A cell is an active, bustling place. Enzymes are performing chemical reactions. Macromolecules--DNA, RNA, proteins, lipids, and carbohydrates--are being built and broken down. Molecular motors are carrying organelles around the cell. DNA damage is being repaired. And the cell membrane is selectively letting molecules and ions in and out. Everything is moving. Countless processes are unfolding at once. All of this requires energy. A lot of it. This, then, raises a fundamental

Two human genomes--together totaling about 6 billion base pairs--are packed into the nucleus of every cell in your body. Every cell contains the full genome, even though different cells use different genes depending on their function. That creates a problem. If we could stretch out the DNA in a single human cell--laying all 6 billion base pairs end to end--it would extend nearly six feet. Yet the nucleus that holds it is only about 5 micrometers in diameter, roughly 10,000 ti

Once an mRNA has been processed, it's packaged with proteins and transported through nuclear pores--large channels in the nuclear membrane--into the cytoplasm, where it can be translated into protein. The central player in translation is a remarkable molecular machine called the ribosome. Ribosomes are made out of proteins and rRNAs (the "r" stands for "ribosomal"). I mentioned a few chapters ago that RNA can sometimes possess enzymatic activity. The translation process featu

9 Transcription: DNA polymerase (light blue) opens the double helix to make the template strand (dark gray) accessible. It then successively adds RNA nucleotides complementary to the DNA template to build the final mRNA (red). A Poly A tail will be added after the mRNA is released by the RNA polymerase (not shown). In 1970, an enzyme called reverse transcriptase was discovered. It used an RNA template to synthesize DNA, enabling information flow from right to left. This is an

How does a string of letters--really, a string of molecules--actually do anything? In this chapter, we shift our focus to function: how DNA works. The Genetic Code If we were to walk down a stretch of double stranded DNA we could announce the letters on one or the other of the strands as we passed: "ATGTCGGATAGATGA", for example. A code is contained in these 15 letters. Every protein in your body--from enzymes to muscle fibers--starts as a sequence like this. A primary purpos

At first glance, DNA doesn’t look like much--a long, repetitive chain of simple parts. And yet, this structure is capable of storing information and copying itself with extraordinary reliability. Structure strongly shapes function--and DNA is one of the clearest examples of that principle in action. When the structure of DNA was discerned by James Watson and Francis Crick (with key x-ray crystallographic data supplied by Rosalind Franklin), they understood immediately how it

In the last chapter, we described a system that seems almost paradoxical: a cell filled with molecules moving randomly--colliding, binding, and separating--and yet somehow producing precise, coordinated behaviors. The question is: what makes that possible? What are the molecules that take these countless random interactions and turn them into consistent, functional outcomes? The answer is not a central controller. The answer is proteins. A typical human cell contains tens of

If we were to shrink down by a factor of about 100,000 and enter a human cell, the first thing we'd notice wouldn't be structures. It would be motion. Not slow, orderly motion--but incessant, chaotic movement. In the cytosol, the water-based liquid in the cell, countless molecules crowded together would be crashing into each other at high speeds. No guidance system is telling these molecules where to go. High speeds plus dense crowding guarantee an extraordinarily large numbe

Cells are often compared to factories. At first glance, the analogy seems almost perfect. But it breaks down in a way that reveals something essential about how life works. We'll use it to orient ourselves. Then we'll critique it. Factories bring in raw materials and use workers, tools, and energy to produce product. To do that, they need power, an assembly line, instructions, storage facilities, and systems for waste disposal. At first glance, the parallels are easy to dra

To understand life, we need to start with the simplest living thing: a cell. But that immediately raises a deeper question--what does it mean for something to be alive at all? What counts as alive? Animal and plant cells are the smallest things that we unreservedly consider to be alive--that is, that display the hallmark characteristics of life. Arguments can be made for and against viruses. We'll skip over that philosophical debate. Cells have the ability to process food to

It's easy to underestimate how incredible a single cell in your body really is. A typical human cell is vanishingly small, yet contains an enormous number of molecules packed into a tiny space. The molecules aren't static. They move constantly and randomly, jostling and colliding, driven by thermal energy. Nobody is running the show. There are only molecules colliding and interacting with each other through local, physical forces. From this activity, life emerges. Cells build