Outside the plasma membrane of most prokaryotes is a fairly rigid wall which gives the organism its shape. The walls of bacteria consist of peptidoglycans. Sometimes there is also an outer capsule. Note that the cell wall of prokaryotes differs chemically from the eukaryotic cell wall of plant cells and of protists. Eukaryotic cells contain a membrane-bound nucleus and numerous membrane-enclosed organelles e. Animals, plants, fungi, and protists are all eukaryotes.
Eukaryotic cells are more complex than prokaryotic cells and are found in a great many different forms. The nucleus contains most of the genetic material DNA of the cell. Additional DNA is in the mitochondria and if present chloroplasts.
The nuclear DNA is complexed with proteins to form chromatin, which is organized as a number of linear chromosomes. Genetic control of the cell is carried out by the production of RNA in the nucleus the process of transcription and the subsequent transfer of this RNA to a ribosome in the cytoplasm, where protein synthesis the process of translation is directed.
The resulting proteins carry out cell functions. Also located in the nucleus is the nucleolus or nucleoli, organelles in which ribosomes are assembled. The nucleus is bounded by a nuclear envelope, a double membrane perforated with pores and connected to the rough endoplasmic reticulum membrane system.
The cytoskeleton consists of microtubules, intermediate fibers, and microfilaments, which together maintain cell shape, anchor organelles, and cause cell movement. The microtubules and microfilaments are frequently assembled and disassembled according to cellular needs for movement and maintaining cell shape.
Intermediate filaments are more permanent than microtubules and microfilaments. The cell diagrams shown here represent intestinal epithelial cells with fingerlike projections, the microvilli.
The location and appearance of cytoskeletal fibers in different cell types will vary. A ribosome is the site of protein synthesis in the cell. Each ribosome consists of a large subunit and a small subunit, each of which contains rRNA ribosomal RNA and ribosomal proteins. The amino acids are joined to produce the protein. You may access more information on From Gene to Protein: Translation.
Ribosomes exist free in the cytoplasm and bound to the endoplasmic reticulum ER. Free ribosomes synthesize the proteins that function in the cytosol, while bound ribosomes make proteins that are distributed by the membrane systems, including those which are secreted from the cell. Life is both wonderful and majestic. Yet for all of its majesty, all organisms are composed of the fundamental unit of life, the cell. The cell is the simplest unit of matter that is alive.
From the unicellular bacteria to multicellular animals, the cell is one of the basic organizational principles of biology. Let's look at some of the components of this basic organizer of living organisms.
There are two primary types of cells: eukaryotic cells and prokaryotic cells. Eukaryotic cells are called so because they have a true nucleus. The nucleus, which houses DNA , is contained within a membrane and separated from other cellular structures.
Prokaryotic cells , however, have no true nucleus. DNA in a prokaryotic cell is not separated from the rest of the cell but coiled up in a region called the nucleoid. As organized in the Three Domain System , prokaryotes include archaeans and bacteria. Eukaryotes include animals , plants , fungi and protists ex. Typically, eukaryotic cells are more complex and much larger than prokaryotic cells.
On average, prokaryotic cells are about 10 times smaller in diameter than eukaryotic cells. Eukaryotes grow and reproduce through a process called mitosis. In organisms that also reproduce sexually , the reproductive cells are produced by a type of cell division called meiosis.
Most prokaryotes reproduce asexually and some through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical daughter cells. Some eukaryotic organisms also reproduce asexually through processes such as budding, regeneration, and parthenogenesis.
Both eukaryotic and prokaryotic organisms get the energy they need to grow and maintain normal cellular function through cellular respiration. Cellular respiration has three main stages: glycolysis , the citric acid cycle , and electron transport. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell.
In unicellular organisms, daughter cells are individuals. To achieve the outcome of identical daughter cells, some steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells; the cytoplasmic contents must also be divided to give both new cells the machinery to sustain life.
In bacterial cells, the genome consists of a single, circular DNA chromosome; therefore, the process of cell division is simplified. Mitosis is unnecessary because there is no nucleus or multiple chromosomes. This type of cell division is called binary fission. The cell division process of prokaryotes, called binary fission , is a less complicated and much quicker process than cell division in eukaryotes.
Because of the speed of bacterial cell division, populations of bacteria can grow very rapidly. The single, circular DNA chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell. As in eukaryotes, the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size. The packing proteins of bacteria are, however, related to some of the proteins involved in the chromosome compaction of eukaryotes.
The starting point of replication, the origin , is close to the binding site of the chromosome to the plasma membrane Figure 1.
As the new double strands are formed, each origin point moves away from the cell-wall attachment toward opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. A septum is formed between the nucleoids from the periphery toward the center of the cell.
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