In mitosis, I understand that the centromeres line up on the spindle. I also know that the centrioles form microtubles between the centromeres during mitosis in the metaphase.
But, are microtubles and spindles the same thing? Or do microtubles make up spindles?
The spindle is made up of microtubules
Spindle fibers form a protein structure that divides the genetic material in a cell. [… ] At the beginning of nuclear division, two wheel-shaped protein structures called centrioles position themselves at opposite ends of the cell forming cell poles. Long protein fibers called microtubules extend from the centrioles in all possible directions, forming what is called a spindle. Some of the microtubules attach the poles to the chromosomes by connecting to protein complexes called kinetochores. Kinetochores are protein formations that develop on each chromosome around the centromere, which is a region located near the middle of a chromosome. Other microtubules bind to the chromosome arms or extend to the opposite end of the cell. During the cell division phase called metaphase, the microtubules pull the chromosomes back and forth until they align in a plane along the equator of the cell, which is called the equatorial plane.
What does spindle mean in biology?
The centromere is also known as the microtubule organizing center. The spindle fibers provide a framework and means of attachment that keep chromosomes organized, aligned and assorted during the entire process of mitosis, lessening the occurrence of aneuploidy, or daughter cells with incomplete sets of chromosomes.
Additionally, what is the definition of centromere in biology? Definition of Centromere In eukaryotes, a centromere is a region of DNA that is responsible for the movement of the replicated chromosomes into the two daughter cells during mitosis and meiosis. There is one centromere on each chromosome, and centromeres are responsible for two major functions.
In this way, how do spindle fibers work?
Spindle fibers are highly active during mitosis. Anaphase: Spindle fibers shorten and pull sister chromatids toward spindle poles. Separated sister chromatids move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell to make room for the cell to separate.
What is the purpose of spindle?
Spindle fibers form a protein structure that divides the genetic material in a cell. The spindle is necessary to equally divide the chromosomes in a parental cell into two daughter cells during both types of nuclear division: mitosis and meiosis. During mitosis, the spindle fibers are called the mitotic spindle.
Mitotic Spindle: Definition, Formation & Function
Chapter 12 / Lesson 6 Transcript
Quiz & Worksheet - The Mitotic Spindle Quiz
The mitotic spindle is a structure of the eukaryotic cytoskeleton involved in mitosis and meiosis. It consists of a bundle of microtubules joined at the ends but spread out in the middle, vaguely resembling an American football in shape.
Mitotic Spindle Apparatus
The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo mitosis and therefore have no need for a mitotic spindle.
The structure in a eukaryotic cell that assembles during mitosis and is responsible for pulling the chromosomes from the metaphase plate to the cell poles during anaphase.
by the time of this checkpoint, the onset of anaphase will be delayed.
A network of microtubules formed during prophase. Some microtubules attach to the centromeres of the chromosomes and help draw the chromosomes apart during anaphase. PICTURE .
apparatus - microtubule-based structure present during mitosis to which chromosomes attach and are separated toward opposite poles of the dividing cell.
morphogenesis - creation of form or structure during development.
- A complex of microtubules that form between opposite poles of a cell during mitosis. Serve to separate and move sister chromatids to opposite ends of the cell for division.
distributes chromosomes to daughter cells: a closer look.
that begins to form in early prophase is a bipolar structure composed of microtubules and associated proteins. As the spindle grows, the centrosomes begin to translocate to opposite ends of the nucleus, apparently driven by the addition of new tubulin monomers to the existing filament network.
in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red.
fiber complex will bind to that DNA sequence without regard for its location or other factors.
Regional Centromeres .
are found next to each other, they are usually at right angles. The centrioles are found in pairs and move towards the poles (opposite ends) of the nucleus when it is time for cell division. During division, you may also see groups of threads attached to the centrioles. Those threads are called the
s are visible in living cells with the polarizing light microscope. Some of the spindle microtubules become attached to the chromosomes at sites known as kinetochores.
In prophase, the nuclear envelope begins to break down, the nuclear material (or chromatin) condenses into rod-shaped chromosomes consisting of two sister chromatids, and the
For example, although a plant cell creates a
and has a centrosome, it lacks centrioles. The other major difference in plants is the way in which cytokinesis occurs.
And when the chromosomes are condensing to undergo mitosis, the centrioles form the areas that
s go and attach to each of the chromosomes and pull the chromosomes to opposite ends of the cell to allow cytokinesis, then, to occur.
The centrioles begin to migrate to opposite ends of the cell where they will serve as the organizing site for the
microtubules. Metaphase is defined as the phase of mitosis when the centromeres and kinetochores are pulled and pushed into the center of the cell.
A phase of mitosis in when the chromosome pairs have lined up at the equator of the
. Lecture - Cell Division
A technique that allows the identification of overall cellular gene expression at the messenger RNA level. Lab 12 - Microarray .
Mitotic stage at which chromosomes are fully condensed and attached to the
at its equator but have not yet started to segregate toward the opposite spindle poles. (Figure 19-34)
Full glossary .
metaphase Stage in mitosis when chromosomes become aligned in the middle of the cell and firmly attached to the
but have not yet segregated toward opposite poles.
metapodosoma Portion of the podosoma that bears the third and fourth pairs of legs of a tick or mite.
A specialized region on the centromere that links each sister chromatid to the
A taxonomic category, the second broadest after domain.
One of the two foci of a cell during mitosis, defined by a centriole, from which half the
radiates towards the other pole.
Return to Search Page .
. This group of organisms is more correctly called Eucarya.
(kih-net-oh-kor) [Gk. kinetikos, putting in motion + choros, chorus]
A specialized region on the centromere that links each sister chromatid to the
A taxonomic category, the second broadest after domain.
Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the
where sister chromatids are attached in mitotic chromosomes. The centromere is generally flanked by repetitive DNA sequences and it is late to replicate. The centromere is an A-T region of about 130 bp. It binds several proteins with high affinity to form the kinetochore which is the anchor for the
What is the difference between the mitotic spindle and microtubules? - Biology
Perhaps the most recognizable phase of mitosis is termed metaphase , a stage where the chromosomes, attached to the kinetochore microtubules, begin to align in a single plane (known as the metaphase plate ) midway between the spindle poles. The kinetochore microtubules exert tension on the chromosomes, which move back and forth in rapid erratic motion as a result, and the entire spindle-chromosome complex is now ready for the next event, separation of the daughter chromatids. Metaphase, one of the most critical stages in mitosis, occupies a substantial portion of the division cycle. The primary reason for this extended interval is that dividing cells pause until all of their chromosomes are completely aligned at the metaphase plate.
View a second, third, and fourth fluorescence image of metaphase.
Presented in the fluorescence digital microscopy image above is a single rat kangaroo ( PtK2 ) kidney cell in the final stages of metaphase. The chromatin is stained with a blue fluorescent probe (DAPI), while the microtubule network (mitotic spindle) is stained green (Alexa Fluor 488). Cellular mitochondria, which surround the mitotic apparatus, are stained with a red dye (MitoTracker Red CMXRos). As the dividing cell approaches metaphase, the chromosomes become highly condensed and the mitotic spindle appears (visible as a series of green tubules in the digital images of PtK2 cell metaphase). In fluorescence images of mitosis, the two poles of the spindle and the spindle fibers are clearly visible, as are the asters , which radially migrate away from the metaphase plate. The mitotic spindle contains the fibers (microtubules) responsible for translocating and separating the chromosomes.
Each spindle fiber is composed of a bundle of microtubules, which are assembled from tubulin molecules that become available as the cytoplasmic microtubular network is disassembled for cell division. The microtubules converge at each end of the spindle to a point termed the spindle pole or mitotic center . In animal cells each spindle unit contains a pair of centrioles , which are self-replicating organelles about 150 nanometers in diameter, surrounded by a radial array of aster microtubules. Plant cells, in contrast, do not contain centrioles, and their mitotic spindles are not as clearly defined at the poles. Plants lack the radial astral array characteristic of animal cells.
Fibers comprising the mitotic spindle are functionally divided into two species. The polar fibers extend to the center of the spindle pole towards the metaphase plate, while the chromosomal fibers (more commonly referred to as kinetochore fibers ) travel from individual condensed chromosomes to the poles. Kinetochore fibers are attached to the chromosomes at the kinetochores , which form at specialized regions on opposite sides of the centromere. During metaphase, the chromosomes are gradually aligned at the metaphase plate, an imaginary plane located an equal distance from the poles, by forces exerted through the mitotic spindle. The chromosomes continually readjust their position to ensure that they are in the precise location for chromatid separation at the commencement of anaphase.
Protein complexes that form kinetochores in the centromere regions of sister chromatids are attached to specific repetitive DNA sequences, known as satellite DNA , which are similar in each chromosome. Mammalian centromeres form kinetochores that bind between 30 and 40 microtubules to each chromatid of the metaphase chromosome. Kinetochore microtubules from the spindle are able to traverse the nuclear region and attach to individual chromosomes when breakdown of the nuclear membrane occurs during prometaphase. Prior to this event, microtubules emanating from the spindle poles continuously probe the cytoplasm in search of kinetochores. When a kinetochore is finally captured by a microtubule, it is dragged toward the spindle pole to a region of higher microtubule density where the attachment probability is substantially increased. The kinetochore belonging to the other chromatid eventually becomes attached to microtubules from the opposite spindle pole, thus initiating final positioning of the chromosome at the metaphase plate.
Condensed chromatid pairs are held together at the metaphase plate by balanced bipolar forces exerted on the chromosomes by kinetochore microtubules in the mitotic spindle. These forces continue to operate throughout metaphase, as evidenced by the continued oscillatory motion of the chromosomes as they fine-tune their positions. Attachment of the individual chromatid kinetochores to the microtubular spindle array initiates the play of forces acting on the chromosomes. Severing one of the kinetochore microtubules from a metaphase chromosome with laser microsurgery causes the entire chromosome to immediately move to the opposite pole (to which it still remains attached). Likewise, artificially splitting the biochemical bonds between sister chromatids in metaphase will result in early migration of the chromosomes to their respective poles. Thus, the forces that act to separate chromatids in anaphase are first established when the microtubular network attaches to the kinetochores during metaphase.
Difference Between Plant Mitosis and Animal Mitosis
The cell division is the process by which organisms generate new cells. By this process, a single parent cell divides and creates identical daughter cells. During cell division, the parent cell duplicates its genetic material (DNA) and transmits to the daughter cells.
There are three types of cell division such as amitosis, mitosis and meiosis. In this case, amitosis occurs in the lower animals like bacteria while mitosis occurs in the body or somatic cells and meiosis in germ cells producing sperm or egg cells.
Mitosis is a vital cell division process that occurs in the body repeatedly where one cell divides and produces two identical daughter cells. In this case, both daughter cells contain the same number of chromosomes or the same amount of genetic materials.
Animal mitosis and plant mitosis are the reproductive nuclear divisions which occur in animals and plants, respectively. During the mitosis, the newly produced cells contain the same number of genetic materials as a result, the number of cells increases in the body which are crucial for growth, repair and regeneration.
There are four significant steps in mitosis such as prophase, metaphase, anaphase and telophase. The mitotic spindle occurs in both animal and plant mitosis. In animal, mitotic spindle occurs with the support of two centrioles, but in plants, it happens through without the assistance of any centrioles due to lack of centrioles.
During the cytokinesis, animal cells form furrow cleavage and finally produces two daughter cells. In contrast, plant cells contain a rigid cell wall that doesn’t create furrows, but it forms a cell plate at the centre of the cells that separate the two cell components.
Besides, all the time and everywhere, animal mitosis occurs while the plant mitosis occurs in the meristem tissues.
The following table shows some significant difference between plant mitosis and animal mitosis:
Binary Fission Steps
While a bacterial cell lacks a nucleus, its genetic material is found within a special region of the cell called a nucleoid. Copying the round chromosome starts at a site called the origin of replication and moves in both directions, forming two replication sites. As the replication process progresses, the origins move apart and separate the chromosomes. The cell lengthens or elongates.
There are different forms of binary fission: The cell can divide across the transverse (short) axis, the longitudinal (long) axis, at a slant, or in another direction (simple fission). Cytokinesis pulls the cytoplasm toward the chromosomes.
When replication is complete, a dividing line—called a septum—forms, physically separating the cytoplasm of the cells. A cell wall then forms along the septum and the cell pinches in two, forming the daughter cells.
While it's easy to generalize and say binary fission only occurs in prokaryotes, this isn't exactly true. Certain organelles in eukaryotic cells, such as mitochondria, also divide by fission. Some eukaryotic cells can divide via fission. For example, algae and Sporozoa may divide via multiple fission in which several copies of a cell are made simultaneously.
Microtubules: Assembly, Function and Centrioles (With Diagram)
Microtubules have many features that distinguish them from microfilaments and intermediate filaments.
To begin with, the outside diameter of a microtubule (usually about 25 nm) is much greater than that of microfilaments. Furthermore, microtubules are hollow, containing a central lumen about 15 nm in diameter.
Microtubule length is quite variable. Some microtubules are less than 200 nm long, but in the long processes of nerve cells their lengths may be as great as 25 μm (i.e., 25,000 nm). Microtubules can also be distinguished from microfilaments chemically. Microtubules contain two major proteins called a tubulin and β tubulin.
Each protein consists of a single polypeptide chain about 500 amino acids long (MW 55,000) and both are similar in primary structure, indicating that they are probably derived from a common ancestral protein. Not only are the α and β tubulins nearly identical but tubulins from diverse species of cells are very similar, suggesting either that they have hardly changed since they first appeared in eukaryotic organisms or that tubulin is a highly conserved protein.
α and β tubulin molecules combine to form heterodimers and these serve as the basic building blocks of microtubules. The model of heterodimer organization shown in Figure 23-14 is based on both chemical studies of microtubules and transmission electron microscopy. The microtubule is formed from a helical array of heterodimers with 13 subunits per turn of the helix. Neighboring heterodimers are linked to one another not only longitudinally but laterally as well.
Assembly and Functions of Microtubules:
The current model for the manner in which tubulin subunits are assembled into a microtubule is based on in vitro studies. Under carefully controlled conditions (e.g., the appropriate concentration of tubulin and the absence of calcium), alpha and beta subunits spontaneously form dimers that when present in high concentrations assemble into chains (Fig. 23-15) the chains then form a variety of intermediate structures including single and double rings, spirals, and stacked rings.
The rings eventually open up to form linear chains or proto-filaments that associate side-by-side to form sheets. When a sheet is sufficiently wide, it is curled to form a tube. The end result is the formation of short cylinders of dimers (Fig. 23-14). After a short cylinder is formed, continued growth occurs by the direct addition of more dimers. Growth occurs primarily by addition of dimers at one end of the tubule. It is believed that during certain micro-tubular functions (such as the operation of the anaphase spindle) the addition of dimers to one end of a microtubule is accompanied by the loss of dimers from the other end.
Assembly of tubulin into dimers requires that these polypeptides bind GTP. GTP-activated dimers can then combine with other dimers or with the growing microtubule. Attachment of a dimer to the microtubule is accompanied by the hydrolysis of the GTP, but the resulting GDP and phosphate remain bound to the tubule. The tubulin dimer also has sites that can bind the drugs colchicine, vincristine, and vinblastine (Fig. 23-16) and these substances inhibit microtubule assembly.
Tubules that are already present at the time of addition of these inhibitors disassemble. Calcium has long been recognized as an important ion in the microtubule assembly and disassembly process. Calcium may influence microtubules either directly or in association with the regulatory protein calmodulin. Recently, a number of proteins have been identified that associate with the surface of microtubules (Fig. 23-17) these proteins are called microtubule- associated proteins, or MAPs. Two families of MAPs have been separated by polyacrylamide gel electrophoresis: MAP-1 and MAP-2.
The MAPs facilitate microtubule assembly that is, microtubules are formed considerably faster and at lower tubulin concentrations in the presence of MAPs. MAPs also protect microtubules from disassembly by colchicine and low temperatures. Interactions among microtubules and between microtubules and other cell components also involve MAPs. For example, microtubules may be interconnected via the micro-trabecular lattice in these regions the lattice is rich in both MAP-1 and MAP-2. MAP-2 appears to be involved in cross-linking microfilaments and intermediate filaments with microtubules.
Although the extensive interactions between microtubules, cytoplasmic filaments, and the micro-trabecular lattice give support and shape to cells, microtubules play more than a supportive role for they are also intimately involved with cell motility, endocytosis and exocytosis, chromosome movements during mitosis, and the actions of cilia and flagella.
Centrioles and basal bodies belong to a group of cell structures referred to as microtubule-organizing centers. These centers are involved in the elaboration of microtubules. Whereas the basal bodies are located at the bases of cilia and flagella, centrioles are usually found near the cell nucleus and occur in pairs structurally, both organelles are identical.
The typical centriole is composed of nine sets of triplets, each triplet consisting of one complete microtubule and two incomplete, C-shaped ones. The triplets are arranged parallel to one another and create a cylindrical body having a diameter of 150 to 250 nm. Although rigorous proof is still lacking, it is generally believed that centrioles are involved in the production of the microtubules that form the spindle of a dividing cell. However, not all cells that form a spindle during nuclear division have centrioles for example, cone-bearing and flowering plants do not.
In cells that do have paired centrioles, the centrioles separate at the onset of nuclear division and move to diametrically opposite positions around the nucleus. Subsequently, as the chromatin condenses to form chromosomes and the nuclear envelope disappears, fibers of the spindle make their appearance, extending from an area adjacent to one centriole through the cell to the other centriole (Fig. 23-18). As division proceeds, a new centriole appears near each original one growth of the new centriole is always perpendicular to the long axis of the original centriole. By the time division is complete, each daughter cell has two complete centrioles.
Centrioles also play a role in the formation of the microtubules present, in flagella and cilia. Here the centrioles are more often referred to as basal bodies or kinetosomes (also blepharoplasts, basal granules, or basal corpuscles).
Structure of the Centriole:
Though not all cells contain centrioles, in those that do the structure of the centriole is the same. Most algal cells (but not red algae), moss cells, some fern cells, and most animal cells have centrioles, but cone-bearing and flowering plants, red algae, and some nonflagellated or nonciliated protozoans (like amoebae) do not. Some species of amoebae have a flagellated stage as well as an amoeboid stage a centriole develops during the flagellated stage but disappears during the amoeboid stage.
The most notable structural characteristic of a centriole is its nine sets of triplets. Each triplet contains three microtubules, which in cross section appear to be arranged like the vanes on a “pinwheel” (Fig. 23- 19). Although there is no surrounding membrane, the nine triplets appear to be embedded in an electron- dense material.
The nine triplets are identical. The innermost (or a) microtubule of each triplet is a complete, round microtubule, but the middle (b) and outer (c) microtubules are incomplete, C-shaped, and share the wall of the neighboring microtubule. Also, the outermost (i.e., c) microtubules may not run the full length of the centriole. The triplets, although generally parallel to each other, may be closer together at the proximal end of the centriole (that end when observed “end-on” that has the triplets tilted inward in a clockwise direction, as shown in Fig. 23-19).
The triplets may also spiral somewhat about the central axis of the centriole. Strands of material extend inward from each a tubule and join together at the central hub. These strands, when seen in cross section, give the centriole the appearance of a cartwheel (Fig. 23-19b).
The idea prevalent years ago that new centrioles arise by the division of existing centrioles is no longer accepted. Rather it appears that new centrioles are either produced de novo or are synthesized using an existing centriole as some form of template. In the latter case, growth of the new centriole occurs at right angles to the long axis of the existing centriole, the two organelles separated from each other by a distance of 50 to 100 nm.
Basal body (i.e., centriole) development has been studied in the ciliates Paramecium and Tetrahymena and in tracheal epithelium of Xenopus and chicks. The stages of development are virtually the same in all of these. Development of the basal body begins with the formation of a single microtubule in an amorphous mass.
Microtubules are added one at time until there is an equally spaced ring of nine (Fig. 23-20). As the microtubules appear the amorphous mass is lost, as though it were being consumed in the production of the microtubules. There is some evidence that “connectives” exist between the microtubules, which could act to set the distance between them.
Each of the nine microtubules in the ring is microtubules. The b microtubules develop next and, finally, the c microtubules. Before the microtubules reach the doublet stage, the cylinder is rarely longer than 70 nm, but after this stage, the microtubules elongate. At the same time, the hub and “cartwheel” are added in the center (Fig. 23-20b). The a-c links are not formed until the end of development.
Basal bodies act as organizing centers for the development of the microtubules of cilia and flagella. New basal bodies form adjacent to centrioles. While still not associated with a flagellum or cilium, the basal body is more properly called a centriole, but after it migrates to a position just underneath the plasma membrane and acts as a center for flagellum or cilium development, it is called a basal body. The synthetic functions of centrioles and basal bodies are not clear. It has been suggested that these bodies may contain DNA and carry out transcription, but so far only RNA has been reported to be present.
Cilia and Flagella:
Cilia and flagella are organelles that project from the surface of certain cells and beat back and forth or create a corkscrew action (Fig. 23-21). In many instances, ciliary or flagellar movements propel cells through their environment. In other cases, the cell remains stationary and the surrounding medium is moved past the cell by the beating of its cilia (as in the layer of epithelial cells that lines the trachea or the collar cells lining the internal chambers of sponges).
Cilia are generally shorter than flagella (i.e., 5-10 μm versus 150 μm or longer) and are present in far larger numbers per cell. Flagella usually occur alone or in small groups occasionally they are present in large numbers, as in a few protozoa and the sperm cells of more advanced plants. The distinction between cilia and flagella is somewhat arbitrary, because other than differences in their lengths, the structure and action of cilia and flagella of eukaryotic cells are identical. (Bacterial flagella differ in structure and action see below.)
A eukaryotic cilium or flagellurn is composed of three major parts: a central axoneme or shaft, the surrounding plasma membrane, and the interposed cytoplasmic matrix (Fig. 23-22). The axonemal elements of nearly all cilia and flagella (as well as the tails of sperm cells) contain the same + 2” arrangement of microtubules. In the center of the axoneme are two singlet microtubules that run the length of the cilium (Fig. 23-23). Projections from the central microtubules occurring periodically along their length form what appears to be an enclosing sheath. Each of the central microtubules is composed of 13 proto-filaments.
Nine doublet microtubules surround the central sheath. One microtubule of each doublet (i.e., the A subfiber) is composed of 13 proto-filaments. The adjoining B sub-fiber is “incomplete,” consisting of 11 proto-filaments (Fig. 23-23). Radial spokes having a periodicity of 24, 32, and 40 nm extend from each Asubfiber inward to the central sheath (i.e., they occur at repeating 24-, 32-, and 40-nm intervals along the axoneme’s length). Adjacent doublets are joined by nexin or inter-doublet links the nexin links have a periodicity of 86 nm.
Extending from each A subfiber are two dynein arms—an “outer” arm and an “inner” arm (see Fig. 23-23). Thin projections from the ends of the dynein arms touch the B sub-fibers of the neighboring doublets. The outer dynein arms occur with a periodicity of 24 nm, whereas the inner arms have a periodicity of 24, 32, and 40 nm (the same as the radial spokes).
The cross section of the cilium depicted in Figure 23-23 is at a level that includes radial spokes, nexin links, and both dynein arms. Each beat of a cilium or flagellum involves the same pattern of microtubule movement. The beat may be divided into two phases, the power or effective stroke and the recovery stroke (Fig. 23-24). The power stroke occurs in a single plane, but recovery may not occur in the same plane as the power stroke.
The sliding microtubule model of ciliary movement is accepted by most investigators. In this model, the doublet microtubules retain a constant length and slide past one another in such a manner as to produce localized bending of the cilium. This activity is powered by ATP hydrolysis and the outer and inner dynein arms have been shown to contain most of the cilium’s ATPase activity.
The localized bending takes the form of a wave that begins at one end of the organelle and proceeds toward the other (usually, but not always, from base to tip). The localized bending is produced through the cyclic formation and breakage of links between the dynein arms of one doublet and the neighboring doublet.
The protein filaments that make up each doublet are rows of tubulin molecules that apparently contain the sites to which the dynein binds. The fact that the sliding of microtubules past one another results in bending of the cilium may be explained by the behavior of the radial spokes that connect the outer nine doublets to the central sheath.
In straight regions of the axoneme, the radial spokes are aligned perpendicular to the doublets from which they arise, whereas in the bent regions they are tilted and stretched (Fig. 23-25). Firm attachment of the radial spokes at both ends provides the resistance necessary to translate the sliding of the doublets into a bending action. Indeed, if the radial spokes and nexin links of sperm tails are destroyed by exposure to trypsin, addition of ATP results in the axonemes becoming longer and thinner, for microtubule sliding is no longer resisted. In effect, sliding is uncoupled from bending by elimination of the connections between the doublets and the central sheath.
Analogies are evident between the dynein-tubulin system of cilia and flagella and the actin-myosin system of muscle. However, whereas muscle fibers can only shorten and relax, cilia are capable of a much larger variety of movements. Ciliary and flagellar activity can be in a single plane, in three-dimensional or helical strokes, and can move the cell forward or backward using waves that are propagated from base to tip or from tip to base.
At the chemical level, whereas Ca 2+ appears to activate the actin-myosin system, these ions have the opposite effect on the dyneintubulin system. The regulation of the Ca 2 + level in a cilium or flagellurn probably involves the plasma membrane surrounding the axoneme. Under normal circumstances (i.e., during periods of continuous beating), the internal Ca 2+ level is low (about 0.1μm) whereas Mg 2 + (necessary to stimulate the ATPase of the membrane) remains in the millimole range.
When the membrane is depolarized, the Ca 2 + level inside the cilium increases and beating ceases. ATP is clearly the source of energy for movement and is produced by cellular respiration. In many cells, mitochondria are located adjacent to the basal body of the cilium or flagellurn, and ATP diffuses toward the tip of the organelle. In sperm, a large mitochondrion is an integral part of the tail (Fig. 23-26) and is wrapped in a spiral about the middle piece of the axoneme.
Flagella of bacteria are different from cilia and flagella of eukaryotic cells. The bacterial flagellum is not covered by the plasma membrane rather it consists of a naked spiral filament about 13.5 nm in diameter and 10-15 nm long. The filament is composed of a chain of repeating protein subunits called flagellin. At its basal end, the spiral filament is attached to a “hook,” which in turn is connected to a rod that penetrates the bacterial cell wall and plasma membrane (Fig. 23-27). A number of rings connect the rod with the membrane and wall layers.
Bacterial flagella work by rotation of the rod and hook, which causes the filament to spin. When the filament spins in a counterclockwise direction, the cell is propelled smoothly and in a straight line, but when the spin is clockwise, a chaotic tumbling motion of the cell is observed.
A period of counterclockwise rotation is followed by a burst of clockwise rotation, so that the bacterium is continuously set off along new linear paths. The source of energy for the rotation (believed to be generated at the cell membrane) is an electrochemical gradient established by an electron transport system that acts across the plasma membrane.
The Mitotic Spindle:
Most studies of chromosome movement during mitosis have focused on the role played by the microtubules that make up the mitotic spindle fibers.
The spindle fibers cause three distinct chromosome movements during mitosis:
(1) orientation of sister chromatids,
(2) alignment of the centromeres on the metaphase plate, and
(3) separation of centromeres and movement of sister chromatids (segregation) to opposite poles of the spindle.
The microtubules that occur in the spindle include:
(1) The centromere microtubules, which terminate in a centromere
(2) The polar microtubules, which terminate at the poles and
(3) The free microtubules, which do not terminate in either a pole or a centromere.
All three types can be dissociated into tubulin subunits by colchicine or cold temperatures. Over the years, several models have been proposed to account for the movements of the chromosomes during anaphase. For example, it has been suggested that the chromosomes are “pushed apart by spindle fibers developing between centromeres that they are pulled apart by spindle fibers extending between the centromeres and the poles of the spindle, and that chromosomes migrate along spindle fibers.
Although individual spindle fibers do not stretch or contract per se, they do change in length through either addition or removal of subunits. S. Inoue has shown that free microtubules alternately grow and decrease in length. His in vitro studies indicate that the microtubules assemble at one end and disassemble at the other.
Other Cell Movements:
Regardless of whether the movements are “internal” (such as cyclosis in plant cells) or result in a major change in shape or position of the cell, the present evidence indicates that microfilaments and/or microtubules are fundamental to these activities. In most cases, an interaction between proteins—such as in the actin-myosin or dynein-tubulin systems—with the simultaneous involvement of an ATPase is the underlying biochemical phenomenon.
Regulation at Internal Checkpoints
It is essential that daughter cells be exact duplicates of the parent cell. Mistakes in the duplication or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced from the abnormal cell. To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell cycle checkpoints at which the cell cycle can be stopped until conditions are favorable. These checkpoints occur near the end of G1, at the G2&ndashM transition, and during metaphase (Figure 6.2.5).
Figure 6.2.5: The cell cycle is controlled at three checkpoints. Integrity of the DNA is assessed at the G1 checkpoint. Proper chromosome duplication is assessed at the G2 checkpoint. Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint.
The G1 Checkpoint
The G1 checkpoint determines whether all conditions are favorable for cell division to proceed. The G1checkpoint, also called the restriction point, is the point at which the cell irreversibly commits to the cell-division process. In addition to adequate reserves and cell size, there is a check for damage to the genomic DNA at the G1 checkpoint. A cell that does not meet all the requirements will not be released into the S phase.
The G2 Checkpoint
The G2 checkpoint bars the entry to the mitotic phase if certain conditions are not met. As in the G1checkpoint, cell size and protein reserves are assessed. However, the most important role of the G2checkpoint is to ensure that all of the chromosomes have been replicated and that the replicated DNA is not damaged.
The M Checkpoint
The M checkpoint occurs near the end of the metaphase stage of mitosis. The M checkpoint is also known as the spindle checkpoint because it determines if all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to spindle fibers arising from opposite poles of the cell.
Watch what occurs at the G1, G2, and M checkpoints by visiting this animation of the cell cycle.
Briefly describe the events that occur in each phase of interphase.
During G1, the cell increases in size, the genomic DNA is assessed for damage, and the cell stockpiles energy reserves and the components to synthesize DNA. During the S phase, the chromosomes, the centrosomes, and the centrioles (animal cells) duplicate. During the G2 phase, the cell recovers from the S phase, continues to grow, duplicates some organelles, and dismantles other organelles.
Chemotherapy drugs such as vincristine and colchicine disrupt mitosis by binding to tubulin (the subunit of microtubules) and interfering with microtubule assembly and disassembly. Exactly what mitotic structure is targeted by these drugs and what effect would that have on cell division?
The mitotic spindle is formed of microtubules. Microtubules are polymers of the protein tubulin therefore, it is the mitotic spindle that is disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during mitosis. The cell will arrest in mitosis and die.
Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.
There are very few similarities between animal cell and plant cell cytokinesis. In animal cells, a ring of actin fibers is formed around the periphery of the cell at the former metaphase plate (cleavage furrow). The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two. In plant cells, a new cell wall must be formed between the daughter cells. Due to the rigid cell walls of the parent cell, contraction of the middle of the cell is not possible. Instead, a phragmoplast first forms. Subsequently, a cell plate is formed in the center of the cell at the former metaphase plate. The cell plate is formed from Golgi vesicles that contain enzymes, proteins, and glucose. The vesicles fuse and the enzymes build a new cell wall from the proteins and glucose. The cell plate grows toward and eventually fuses with the cell wall of the parent cell.
List some reasons why a cell that has just completed cytokinesis might enter the G0 phase instead of the G1 phase.
Many cells temporarily enter G0 until they reach maturity. Some cells are only triggered to enter G1 when the organism needs to increase that particular cell type. Some cells only reproduce following an injury to the tissue. Some cells never divide once they reach maturity.
What cell cycle events will be affected in a cell that produces mutated (non-functional) cohesin protein?
If cohesin is not functional, chromosomes are not packaged after DNA replication in the S phase of interphase. It is likely that the proteins of the centromeric region, such as the kinetochore, would not form. Even if the mitotic spindle fibers could attach to the chromatids without packing, the chromosomes would not be sorted or separated during mitosis.