The making of a living organism

The structure and composition of living organisms varies greatly - from single celled bacteria to complex multi-cellular organisms with differentiated cell types and interconnected organ systems. Regardless of the complexity, every living entity contains a blueprint for its construction in the form of a double-helical chain of molecules called deoxyribonucleic acid (DNA) .

In all living organisms, this DNA is housed inside the cell, a membrane-enclosed unit that contains the machinery and supplies for the life functions or metabolic processes of the cell. Prokaryotes (meaning before nucleus) house their DNA in a loosely defined region of the cell called a nucleoid . Eukaryotes (meaning true nucleus) sequester the DNA into a separate, membrane-bound compartment called a nucleus .

DNA takes on different forms depending on the organism. In bacteria, for example, the DNA forms a continuous loop referred to as a circular chromosome. In more complex organisms the DNA is arranged linearly, but is tightly wound around a complex of proteins that form a sort of scaffold for higher orders of chromosomal structure. Chromosome architecture is unique to every species and plays a significant role in the reproductive isolation of species.

Simple unicellular organisms consist of a single cell that functions independently within its environment. In multi-cellular beings, different cell types might make up distinct tissues and organs, and must coordinate their functions to create a viable organism.