Interactive · 2 questions
One of the defining characteristics of life is the ability to reproduce. But what do we really mean by that? Stop for a minute and try to define carefully what it means for an entity to reproduce.
You probably said something like "make an identical copy of itself".
But is that quite right?
Living things don't generally reproduce themselves perfectly. In fact, for evolution to occur, there needs to be a constant source of imperfections in the reproduction process. More on that here. However, by and large, the new copy (we'll call it the child) needs to pretty much resemble the original living creature (we'll call it the parent). What's more, the child needs to be able to continue making more children approximately as similar to it as it was to the parent.
To reproduce in this manner, a living thing must store some basic set of information that defines itself and that it is capable of passing down to its progeny. But how would one store this information?
We're used to thinking of information stored linearly, as marks on a piece of paper or ones and zeros on a computer. It turns out, all living things on earth, also store information in this way. They use linear polymers -- long molecules composed of chains of small subunits.
Think back to your grade school science classes. What do we call these information storing molecules found in all living things?
The general term for the particular long polymers used by all the living creatures we know of is "nucleic acids". There are two types, called DNA and RNA. Both are composed of 4 types of subunits joined together in long chains. These polymers spell out the code that defines a human, a slug, or a smallpox virus. They define your eye color and skin color and whether you have hemophilia and whether you like cilantro. And they are structured in such a way as to enable them to be copied via chemical reactions.
The rest is all details -- how to copy these molecules, physically separate the copies, protect them from damage in a hostile environment, maintain the machinery for upkeeping a supply of raw materials to continue making more copies.
Ultimately, you, your dog, and your dog's flea are just mind-bogglingly complicated mechanisms for a handful of long linear molecules to keep making more copies of themselves.