Flex your arm. It feels smooth, controlled, almost effortless. But beneath that motion, billions of molecular events are unfolding--tiny proteins pulling, releasing, and resetting in rapid succession. Let's zoom in. Meet myosin, a protein complex in skeletal muscle cells. In this chapter, we'll see how ATP enables molecular-scale myosin movements that cause skeletal muscle contractions. A muscle contraction is not a single event--it’s the result of billions of coordinated mo

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

If DNA is locked away in the nucleus, how does its information get out and turned into something the cell can use--namely, a protein? Francis Crick tackled this question and, in 1958, came up with an answer and a simple drawing to go along with it. He called his idea molecular biology's central dogma. Central dogma Crick originally presented central dogma as a law-like generalization or regularity. At it's core, it's just a diagram of how information flow in cells. But rememb

How does a string of letters--really, a string of molecules--actually do anything? In this chapter, we shift our focus from structure to function: how DNA works. The genetic code If we were to walk down a stretch of DNA we could announce the letters on the strand as we passed: "ATGTCGGATAGATGA", for example. A code is contained in these 15 letters. Every protein in your body--from enzymes to structural proteins--starts as a DNA sequence like this, but longer. A primary purpos

At first glance, DNA doesn’t look like much--a long, repetitive chain made of just four kinds of parts. And yet, this simple structure is capable of both storing an enormous amount of information and replicating itself with incredible accuracy. Structure shapes function, and DNA is a clear example. When the structure of DNA was discovered by James Watson and Francis Crick and with key x-ray crystallographic data supplied by Rosalind Franklin, they understood immediately how i

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 proteins. A typical human cell contains tens of millions of protein molecules. Some cut

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 at its most basic level, we need to start with the simplest living thing: a cell. Every living thing, be it plant, animal, insect, or you, is made of cells that are also living things. But that raises a deeper question: What does it mean for something to be alive? What counts as alive? Animal and plant cells and unicellular organisms like bacteria are the simplest things that we unreservedly consider to be alive--that is, that display the hallmark character

A single human cell is easy to overlook. It’s so small that roughly 1,000 could fit on the period at the end of this sentence. Yet, inside that space exists a world of astonishing activity and complexity. That cell contains billions of protein molecules along with DNA, RNA, sugars, fats, countless smaller compounds and ions. These molecules aren't static. They move randomly at very high speeds--that is, very high relative to the size of the cell. They move because they are