DNA is present in almost all living cells in all living things. Its main task is to act as an archive of information about the physical nature of the organism. Cells need this information to take on new functions or reproduce. With the exception of a few viruses, no living thing could be made without DNA.
In the human body, DNA controls and maintains every cell - nothing gets done without DNA's permission! DNA uses a 'messenger', the molecule RNA or ribonucleic acid, to apply this 'coded information' and manage day-to-day cell activities.
Every DNA molecule contains all of the instructions for making a complete living thing. In the human body, cells contain the DNA that was in the original fertilised egg. This complete blueprint is known as the genome, and DNA is sometimes referred to as the 'genetic code'.
Not all cells contain a complete DNA copy, although most do.
Cells may contain all of the DNA information, but only need the bits of information that are specific to their function. The cells in eyes need to know what colour the eyes should be, but don't need to know what shape the nose is, how to make hair or digest food. Cells in blood need to know how to make haemoglobin, but don't need to know what colour skin is or how long the legs are. The small part of the genome needed for a specific function or characteristic is known as the gene.
So, although very similar in structure to DNA, RNA has a more specialised function: it carries only the information that is needed during the lifetime of a single cell. In this respect, DNA could be thought of as a book, and RNA as a photocopied page that describes a specific piece of information about building, management or maintenance functions.
This helps to explain why humans continue to look more or less the same, even as cells in the body wear out and are replaced. DNA remains stable in humans throughout their lives. How people age and die remains something of a mystery.