Bacteria cells Have a Nucleus
Photo by: icholakov
Hundreds of thousands of bacterial species exist on Earth. They can be found in very diverse environments ranging from cold to hot and alkaline to acid. They live in soil, in water, and on rocks. They exist deep in the earth, high on mountains, and in deep-sea vents. They grow on and in other bacteria, worms, insects, plants, animals, and people.
Bacteria are prokaryotes . Prokaryotic cells possess simpler structures than eukaryotic cells , since they do not have a , other membrane bound organelles , or a . Bacterial cells have two major compartments, the cytoplasm and cell envelope, and may also have exterior appendages , such as flagella or pili. There are two major types of prokaryotes: bacteria and archaea. Archaea (also called archaebacteria) are often found in extreme environments, and while they are clearly prokaryotic, they have evolved separately from bacteria. Mitochondria and chloroplasts are two membrane-bound organelles carried within eukaryotic cells that are thought to have been derived from free-living prokaryotic organisms that became irreversibly engulfed by ancestral eukaryotes.
A colored transmission electron micrograph of Streptococcus bacteria attached to a human tonsil cell.
Growth and Reproduction
Bacterial cells grow by a process called binary fission: One cell doubles in size and splits in half to produce two identical daughter cells. These daughter cells can then double in size again to produce four sibling cells and these to produce eight, and so on. The time it takes for a bacterial cell to grow and divide in two is called the doubling time. When nutrients are plentiful, the doubling time of some bacterial species can be as short as twenty minutes. However, most bacterial species show a doubling time between one and four hours. A single bacterial cell with a one-hour doubling time will produce over 1 million offspring within twenty hours. If left unchecked, a single E. coli bacterium replicating once every twenty minutes could replicate to equal the mass of Earth in twenty-four hours. The enormous increase in cell numbers that accompanies this exponential growth provides these simple unicellular organisms with an incredible growth advantage over other unicellular or multicellular organisms. Luckily, there are always limits to bacterial growth.
The cytoplasm of a bacterial cell contains the deoxyribonucleic acid (DNA) molecules that make up the bacterial (or nucleoid), the transcriptional machinery that copies DNA into ribonucleic acid (RNA), and the ribosomes that translate the messenger RNA information into protein sequence. Since there is no nucleus, all of these processes occur simultaneously. The rapid growth rate of the bacterial cell requires constant DNA replication and ways to segregate the two new chromosomes into the two daughter cells without tangling them.
Structure and Diversity
Bacterial cells express a variety of shapes and sizes. The smallest bacteria are the Mycoplasmas, which range from about 0.1 to 0.25 micrometers in diameter, while the gigantic Epulopiscium fishelsoni is 250 micrometers long and visible to the naked eye. Some bacteria have a coccal (spherical) shape. Others are shaped as bacilli (rods), vibrio (curved rods), or spirochetes (spirals).