Summary
Highlights
Genes, DNA, and chromosomes are the fundamental instructions passed from parents that define an individual. These instructions are found within cells, which are the basic units of all living organisms.
All cells, regardless of their varied forms and functions, share basic parts. Each cell is enclosed by a membrane, containing cytoplasm, within which lies the nucleus. Inside the nucleus are chromosomes. Humans typically have 23 pairs of chromosomes (46 total), with 22 pairs being autosomes and the 23rd pair being sex chromosomes (XX for females, XY for males).
Chromosomes are long strings of DNA (deoxyribonucleic acid), which is shaped like a twisted ladder called a double helix. The 'steps' of this ladder are made from four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). A segment of DNA is called a gene. The body uses genes as a recipe to build proteins, with the order and sequence of bases determining the protein's size and shape, which in turn dictate its function.
Proteins form cells, cells form tissues, and tissues form organs like eyes and skin. Ultimately, genes determine not only what an organism is (e.g., a human, a cow, an apple) but also its specific physical characteristics, such as hair, skin, and eye color.