What is a Cell?
The simplest organisms are single-celled creatures such as bacteria and algae while more complex organisms, such as humans and trees, are comprised of many cells with specialized functions. Cells consist of membrane bound collections of DNA, proteins, sugars and cytosol –the jelly-like liquid inside the cell. The semi-porous cell membrane controls what enters and exits the cell. Scientists split cells into two main types: prokaryotic cells, such as those comprising bacteria and archae that do not have a well-defined nucleus, and eukaryotic cells that have a distinct nucleus surrounded by a nuclear membrane.
Small cells, like bacteria, don’t need to separate or centralize their DNA because it’s never far from the action! What they just isn’t that complex. But large cells such as mammals and plants have, many with specialized organelles — small specialized membranous parts of the cell — need to round up those long threads of DNA and keep them located near each other and centralized so they can quickly get messages to and from even the furthest edges of the cell membrane. Not only does keeping the DNA centralized make it more convenient it also protects it from foreign and domestic assault! When conditions are adverse to DNA survival such as when the cell’s pH is off, that extra protection by the nuclear membrane can slow down damage to your DNA. And when invasive attackers, such as viruses, strike, not only do they have to get past the cell membrane, they have to work their way to the very center of the cell and pass through the nuclear membrane too. The whole time the invaders are seeking out your DNA, natural defenses that exist only outside the nuclear membrane are attempting to destroy the invader’s DNA. That wouldn’t be possible if you didn’t have your own DNA within internal fortifications because that protective DNA destroying machinery could just as easily chew up your own genetic code!
And it’s not just in times of trouble that this double walled system is beneficial. Some chemical processes that the cell wants to pursue are harmful to your DNA. By isolating the nuclear DNA away from the cell’s manufacturing plants, it allows the cell to do things that might otherwise be self-destructive. Plus, remember that virus defending mechanism that eats foreign DNA? There is also a mechanism that breaks down our own DNA, on purpose. This mechanism recycles your RNA and mRNA, breaking it into it’s component parts so it can be used again and again. Without that kind of recycling the cell would need to come up with a lot more raw materials. It’s a very energy efficient process that helps give eukaryotes a big advantage. This same mechanism also serves as an efficient communication feedback loop. More mRNA has to be produced by the DNA than is broken down by other processes in the cell before the ‘notes’ get from the foreman sitting in the nucleus reading your DNA blueprints to the workers in the organelles saying to make more proteins — the cellular bricks.
Simplified Summary: The cell is the basic and smallest unit of life that consists of an outer porous membrane, cellular bodies and, in eukaryotes like us, an inner membrane that surrounds the nucleus where our basic building instructions, called nuclear DNA, resides. Double-stranded nuclear DNA consists of autosomal and the sex chromosomes (X and Y.) An important cellular body in the outer cell is the mitochondria (the bodies energy generator!) and it has it’s very own single-stranded DNA called mtDNA.
Factoid: Viruses are not classified as alive because they are DNA floating around without their own cells but bacteria are classified as alive because they are cells!
Image Credits: National Institute of General Medical Sciences