An organism is formed based from an architectural blueprint called DNA. In eukaryotes including human cell, the nucleus holds the DNA. Humans contain 46 chromosomes and for each chromosome, DNA is organized into a strand. Each shorter segment of the DNA strand is called genes. This is analogous to organizing a long paragraph into a different clear sentences. Each gene is an instruction to make a protein. Analogically speaking, DNA in eukaryotic cells are like the apps (application software) in mobile phone. If you want to know the current date, you would only access the calendar app and not run the other programs. The same idea applies in eukaryotic DNA. A cell does not express all the gene stored in the nucleus and only express gene that are needed depending on the type of cell. For example, each cell in the human body regardless of where it is located contains the gene to make the hair, skin, teeth, liver, and ovary, but these genes are turned off in the stomach cells. If genes for making the teeth were turned on in the stomach cells, you will have teeth forming in the stomach.
Blankenship-Williams, L. (2015). What You Really Need To Know Before Anatomy, Physiology, and Microbiology. Carbohydrates and Nucleic Acids. Accessed November 18, 2019.