How Cells Work: Prokaryotes and Eukaryotes
Knowing how cells work is critical in the genetics field. All living things consist of one or both of two cell types: prokaryotes and eukaryotes. The basic biologies of prokaryotes and eukaryotes are similar but not identical, so understanding the differences and similarities between them is important.
The process of passing genetic material from one generation to the next depends completely on how cells grow and divide. To reproduce, a simple organism such as bacteria or yeast simply copies its DNA (through a process called replication) and splits in two. But organisms that reproduce sexually go through a complicated dance that includes mixing and matching strands of DNA (a process called recombination) and then reducing the amount of DNA in special sex cells to arrive at completely new genetic combinations for their offspring.
There are two basic kinds of organisms — ones with a nucleus and those without a nucleus (a compartment filled with DNA surrounded by a membrane called a nuclear envelope):
- Prokaryotes: Organisms whose cells lack a nucleus and therefore have DNA floating loosely in the liquid center of the cell. Prokaryotes divide, and thus reproduce, by simple mitosis.
- Eukaryotes: Organisms that have a well-defined nucleus to house and protect the DNA. Eukaryotes divide by meiosis for sexual reproduction.
Prokaryotes: Cells without a nucleus
Organisms composed of cells without nuclei are classified as prokaryotes, which means "before nucleus." Prokaryotes are the most common forms of life on earth. You are, at this very moment, covered in and inhabited by millions of prokaryotic cells: bacteria. Much of your life and your body's processes depend on these arrangements; for example, the digestion going on in your intestines is partially powered by bacteria that break down the food you eat. Most of the bacteria in your body are completely harmless to you. Other species of bacteria, however, can be vicious and deadly, causing rapidly transmitted diseases such as cholera.
All bacteria, regardless of temperament, are simple, one-celled prokaryotic organisms. None have cell nuclei, and all are small cells with relatively small amounts of DNA.
The exterior of a prokaryotic cell is encapsulated by a cell wall that serves as the bacteria's only protection from the outside world. A plasma membrane (membranes are thin sheets or layers) regulates the exchange of nutrients, water, and gases that nourish the bacterial cell. DNA, usually in the form of a single hoop-shaped piece (segments of DNA like this one are called chromosomes), floats around inside the cell. The liquid interior of the cell is called the cytoplasm. The cytoplasm provides a cushiony, watery home for the DNA and other cell machinery that carries out the business of living. Prokaryotes divide, and thus reproduce, by simple mitosis.
Eukaryotes: Cells with a nucleus
Organisms that have cells with nuclei are classified as eukaryotes (meaning "true nucleus"). Eukaryotes range in complexity from simple one-celled animals and plants all the way to complex multicellular organisms like you. Eukaryotic cells are fairly complicated and have numerous parts to keep track of. Like prokaryotes, eukaryotic cells are held together by a plasma membrane, and sometimes a cell wall surrounds the membrane (plants, for example have cell walls). But that's where the similarities end.
The most important feature of the eukaryotic cell is the nucleus — the membrane-surrounded compartment that houses the DNA that's divided into one or more chromosomes. The nucleus protects the DNA from damage during day-to-day living. Eukaryotic chromosomes are usually long, string-like segments of DNA instead of the hoop-shaped ones found in prokaryotes. Another hallmark of eukaryotes is the way the DNA is packaged: Eukaryotes usually have much larger amounts of DNA than prokaryotes, so to fit all that DNA into the tiny cell nucleus, it must be tightly wound around special proteins.
Unlike prokaryotes, eukaryotes have all sorts of cell parts, called organelles, that help carry out the business of living. The organelles are found floating around in the watery cytoplasm outside the nucleus.
Two of the most important organelles are the following:
- Mitochondria: The powerhouses of the eukaryotic cell, mitochondria pump out energy by converting glucose to ATP (adenosine triphosphate). ATP acts like a battery of sorts, storing energy until it's needed for day-to-day living. Both animals and plants have mitochondria.
- Chloroplasts: These organelles are unique to plants. They process the energy of sunlight into sugars that then are used by plant mitochondria to generate the energy that nourishes the living cells.
Eukaryotic cells are able to carry out behaviors that prokaryotes can't. For example, one-celled eukaryotes often have appendages, such as long tails (called flagella) or hair-like projections (called cilia) that work like hundreds of tiny paddles, to help them move around. Also, only eukaryotic cells are capable of ingesting fluids and particles for nutrition; prokaryotes must transport materials through their cell walls, a process that severely limits their culinary options.
In most multicellular eukaryotes, cells come in two basic varieties: body cells (called somatic cells) and sex cells (or gametes). The two cell types have very different functions and are produced in very different ways.
Multicellular eukaryotes: Somatic cells
Somatic cells are produced by simple cell division called mitosis. Somatic cells of multicellular organisms like you are differentiated into special cell types. Skin cells and muscle cells are both somatic cells, for instance, but if you were to examine your skin cells under a microscope and compare them with your muscle cells, you'd see their structures are very different. The various cells that make up your body all have the same basic components (membrane, organelles, and so on), but the arrangements of the elements change from one cell type to the next so that they can carry out various jobs such as digestion (intestinal cells), energy storage (fat cells), or oxygen transport to your tissues (blood cells).
Multicellular eukaryotes: Sex cells (gametes)
Sex cells are specialized cells that are used for reproduction. Only eukaryotic organisms engage in sexual reproduction. Sexual reproduction combines genetic material from two organisms and requires special preparation in the form of a reduction in the amount of genetic material allocated to sex cells — a process called meiosi. In humans, the two types of sex cells are eggs and sperm.