The living cell is a self-organizing, self-replicating, environmentally responsive machine of staggering complexity. The instructions for this complexity are contained within the cell's genetic code, but how this information is accessed, read and interpreted is influenced by development and differentiation.The articles that follow this introduction describe our current understanding of what goes on in every one of the mostly invisibly small 10 trillion cells of our bodies.
To divide, a cell needs to create a second set of its genetic material to donate to the daughter cell. The review by Bloom and Joglekar examines how duplicated chromosomes are divided accurately between mother and daughter cells and packaged by proteins, mainly histones, in the nucleus. This packaging regulates gene expression, and Ho and Crabtree discuss how this occurs during development and differentiation. In eukaryotes, protein-coding genes are transcribed into precursor messenger RNAs that contain non-coding regions. As described by Nilsen and Graveley, these non-coding regions must be removed before the RNA can be translated into protein, in a process known as alternative splicing.
The shape, movement and positioning of organelles within the cell depend on dynamic, polymeric cytoskeletal proteins. Fletcher and Mullins analyze the principles that allow these proteins to produce and respond to mechanical forces, as well as to establish order in the cytoplasm over long distances. In a process called endocytosis, portions of the cell membrane are internalized into the cytoplasm. This enables the cell to capture material from the extracellular environment and to respond to cues detected by externally oriented receptors. Scita and Di Fiore discuss the integral role of the endocytic system in the cell's signaling network.
Now I know I've written about this before, but I keep coming back to it: The unceasing hive of activity that is our body, every cell like a hugely complex petrochemical factory on full blast, none of which requires the slightest conscious attention from me. Breath. Heartbeat. Digestion. Scratching an itch. The healing of wounds. The maintenance of memories. Dreams. It all happens by autopilot. This huge colony of multiplying, replenishing, differentiated cells which is me. And in it and on it flickering that thing which is self-awareness, the thing I think of as the "real" me, which exists only by the grace of all this other unconscious biochemical activity.
Yes, it all happens without our thinking about it. But it's worth thinking about. It's easy enough to see why our ancestors imagined an immaterial "me" that came into the world full-blown at birth and goes on living after death. They had no clue of the frenetic machinery of life, the buzz of self-sustenance that goes on in every cell of our bodies, the tiny furnaces of the lungs, the throbbing pistons of the heart, the ever-ready mousetraps of the immune system, the recycling plant of the gut. And there, at the top of the spine -- the flame on the wick -- a scintillating ball of neurons, firing, recharging, firing, like the constantly regenerative pixels of a television screen that give the illusion of continuity.
And when it all stops, the self goes off like a light.