Within the cell lies the nucleus. Inside the nucleus lie the chromosomes that carry the genetic instructions. They are a set of instructions that specify how the cell should develop and operate. It would take many millions of these separate commands to write down all the instructions for even one cell, so a special code is used, based on very long chain molecules of DNA - deoxyribonucleic acid.
A DNA molecule is like a double-helix necklace of two entwined strands. On each strand are strung four different kinds of ‘beads’, which are called bases. It is the sequence of these bases that makes up the genetic code. The DNA molecules are organised into units of libraries called chromosomes.
For the information in the DNA to be used by the cell, portions of it first have to be copied. Short single stranded copies of one of the two DNA stands are made. These copies are called messenger RNA, and are transported out of the nucleus, through special holes or pores, into cells where they make their way to the ribosomes. Each strand of messenger RNA contains the information to manufacture a particular protein.
A city needs a central place from where things are being run or controlled, often the city or town hall. This is where the plans are stored for city developments and sometimes where the central library facility and the central registers are housed. Much of the city’s municipal maintenance work is also controlled from here. Clearly our cities don’t run along quite the same lines as a cell, but if they did, each factory manager would collect the plans for each day's work from the city hall each day. Every minute there would be messages coming from the city walls to tell the factories what to make next or whether to shut down certain production lines. The city hall would also govern control what the gatekeepers at the city walls allow in and out of the city.