. Eventually, some environmental signal tells the cells to start copying viral DNA and producing viral proteins as fast as they can. Even as its own processes wither away, the cell continues making new viruses, until it finally bursts and releases thousands upon thousands of viruses to seek new targets.

If we think of DNA as a cell’s source code, then a virus like the lambda phage is a sort of self-modifying program. Not only does it hijack the cell’s machinery to make copies of itself, but it permanently changes the original code. Often a phage’s alterations will eventually be corrected by the cell or its descendants, but sometimes non-executable fragments of phage “code” stay in the genome. After millions of years of evolution, our genome is littered with the remains of viral insertions, sequences known as transposons.

Plasmids

And yet, viruses still aren’t the simplest possible things that can replicate themselves, however. If a virus is nothing but a bit of DNA or RNA in a protein capsule, a plasmid ditches its shell in favor of existence as a free-floating loop of nucleic acid. Once scooped up by a larger cell, the plasmid takes advantage of the same vulnerabilities as the virus and uses the cell’s own machinery to make new copies of itself.

Plasmids are particularly common in prokaryotes, where they serve as a way for bacteria to swap bits of genetic information back and forth. A common—and troublesome—example of this is the transmission of antibiotic resistances from one species of bacteria to another. Experimental biologists can also exploit the process to quickly introduce genes they are interested in.

Prions

Plasmids may be the simplest things that qualify as “sort of alive,” but there is another type of ultra-simple self-replicating biomolecule out there—prions, misfolded proteins most famous as the cause of “mad cow disease,²” along with a number of similar neurodegenerative diseases such as kuru and Creutzfeldt-Jakob disease.

A normal, “living” copy of something called the major prion protein (PrP) is an important player in the nervous and immune systems. Its exact roles are still unknown, but it’s been connected to circadian rhythm, long-term memory, neural plasticity (the brain’s ability to change and adapt), and the activation of various immune cells. Whatever it does is clearly vital, though—prion diseases inevitably lead to death.

It is possible, however, for healthy PrP (referred to as PrP^C, for cellular) to mutate, dramatically changing its three-dimensional structure. The new, misfolded variants (or PrP^Sc, after one of the earliest known prion diseases, scrapie) proceed to attach themselves to healthy PrP^C molecules and somehow twist them into the same misfolded shape. PrP^C becomes PrP^Sc, and both copies are released to infect more proteins.

So basically, prions are like zombies—they used to be “living,” healthy proteins, but after mutation, they begin to “bite” other healthy proteins and turn them into more zombies…until the brain is nothing but a zombie-infested wasteland.

Cellular Activity

The Central Dogma

Regardless of whether an organism is prokaryotic or eukaryotic, at the cellular level everything boils down to the same process, known as the central dogma of biology (Fig. 3).

Fig. 3

The “central dogma” of biology

Long, helical molecules of DNA form a sort of cellular blueprint, containing all the plans and instructions necessary for a cell to function. Individual sections, or genes, are then transcribed to smaller molecules of messenger RNA. These, in turn, travel to giant molecular factories known as ribosomes, where they will be read and translated into the proteins that make up most of a cell’s machinery. At that point, the gene is considered to have been expressed.

Or, in computer terms, DNA is a stored program, which is “executed” by transcription to RNA and expression as a protein.

Types of RNA

Messenger RNA, ribosomal RNA, and transfer RNA are abbreviated as mRNA, rRNA, and tRNA, respectively. As time goes by, more and more internal uses are discovered for RNA molecules. (Some of these are discussed below, such as CRISPR and gene silencing.) A gene product is a generic term for a molecule (RNA or protein) that is coded for by a gene.

This basic process, the progression from DNA to RNA to protein, is carried out by all living organisms. In fact, many of the genes involved in transcription and translation are identical across wildly different species, or at least highly similar.

This is, of course, a dramatic oversimplification. The more complicated eukaryotic cells add in an entirely new step of messenger RNA editing—splicing—in which sequences known as introns are removed and discarded. (Not always the same introns, though—there can be multiple ways to splice the mRNA for a gene, so a single gene can produce many different proteins. Just in case eukaryotes weren’t complicated enough.)

Additionally, some RNA molecules perform useful functions without ever being translated into proteins—for example, key parts of ribosomes are made of pure “ribosomal RNA”, rather than proteins or lipids.

Even the basic structure of DNA can influence gene expression. Different cells pack the genome in different ways, exposing some segments and hiding others away in densely packed nucleosomes where they cannot be accessed by the cell’s transcription machinery.

Cellular Signaling

It would be impossible for multicellular organisms to exist without some way of coordinating the activity of the cells they are made of. Cells are too small and too simple to use things like words and sounds, so instead they “talk” through a bewildering variety of chemical signals (typically some variety of specialized protein).

A ligand is a molecule that binds to a specific place on another molecule. The shape of a protein is called its conformation.

Once released into the environment, signals float around until they encounter matching receptor sites on proteins sticking out of other cells’ plasma membranes. The signal binds to the receptor, triggering some sort of chemical change that the cell “understands”—typically, this means releasing more signals, which bind to more receptors, which release more signals, and on and on in long cascades known as pathways.

There are four well-studied categories of receptor:

• Enzyme-linked receptors stretch across the plasma membrane. When a signal—the receptor protein’s ligand—binds to the receptor site outside the cell, it toggles the enzyme inside the cell on or off, directly changing what sorts of chemical reactions occur inside the cell.

• G-protein coupled receptors (GPCRs) are complexes of multiple proteins, both inside and outside the plasma membrane. When the exterior receptor is activated, the protein changes its conformation (or shape), releasing a smaller G protein inside the membrane to float off and activate other processes. (Although it’s worth noting that the term “G protein” is something of a misnomer—like the receptor proteins, a single G protein is typically made of several smaller proteins bound together.)

• Ion channels are essentially doorways. They form pores in the plasma membrane; when activated, the pores open to allow the passage of molecules that wouldn’t normally be able to cross the barrier. Some are activated by signal molecules of one sort or another, but other varieties exist, such as the voltage-gated ion channels found in nerve cells that respond to changes in the cell’s electrical charge.

• Nuclear receptors respond to signals by binding directly to strands of DNA or RNA, changing how that particular gene is expressed.

Receptors—and signaling pathways in general—tend to be of particular interest to biologists working on drug development. It’s hard to directly affect the inner workings of a cell: after all, they’ve had billions of years of evolution to teach them how to seal off their interiors to all but a few specific compounds. If you want to affect a cellular process with a new drug, it’s much easier to target receptors—the cell’s existing API, so to speak.

Cell Division

Perhaps the most important property separating living organisms from nonliving ones is their ability to reproduce. At the cellular level, the process is called division—the separation of a single cell into two identical daughter cells, each with their own fresh copy of their parent’s genome.

In prokaryotes, the process is simple. First, the DNA “unzips,” separating into its two component strands. Proteins called DNA polymerases travel along each strand and assemble a new matching strand, nucleotide by nucleotide. When they’re done, each loop attaches to a different point on the plasma membrane, which then pinches inward until it splits off into two separate bubbles. This final division is known as cytokinesis.

In eukaryotes, however, the presence of multiple chromosomes complicates things. The cell not only has to copy its DNA, it has to make sure that each daughter cell receives one copy of every chromosome—no more, no less. The process is controlled by a set of proteins known as cyclins and cyclin-dependent kinases (CDKs), and can be divided into the four distinct phases of the cell cycle:

A kinase is a protein that modifies another protein by adding a phosphate group. This process is called phosphorylation.

• During the G1 phase—also known as the growth phase—the cell prepares for division by growing larger and making extra copies of organelles like mitochondria. At the end of the phase, the cell can either return to its normal state (G0 phase), or proceed to the next step of the cell cycle.

• During the S phase, the chromosomes are duplicated. The two copies, however, remain attached at a point called the centromere.

• During the G2 phase, the cell assembles a temporary protein scaffold, known as the mitotic spindle. At the same time, tumor-suppressant genes such as p53 check for damaged DNA. If they find any, they’ll either repair it or cause the cell to self-destruct; if not, the cell begins to physically divide.

• During the M, or mitosis phase, the cell separates the two copies of its DNA, pulling one set of chromosomes to each side of the cell before undergoing cytokinesis and finally splitting into two daughter cells.

Mitosis doesn’t take long, but it’s an impressively complicated process with its own set of distinct, named steps. They are, in sequence, as follows:

• Prophase—the DNA packs itself into its most condensed form, creating the familiar “x” shape, and the nucleus dissolves.

• Prometaphase—the nucleus breaks apart, allowing the chromosomes to move separately.

• Metaphase—the chromosomes form a straight line across the center of the cell and attach themselves to the mitotic spindle assembled during the G2 phase.

• Anaphase—the mitotic spindle contracts, pulling the chromosomes apart at the centromere and dragging one copy to each side of the cell.

• Telophase—a pair of new nuclei forms around each set of chromosomes.

As with most things in life, everything gets much more complicated when sex is involved. Pretty much every organism capable of sexual reproduction keeps multiple copies of its DNA around in its normal, or somatic, cells. Take us, for example—we have two complete sets of chromosomes, one inherited from each parent, making us diploid organisms. That’s only the bare minimum, though, as organisms have been discovered with three, four, five, or more sets. (The black mulberry, for some reason, has a whopping 44 copies of each chromosome.)

Prokaryotic Sex

Prokaryotes might not enjoy the benefits—and complexities—of sexual reproduction and meiosis, but that doesn’t mean they don’t have ways of trading genes with each other. Remember when we mentioned plasmids earlier? One important subtype of those is fertility, f-plasmids, which contain all the genes necessary for a process called conjugation.

The prokaryote with the f-plasmid—the “male,” if you will—constructs a sort of protein grappling hook called a sex pilus, which it uses to grab onto another, “female” cell. The two prokaryotes then form a conjugate bridge and freely exchange plasmids—including copies of the f-plasmid.

And if you think sex was complicated for humans, be glad you’re not a bacteria. Conjugation typically involves groups of up to ten separate bacteria, with the “females” becoming “male” afterward, thanks to their new copies of the f-plasmid.

But wait. Fertilization involves two cells fusing into one. If both sperm and egg were diploid, the baby would end up with four complete copies of its DNA. Except that doesn’t happen—a fertilized egg is diploid, with the usual two copies of DNA. What happens to the extra?

It turns out that our bodies get rid of the excess long before we actually get to the fun parts. Our egg and sperm cells, our gametes, have only a single set of DNA each. These haploid cells, as they’re known, are produced by a variant of the normal cell division process called meiosis that results in two sets of daughter cells. The full process is broken down in Fig. 4.

Fig. 4

How meiosis produces haploid cells. (a) A diploid cell, with one pair of homologous chromosomes. (b) After DNA replication the cell has a two pairs of sister chromatids. (c) The homologous chromatids pair to form a bivalent containing four chromatids. (d) DNA fragments recombine. (e) Bivalents are separated in preparation for division I. (f) The cell divides. Each daughter has two copies of a single parent’s chromosome. (g) The sister chromatids in each daughter cell separate from each other in preparation for division II. (h) The daughter cells divide, producing four haploid cells, each of which contains a single representative of each chromosome pair from the original diploid cell. (i) In sexual reproduction, two haploids fuse to form a diploid cell with two homologous copies of each chromosome—one from each parent. Shown here is a cell formed from one of the daughter cells in (h), and a second haploid cell from another parent

But there is, as usual, another layer of complexity. Unlike the cell in Fig. 4, we have more than one chromosome pair. (Twenty-three, to be precise, at least in humans—chromosome numbers vary wildly between different species.)

Consider a diploid cell with N chromosome pairs. For convenience’s sake, we’ll label each chromosome as either A_N or B_N, with N representing the chromosome number and A and B representing the parent it was originally inherited from. When the cell undergoes meiosis, two of the four daughter cells will receive A1, and the other two will receive B1. But just because a particular cell happened to get A1 doesn’t mean it’ll get A2, A3, and all the rest of the set—chromosome pairs are distributed randomly. The daughter cells will get either A1 or B1, either A2 or B2, and so on, all the way down the line. That adds up to a whopping 2^N potential sets of DNA, meaning each gamete is almost guaranteed to be different.

And that matters, because there’s nothing to say that A1 and B1 are identical sequences. Populations often have multiple varieties, or alleles, of many of their genes. (Among many other things, that’s why we have different eye and hair colors.) Typically, one allele will be dominant over the other—the cell will make its version of the protein, while the other, recessive allele is effectively hidden.

And just to make things worse, meiosis also often involves some amount of genetic recombination, where individual genes are swapped between chromosome pairs. In humans, such crossover events typically happen two to three times per chromosome.

All of this variety and randomization is a big part of the reason why diploid species can be so genetically diverse, and why evolution can occur so much faster than in monoploids—there are far, far more potential genetic outcomes of any given mating than there are of a single cell division.