Bacterial Reproduction

Bacteria are prokaryotic organisms that reproduce asexually. Bacterial reproduction most commonly occurs by a kind of cell division called binary fission. Binary fission results in the formation of two bacterial cells that are genetically identical.

Bacterial Cell Structure

Bacterial cells typically contain the following structures: a cell wall, cell membrane, cytoplasm, ribosomes, plasmids, flagella, and a nucleiod region.

• Cell Wall - Outer covering of the cell that protects the bacterial cell and gives it shape.
• Cytoplasm - A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.

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• Cell Membrane or Plasma Membrane - Surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
• Flagella - Long, whip-like protrusion that aids in cellular locomotion.
• Ribosomes - Cell structures responsible for protein production.
• Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction.
• Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Bacterial Reproduction:

AsexualMost bacteria reproduce by binary fission. During binary fission, the single DNA molecule replicates and both copies attach to the cell membrane.The cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward.A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

Bacterial Recombination:

Binary fission is an effective way for bacteria to reproduce, however it does produce problems. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics. In order to incorporate some genetic variation, bacteria use a process called recombination. Bacterial recombination can be accomplished through conjugation, transformation, or transduction.

Conjugation

Some bacteria are capable of transferring pieces of their genes to other bacteria that they come in contact with. During conjugation, one bacterium connects itself to another through a protein tube structure called a pilus. Genes are transferred from one bacterium to the other through this tube.

Transformation

Some bacteria are capable of taking up DNA from their environment. These DNA remnants most commonly come from dead bacterial cells. During transformation, the bacterium binds the DNA and transports it across the bacterial cell membrane. The new DNA is then incorporated into the bacterial cell's DNA.

Transduction

Transduction is a type of recombination that involves the exchanging of bacterial DNA through bacteriophages. Bacteriophages are viruses that infect bacteria. There are two types of transduction: generalized and specialized transduction.Once a bacteriophage attaches to a bacterium, it inserts its genome into the bacterium. The viral genome, enzymes, and viral components are then replicated and assembled within the host bacterium. The newly formed bacteriophages then lyse or split open the bacterium, releasing the replicated viruses.During the assembling process however, some of the host's bacterial DNA may become encased in the viral capsid instead of the viral genome. When this bacteriophage infects another bacterium, it injects the DNA fragment from the previous bacterium. This DNA fragment then becomes inserted into the DNA of the new bacterium. This type of transduction is called generalized transduction.In specialized transduction, fragments of the host bacterium's DNA become incorporated into the viral genomes of the new bacteriophages. The DNA fragments can then be transfered to any new bacteria that these bacteriophages infect.
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Cilia and Flagella

Cilia and flagella are motile cellular appendages found in most microorganisms and animals, but not in higher plants. In multicellular organisms, cilia function to move a cell or group of cells or to help transport fluid or materials past them. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs. Among other tasks, cilia also generate water currents to carry food and oxygen past the gills of clams and transport food through the digestive systems of snails. Flagella are found primarily on gametes, but create the water currents necessary for respiration and circulation in sponges and coelenterates as well. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. Protozoans belonging to the phylum Ciliophora are covered with cilia, while flagella are a characteristic of the protozoan group Mastigophora.

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In eukaryotic cells, cilia and flagella contain the motor protein dynein and microtubules, which are composed of linear polymers of globular proteins called tubulin. The core of each of the structures is termed the axoneme and contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules. One full microtubule and one partial microtubule, the latter of which shares a tubule wall with the other microtubule, comprise each doublet microtubule (see Figure 1). Dynein molecules are located around the circumference of the axoneme at regular intervals along its length where they bridge the gaps between adjacent microtubule doublets.

A plasma membrane surrounds the entire axoneme complex, which is attached to the cell at a structure termed the basal body (also known as a kinetosome). Basal bodies maintain the basic outer ring structure of the axoneme, but each of the nine sets of circumferential filaments is composed of three microtubules, rather than a doublet of microtubules. Thus, the basal body is structurally identical to the centrioles that are found in the centrosome located near the nucleus of the cell. In some organisms, such as the unicellular Chlamydomonas, basal bodies are locationally and functionally altered into centrioles and their flagella resorbed before cell division.

Eukaryotic cilia and flagella are generally differentiated based on size and number: cilia are usually shorter and occur together in much greater numbers than flagella, which are often solitary. The structures also exhibit somewhat different types of motion, though in both cases movement is generated by the activation of dynein and the resultant bending of the axoneme. The movement of cilia is often described as whip-like, or compared to the breast stroke in swimming. Adjacent cilia move almost simultaneously (but not quite), so that in groups of cilia, wave-like patterns of motion occur. Flagella, however, exhibit a smooth, independent undulatory type of movement in eukaryotes. Prokaryotic flagella, which have a completely different structure built from the protein flagellin, move in a rotating fashion powered by the basal motor.

Defects in the cilia and flagella of human cells are associated with some notable medical problems. For example, a hereditary condition known as Kartagener's syndrome is caused by problems with the dynein arms that extend between the microtubules present in the axoneme, and is characterized by recurrent respiratory infections related to the inability of cilia in the respiratory tract to clear away bacteria or other materials. The disease also results in male sterility due to the inability of sperm cells to propel themselves via flagella. Damage to respiratory cilia may also be acquired rather than inherited and is most commonly linked to smoking cigarettes. Bronchitis, for instance, is often triggered by a build-up of mucus and tar in the lungs that cannot be properly removed due to smoking-related impairment of cilia.

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Peroxisomes

Microbodies are a diverse group of organelles that are found in the cytoplasm of almost all cells, roughly spherical, and bound by a single membrane. There are several types of microbodies, including lysosomes, but peroxisomes are the most common. All eukaryotes are comprised of one or more cells that contain peroxisomes. The organelles were first discovered by the Belgian scientist Christian de Duve, who also discovered lysosomes.

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Peroxisomes contain a variety of enzymes, which primarily function together to rid the cell of toxic substances, and in particular, hydrogen peroxide (a common byproduct of cellular metabolism). These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen (a process termed oxidation). Others are more important for their ability to initiate the production of phospholipids, which are typically used in the formation of membranes.

In order to carry out their activities, peroxisomes use significant amounts of oxygen. This characteristic of the organelles would have been extremely important millions of years ago, before cells contained mitochondria, when the Earth's atmosphere first began to amass large amounts of oxygen due to the actions of photosynthetic bacteria. Peroxisomes would have been primarily responsible at that time for detoxifying cells by decreasing their levels of oxygen, which was then poisonous to most forms of life. The organelles would have provided the cellular benefit of carrying out a number of advantageous reactions as well. Later, when mitochondria eventually evolved, peroxisomes became less important (in some ways) to the cell since mitochondria also utilize oxygen to carry out many of the same reactions, but with the additional benefit of generating energy in the form of adenosine triphosphate (ATP) at the same time.

Peroxisomes are similar in appearance to lysosomes, another type of microbody, but the two have very different origins. Lysosomes are generally formed in the Golgi complex, whereas peroxisomes self-replicate. Unlike self-replicating mitochondria, however, peroxisomes do not have their own internal DNA molecules. Consequently, the organelles must import the proteins they need to make copies of themselves from the surrounding cytosol. The importation process of peroxisomes is not yet well understood, but it appears to be heavily dependent upon peroxisomal targeting signals composed of specific amino acid sequences. These signals are thought to interact with receptor proteins present in the cytosol and docking proteins present in the peroxisomal membrane. As more and more proteins are imported into lumen of a peroxisome or are inserted into its membrane, the organelle gets larger and eventually reaches a point where fission takes place, resulting in two daughter peroxisomes. Illustrated in Figure 2 is a fluorescence digital image of an African water mongoose skin fibroblast cell stained with fluorescent probes targeting the nucleus (red), actin cytoskeletal network (blue), and peroxisomes (green).

Since the early 1980s, a number of metabolic disorders have been discovered to be caused by molecular defects in peroxisomes. Two major categories have been described so far. The first category consists of disorders of peroxisome biogenesis in which the organelle fails to develop normally, causing defects in numerous peroxisomal proteins. The second category involves defects of single peroxisomal enzymes. Studies indicate that approximately one in every 20,000 people has some type of a peroxisomal disorder. The most serious of these disorders is Zellweger syndrome, which is characterized by an absence or reduced number of peroxisomes in the cells. Present in patients at birth (congenital), Zellweger syndrome has no cure or effective treatment and usually causes death within the first year of life.

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Lysosomes

The main function of these microscopic organelles is to serve as digestion compartments for cellular materials that have exceeded their lifetime or are otherwise no longer useful. In this regard, the lysosomes recycle the cell's organic material in a process known as autophagy. Lysosomes break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds, which are then transferred back into the cytoplasm as new cell-building materials. To accomplish the tasks associated with digestion, the lysosomes utilize about 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi apparatus. Lysosomes are often budded from the membrane of the Golgi apparatus, but in some cases they develop gradually from late endosomes, which are vesicles that carry materials brought into the cell by a process known as endocytosis.

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Like other microbodies, lysosomes are spherical organelles contained by a single layer membrane, though their size and shape varies to some extent. This membrane protects the rest of the cell from the harsh digestive enzymes contained in the lysosomes, which would otherwise cause significant damage. The cell is further safeguarded from exposure to the biochemical catalysts present in lysosomes by their dependency on an acidic environment. With an average pH of about 4.8, the lysosomal matrix is favorable for enzymatic activity, but the neutral environment of the cytosol renders most of the digestive enzymes inoperative, so even if a lysosome is ruptured, the cell as a whole may remain uninjured. The acidity of the lysosome is maintained with the help of hydrogen ion pumps, and the organelle avoids self-digestion by glucosylation of inner membrane proteins to prevent their degradation.

The discovery of lysosomes involved the use of a centrifuge to separate the various components of cells. In the mid-twentieth century, the Belgian scientist Christian Ren� de Duve was investigating carbohydrate metabolism of liver cells and observed that that the cells released an enzyme called acid phosphatase in larger amounts when they received proportionally greater damage in the centrifuge. To explain this phenomenon, de Duve suggested that the digestive enzyme was encased in some sort of membrane-bound organelle within the cell, which he dubbed the lysosome. After estimating the probable size of the lysosome, he was able to identify the organelle in images produced with an electron microscope.

Lysosomes are found in all animal cells, but are most numerous in disease-fighting cells, such as white blood cells. This is because white blood cells must digest more material than most other types of cells in their quest to battle bacteria, viruses, and other foreign intruders. Several human diseases are caused by lysosome enzyme disorders that interfere with cellular digestion. Tay-Sachs disease, for example, is caused by a genetic defect that prevents the formation of an essential enzyme that breaks down complex lipids called gangliosides. An accumulation of these lipids damages the nervous system, causes mental retardation, and death in early childhood. Also, arthritis inflammation and pain are related to the escape of lysosome enzymes.

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The Cytoskeleton

What is the cytoskeleton?



The cytoskeleton is a network of fibers throughout the cell's cytoplasm that helps the cell maintain its shape and gives support to the cell.

A variety of cellular organelles are held in place by the cytoskeleton.

Fibroblast cells. Fluorescent light micrograph of two fibroblast cells, showing their nuclei (purple) and cytoskeleton. The cytoskeleton is made up of microtubules of the protein tubulin (yellow) and filaments of the protein actin (blue). The cytoskeleton supports the cell's structure, allows the cell to move and assists in the transport of organelles and vesicles within the cell. Fibroblasts are cells forming connective tissue, and are responsible for secreting connective tissue proteins such as collagen.

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What are some distinguishing characteristics?

The cytoskeleton is composed of at least three different types of fibers: microtubules, microfilaments and intermediate filaments.

These types are distinguished by their size with microtubules being the thickest and microfilaments being the thinnest.

• Microtubules are hollow rods functioning primarily to help support and shape the cell and as "routes" along which organelles can move. Microtubules are typically found in all eukaryotic cells.
• Microfilaments or actin filaments are solid rods and are active in muscle contraction. Microfilaments are particularly prevalent in muscle cells but similar to microtubules, they are also typically found in all eukaryotic cells.
• Intermediate filaments can be abundant in many cells and provide support for microfilaments and microtubules by holding them in place.In addition to providing support for the cell, the cytoskeleton is also involved in cellular motility and in moving vesicles within a cell, as well as assisting in the formation of food vacuoles in the cell.

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The Golgi Apparatus

The Golgi apparatus (GA), also called Golgi body or Golgi complex and found universally in both plant and animal cells, is typically comprised of a series of five to eight cup-shaped, membrane-covered sacs called cisternae that look something like a stack of deflated balloons. In some unicellular flagellates, however, as many as 60 cisternae may combine to make up the Golgi apparatus. Similarly, the number of Golgi bodies in a cell varies according to its function. Animal cells generally contain between ten and twenty Golgi stacks per cell, which are linked into a single complex by tubular connections between cisternae. This complex is usually located close to the cell nucleus.

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Due to its relatively large size, the Golgi apparatus was one of the first organelles ever observed. In 1897, an Italian physician named Camillo Golgi, who was investigating the nervous system by using a new staining technique he developed (and which is still sometimes used today; known as Golgi staining or Golgi impregnation), observed in a sample under his light microscope a cellular structure that he termed the internal reticular apparatus. Soon after he publicly announced his discovery in 1898, the structure was named after him, becoming universally known as the Golgi apparatus. Yet, many scientists did not believe that what Golgi observed was a real organelle present in the cell and instead argued that the apparent body was a visual distortion caused by staining. The invention of the electron microscope in the twentieth century finally confirmed that the Golgi apparatus is a cellular organelle.

The Golgi apparatus is often considered the distribution and shipping department for the cell's chemical products. It modifies proteins and lipids (fats) that have been built in the endoplasmic reticulum and prepares them for export outside of the cell or for transport to other locations in the cell. Proteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the Golgi complex. The vesicles fuse with the Golgi membranes and release their internally stored molecules into the organelle. Once inside, the compounds are further processed by the Golgi apparatus, which adds molecules or chops tiny pieces off the ends. When completed, the product is extruded from the GA in a vesicle and directed to its final destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the cell's function in the organism. Other products are returned to the endoplasmic reticulum or may undergo maturation to become lysosomes.

The modifications to molecules that take place in the Golgi apparatus occur in an orderly fashion. Each Golgi stack has two distinct ends, or faces. The cis face of a Golgi stack is the end of the organelle where substances enter from the endoplasmic reticulum for processing, while the trans face is where they exit in the form of smaller detached vesicles. Consequently, the cis face is found near the endoplasmic reticulum, from whence most of the material it receives comes, and the trans face is positioned near the plasma membrane of the cell, to where many of the substances it modifies are shipped. The chemical make-up of each face is different and the enzymes contained in the lumens (inner open spaces) of the cisternae between the faces are distinctive. Illustrated in Figure 2 is a fluorescence digital image taken through a microscope of the Golgi apparatus (pseudocolored red) in a typical animal cell. Note the close proximity of the Golgi membranes to the cell nucleus.

Proteins, carbohydrates, phospholipids, and other molecules formed in the endoplasmic reticulum are transported to the Golgi apparatus to be biochemically modified during their transition from the cis to the trans poles of the complex. Enzymes present in the Golgi lumen modify the carbohydrate (or sugar) portion of glycoproteins by adding or subtracting individual sugar monomers. In addition, the Golgi apparatus manufactures a variety of macromolecules on its own, including a variety of polysaccharides. The Golgi complex in plant cells produces pectins and other polysaccharides specifically needed by for plant structure and metabolism. The products exported by the Golgi apparatus through the trans face eventually fuse with the plasma membrane of the cell. Among the most important duties of the Golgi apparatus is to sort the wide variety of macromolecules produced by the cell and target them for distribution to their proper location. Specialized molecular identification labels or tags, such as phosphate groups, are added by the Golgi enzymes to aid in this sorting effort.

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The Endoplasmic Reticulum

What is the endoplasmic reticulum?


The endoplasmic reticulum (ER) is a network of flattened sacs and branching tubules that extends throughout the cytoplasm in plant and animal cells. These sacs and tubules are all interconnected by a single continuous membrane so that the organelle has only one large, highly convoluted and complexly arranged lumen (internal space). Usually referred to as the endoplasmic reticulum cisternal space, the lumen of the organelle often takes up more than 10 percent of the total volume of a cell. The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the double-layered nuclear envelope, it further provides a pipeline between the nucleus and the cytoplasm.

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The endoplasmic reticulum manufactures, processes, and transports a wide variety of biochemical compounds for use inside and outside of the cell. Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only transiently as they pass on their way to other locations. Other proteins, however, are targeted to constantly remain in the lumen and are known as endoplasmic reticulum resident proteins. These special proteins, which are necessary for the endoplasmic reticulum to carry out its normal functions, contain a specialized retention signal consisting of a specific sequence of amino acids that enables them to be retained by the organelle. An example of an important endoplasmic reticulum resident protein is the chaperone protein known as BiP (formally: the chaperone immunoglobulin-binding protein), which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations.

There are two basic kinds of endoplasmic reticulum morphologies: rough and smooth. The surface of rough endoplasmic reticulum is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope. This type of endoplasmic reticulum is involved mainly with the production and processing of proteins that will be exported, or secreted, from the cell. The ribosomes assemble amino acids into protein units, which are transported into the rough endoplasmic reticulum for further processing. These proteins may be either transmembrane proteins, which become embedded in the membrane of the endoplasmic reticulum, or water-soluble proteins, which are able to pass completely through the membrane into the lumen. Those that reach the inside of the endoplasmic reticulum are folded into the correct three-dimensional conformation, as a flattened cardboard box might be opened up and folded into its proper shape in order to become a useful container. Chemicals, such as carbohydrates or sugars, are added, then the endoplasmic reticulum either transports the completed proteins to areas of the cell where they are needed, or they are sent to the Golgi apparatus for further processing and modification.

Most proteins exported from the endoplasmic reticulum exit the organelle in vesicles budded from the smooth portion, which has a more even appearance than rough endoplasmic reticulum when viewed through the electron microscope because of the lack of ribosomes. The smooth endoplasmic reticulum in most cells is much less extensive than the rough endoplasmic reticulum and is sometimes alternatively termed transitional. Smooth endoplasmic reticulum is chiefly involved, however, with the production of lipids (fats), building blocks for carbohydrate metabolism, and the detoxification of drugs and poisons. Therefore, in some specialized cells, such as those that are occupied chiefly in lipid and carbohydrate metabolism (brain and muscle) or detoxification (liver), the smooth endoplasmic reticulum is much more extensive and is crucial to cellular function. Smooth endoplasmic reticulum also plays a role in various cellular activities through its storage of calcium and involvement in calcium metabolism. In muscle cells, smooth endoplasmic reticulum releases calcium to trigger muscle contractions. Presented in Figure 2 is a fluorescence digital image taken through the microscope of the endoplasmic reticulum network in a bovine (cow) pulmonary artery endothelial cell grown in culture.

It is the rough endoplasmic reticulum that is directly continuous with the nuclear envelope (as illustrated in Figure 1), which is also studded with ribosomes, and the two organelles are thought to have evolved simultaneously in ancient cells. Due to their physical membranous connection, the lumen of the endoplasmic reticulum and the space between the layers of the nuclear envelope comprise a single compartment. Accordingly, the nucleus has direct access to proteins (many of which are produced by the ribosomes upon its surface) and other materials present in the endoplasmic reticulum lumen, so that transport vesicles are not needed to obtain them. The close association between the endoplasmic reticulum and the nucleus also enables the organelles to share information in a very efficient manner. For instance, if the endoplasmic reticulum begins to undergo functional problems and unfolded proteins accumulate within the organelle, which can be extremely hazardous to the cell, the organelle quickly sends a signal to the nucleus (as well as to the cytoplasm). The nucleus responds by slowing ribosomal translation through a several-step process, thereby giving the endoplasmic reticulum extra time to catch up on its protein folding, thus maintaining cellular health.

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Ribosomes

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic and eukaryotic cells. Now we turn our attention to the protein assemblers of a eukaryotic cell, the ribosomes.

Ribosomes are organelles that consist of RNA an proteins. They are responsible for assembling the proteins of the cell. Depending on the protein production level of a particular cell, ribosomes may number in the millions.

Distinguishing Characteristics:

Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized by the nucleolus. These two units join together when the ribosome attaches to messenger RNA to produce a protein in the cytoplasm.

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Location in the Cell:

There are two places that ribosomes usually exist in the cell: suspended in the cytosol and bound to the endoplasmic reticulum. These ribosomes are called free ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form aggregates called polysomes.

Free ribosomes usually make proteins that will function in the cytosol, while bound ribosomes usually make proteins that are exported or included in the cell's membranes. Interestingly enough, free ribosomes and bound ribosomes are interchangeable and the cell can change their numbers according to metabolic needs.

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Journey Into The Cell: Mitochondria

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic (pro-) and eukaryotic (eu-) cells. Now we turn our attention to the "power houses" of a eukaryotic cell, the mitochondria.

What are mitochondria?

Mitochondria are the cell's power producers. They convert energy into forms that are usable by the cell. They are the sites of cellular respiration which ultimately generates fuel for the cell's activities.

Mitochondria Structural Features

Mitochondrion

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What are their distinguishing characteristics?

Mitochondria are bounded by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins. The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the "productivity" of cellular respiration by increasing the available surface area.

Muscle Cell Mitochondria, Copyright Dennis Kunkel

The double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between the two membranes while the mitochondrial matrix is the part enclosed by the innermost membrane. Several of the steps in cellular respiration occur in the matrix due to its high concentration of enzymes.

Mitochondrion with matrix

Mitochondria are semiautonomous (semi- auto-) in that they can divide and grow to make more of themselves. They also have their own DNA and ribosomes.

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Journey into the Cell: The Nucleus

The Nucleus:

In Journey into the Cell, we looked at the structure of the two major types of cells: prokaryotic and eukaryotic cells. Now we turn our attention to the "nerve center" of a eukaryotic cell, the nucleus.

The nucleus is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction.

It is commonly the most prominent organelle in the cell.

Nucleus

Nucleus with Nuclear Pores

Liver Cell Nucleus with Dark Nucleolus

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Distinguishing Characteristics:

The nucleus is bounded by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm.

The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into and out of the nucleus through nuclear pores.

Chromosomes are also located in the nucleus.

When a cell is "resting" i.e. not dividing, the chromosomes are organized into long entangled structures called chromatin and not into individual chromosomes as we typically think of them.

The Nucleolus:

The nucleus also contains the nucleolus which helps to synthesize ribosomes.

The nucleolus contains nucleolar organizers which are parts of chromosomes with the genes for ribosome synthesis on them. Copious amounts of RNA and proteins can be found in the nucleolus as well.

The nucleus controls the synthesis of proteins in the cytoplasm through the use of messenger RNA. Messenger RNA is produced in the nucleolus of the cell and travels to the cytoplasm through the pores of the nuclear envelope.

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The Nucleus

The presence of a nucleus and other membranous orhanella characterizes eukaryotic celld. The nucleus houses the genetic material, DNA, which id complexed with an array of acidic and basic protein into thin fibers. During nondivisional phase of the cell cycle, these fibers are uncoiled and dispersed into chromatin.

Duriing mitosis and meiosis chromatin fibers coil and condense into structure called chromosomes. Also present in the nucleud id the nucleolus, an amorphous component where ribosomal RNA is synthesized and where the initial stages of ribosomal assembly occur. The areas of DNA encoding rRNA are collectively referred to as the nucleolus control region (NOR).

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The lack of the nuclear envelope and membranaeous organelles is characteristic of prokaryotes. In bacteria such as a E. coli, the genetic material is present as a long, circular DNA molecule that is compacted into an area referred as the nucleotid area. Part of the DNA may be attached to the cell membrane, but in general the nucleotid cinstitutes a larhe area trougout the cell. Although the DNA is compacted, it does not undergo the extensive coiling characteristic of the stages of mitosis were, in eukaryotes, chromosome become visible. Nor is the DNA in these organism associated as extensively with proteins as is eukaryotic DNA. Prokaryotic cells do not have a distinct nucleolus, but do contain genes that specify rRNA molecules.

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