Prokaryotic and eukaryotic cells both include nuclear bodies that contain chromosomes. Chromosomes are strands of genetic material composed of deoxyribonucleic acid, or DNA. Eukaryotes have a true nucleus, which comprises one or more pairs of linear chromosomes, and is separated from the rest of the cell by a nuclear membrane. The nucleus also contains a nucleolus, which is a structure that assembles the components necessary for protein synthesis. By contrast, the prokaryotic nuclear body, or nucleoid, lacks a nuclear membrane. It contains one circular chromosome composed of DNA, and does not have a nucleolus.
Prokaryotic and eukaryotic cells are surrounded by a plasma membrane that keeps the cell intact, and regulates the passage of substances into and out of the cell. All prokaryotes have a rigid cell wall that surrounds the plasma membrane. Some eukaryotes -- such as plants, algae and fungi -- also have cell walls. Others, such as animal and protist cells, do not. In addition to a nucleus, eukaryotic cells contain microscopic organelles, including ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria and different types of vesicles, each of which performs a specialized function. Some eukaryotes also have chloroplasts, in which photosynthesis occurs. With the exception of ribosomes, prokaryotes lack organelles. Because of their simplicity, prokaryotes are much smaller than eukaryotes. This allows nutrients to rapidly reach any part of the cell's interior.
All cells replicate through a process called cell division. Prokaryotes divide through binary fission, in which one cell divides into two nearly equal parts. Eukaryotic cell division requires mitosis, which is a complex process involving duplication of the chromosomes within the nucleus. Eukaryotic sex cells, such as eggs and sperm, divide by a process similar to mitosis, called meiosis. Prokaryotic cells may or may not require oxygen to perform life-sustaining functions, while eukaryotic cells need oxygen. Cells of both types may move using hair-like projections called cilia or flagella. Some eukaryotes have cilia or flagella. Some prokaryotes have flagella, but all of them lack cilia.
Scientists classify living organisms by group, based on shared characteristics. One such classification is the kingdom. The six kingdoms are Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animalia. Archaebacteria and eubacteria are prokaryotes. Archaebacteria are unicellular organisms commonly found in harsh environments. Eubacteria thrive everywhere, and are the type of bacteria with which most people are familiar. Protists (such as protozoa and algae), fungi, plants and animals are eukaryotes.