Are Chromosomes Visible During Interphase

The five phases of mitosis and cell division tightly coordinate the movements of hundreds of proteins. How did early biologists unravel this complex trip the light fantastic toe of chromosomes?

Perhaps the nigh astonishing thing about mitosis is its precision, a feature that has intrigued biologists since Walther Flemming outset described chromosomes in the late 1800s (Paweletz, 2001). Although Flemming was able to correctly deduce the sequence of events in mitosis, this sequence could not be experimentally verified for several decades, until advances in light microscopy made it possible to find chromosome movements in living cells. Researchers now know that mitosis is a highly regulated process involving hundreds of different cellular proteins. The dynamic nature of mitosis is best appreciated when this process is viewed in living cells.

Mitosis Occupies a Portion of the Jail cell Bicycle

In his pioneering studies of mitosis, Flemming noted that the nuclear material, which he named "chromatin" for its ability to take upward stains, did not have the aforementioned appearance in all cells. (We still apply the word "chromatin" today, albeit in a more than biochemical sense to refer to complexes of nuclear DNA and protein.) Specifically, in some cells, chromatin appeared as an amorphous network, although in other cells, it appeared as threadlike bodies that Flemming named "mitosen." Based on his observations, Flemming had the insight to propose that chromatin could undergo reversible transformations in cells. Today, scientists know that Flemming had successfully distinguished chromosomes in the interphase portion of the cell bike from chromosomes undergoing mitosis, or the portion of the cell cycle during which the nucleus divides (Figure 1). With very few exceptions, mitosis occupies a much smaller fraction of the prison cell cycle than interphase.

The difference in DNA compaction between interphase and mitosis is dramatic. A precise approximate of the difference is not possible, simply during interphase, chromatin may be hundreds or even thousands of times less condensed than it is during mitosis. For this reason, the enzyme complexes that copy DNA have the greatest access to chromosomal Deoxyribonucleic acid during interphase, at which time the vast bulk of gene transcription occurs. In addition, chromosomal Dna is duplicated during a subportion of interphase known every bit the S, or synthesis, phase. As the ii girl Deoxyribonucleic acid strands are produced from the chromosomal DNA during S stage, these girl strands recruit additional histones and other proteins to form the structures known equally sister chromatids (Figure 2). The sis chromatids, in turn, become "glued" together by a poly peptide complex named cohesin. Cohesin is a member of the SMC, or structural maintenance of chromosomes, family of proteins. SMC proteins are Dna-binding proteins that touch on chromosome architectures; indeed, cells that lack SMC proteins prove a variety of defects in chromosome stability or chromosome behavior. Current data suggest that cohesin complexes may literally course circles that encompass the two sister chromatids (Hirano, 2002; Hagstrom & Meyer, 2003). At the end of South phase, cells are able to sense whether their Dna has been successfully copied, using a complicated prepare of checkpoint controls that are still not fully understood. For the nearly office, merely cells that have successfully copied their Deoxyribonucleic acid volition continue into mitosis.

Chromatin Is Extensively Condensed as Cells Enter Mitosis

The most obvious difference betwixt interphase and mitosis involves the advent of a cell'southward chromosomes. During interphase, private chromosomes are non visible, and the chromatin appears lengthened and unorganized. Recent research suggests, however, that this is an oversimplification and that chromosomes may actually occupy specific territories inside the nucleus (Cremer & Cremer, 2001). In any case, as mitosis begins, a remarkable condensation procedure takes place, mediated in function by some other member of the SMC family, condensin (Hirano, 2002; Hagstrom & Meyer, 2003). Like cohesin, condensin is an elongated complex of several proteins that binds and encircles DNA. In dissimilarity to cohesin, which binds ii sister chromatids together, condensin is idea to bind a unmarried chromatid at multiple spots, twisting the chromatin into a variety of coils and loops (Effigy 3).

The Mitotic Spindle Aids in Chromosome Separation

During mitosis, chromosomes become attached to the structure known as the mitotic spindle. In the late 1800s, Theodor Boveri created the primeval detailed drawings of the spindle based on his observations of cell division in early on Ascaris embryos (Figure iv; Satzinger, 2008). Boveri'due south drawings, which are amazingly accurate, show chromosomes attached to a bipolar network of fibers. Boveri observed that the spindle fibers radiate from structures at each pole that nosotros now recognize equally centrosomes, and he also noted that each centrosome contains two small, rodlike bodies, which are now known as centrioles. Boveri observed that the centrioles indistinguishable earlier the chromosomes become visible and that the two pairs of centrioles movement to dissever poles before the spindle assembles. We now know that centrioles duplicate during S phase, although many details of this duplication process are withal under investigation.

The composition of the spindle fibers remained unknown until the 1960s, when tubulin was discovered and techniques were developed for visualizing spindles using electron microscopes (Mitchison & Salmon, 2001). It is now well-established that spindles are bipolar arrays of microtubules composed of tubulin (Figure 5) and that the centrosomes nucleate the growth of the spindle microtubules. During mitosis, many of the spindle fibers attach to chromosomes at their kinetochores (Figure six), which are specialized structures in the most constricted regions of the chromosomes. The length of these kinetochore-attached microtubules so decreases during mitosis, pulling sis chromatids to opposite poles of the spindle. Other spindle fibers do not attach to chromosomes, but instead form a scaffold that provides mechanical force to separate the daughter nuclei at the end of mitosis.

Mitosis Is Divided into Well-Divers Phases

From his many detailed drawings of mitosen, Walther Flemming correctly deduced, but could not bear witness, the sequence of chromosome movements during mitosis (Figure 7). Flemming divided mitosis into 2 broad parts: a progressive phase, during which the chromosomes condensed and aligned at the center of the spindle, and a regressive phase, during which the sis chromatids separated. Our modern understanding of mitosis has benefited from advances in light microscopy that have allowed investigators to follow the process of mitosis in living cells. Such live cell imaging non but confirms Flemming's observations, but it too reveals an extremely dynamic process that can only be partially appreciated in still images.

A multi-panel diagram (labeled A through G) shows an animal cell in interphase, prophase, prometaphase, metaphase, anaphase, and cytokinesis. Different colors distinguish newly replicated DNA from original DNA. In addition to the schematic diagrams, panels B through G include photomicrographs of plant cells in each phase.

Today, mitosis is understood to involve five phases, based on the physical state of the chromosomes and spindle. These phases are prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis is the last concrete jail cell segmentation that follows telophase, and is therefore sometimes considered a 6th phase of mitosis. All phases of mitosis, also as the flanking periods of interphase and cytokinesis before and later, are shown in Figure viii. Researchers' biochemical understanding of mitotic phases has greatly increased in recent years (Mitchison & Salmon, 2001), revealing that this highly orchestrated process involves hundreds, if not thousands, of cellular proteins.

Prophase

Mitosis begins with prophase, during which chromosomes recruit condensin and brainstorm to undergo a condensation process that will proceed until metaphase. In most species, cohesin is largely removed from the arms of the sister chromatids during prophase, allowing the private sister chromatids to be resolved. Cohesin is retained, however, at the most constricted part of the chromosome, the centromere (Figure 9). During prophase, the spindle too begins to form as the ii pairs of centrioles move to contrary poles and microtubules begin to polymerize from the duplicated centrosomes.

Prometaphase

Prometaphase begins with the abrupt fragmentation of the nuclear envelope into many pocket-size vesicles that will eventually be divided between the future girl cells. The breakdown of the nuclear membrane is an essential step for spindle associates. Considering the centrosomes are located exterior the nucleus in animal cells, the microtubules of the developing spindle practise not accept access to the chromosomes until the nuclear membrane breaks apart.

Prometaphase is an extremely dynamic part of the cell cycle. Microtubules quickly get together and disassemble as they grow out of the centrosomes, seeking out attachment sites at chromosome kinetochores, which are circuitous platelike structures that assemble during prometaphase on one confront of each sister chromatid at its centromere. Every bit prometaphase ensues, chromosomes are pulled and tugged in reverse directions by microtubules growing out from both poles of the spindle, until the pole-directed forces are finally balanced. Sister chromatids exercise non break apart during this tug-of-war because they are firmly fastened to each other by the cohesin remaining at their centromeres. At the end of prometaphase, chromosomes take a bi-orientation, pregnant that the kinetochores on sister chromatids are connected by microtubules to opposite poles of the spindle.

Metaphase

Side by side, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle. Metaphase is particularly useful in cytogenetics, considering chromosomes can be most easily visualized at this phase. Furthermore, cells can be experimentally arrested at metaphase with mitotic poisons such as colchicine. Video microscopy shows that chromosomes temporarily cease moving during metaphase. A complex checkpoint mechanism determines whether the spindle is properly assembled, and for the most part, but cells with correctly assembled spindles enter anaphase.

Anaphase

A schematic diagram shows cohesin molecules being degraded by the separase enzyme in-between two separating sister chromatids. The chromatids are shown as grey, vertically-aligned noodles arranged beside each other in parallel. At the center of each sister chromatid is a dark grey half-circle, representing part of the centromere where the two sister chromatids were once together.

The progression of cells from metaphase into anaphase is marked by the precipitous separation of sis chromatids. A major reason for chromatid separation is the precipitous degradation of the cohesin molecules joining the sister chromatids by the protease separase (Figure ten).

2 separate classes of movements occur during anaphase. During the first function of anaphase, the kinetochore microtubules shorten, and the chromosomes motility toward the spindle poles. During the second role of anaphase, the spindle poles carve up as the non-kinetochore microtubules movement past each other. These latter movements are currently thought to exist catalyzed by motor proteins that connect microtubules with opposite polarity and then "walk" toward the end of the microtubules.

Telophase and Cytokinesis

Mitosis ends with telophase, or the phase at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes brainstorm to decondense into their interphase conformations. Telophase is followed by cytokinesis, or the sectionalization of the cytoplasm into two daughter cells. The daughter cells that issue from this process have identical genetic compositions.

References and Recommended Reading

Cheeseman, I. M., & Desai, A. Molecular architecture of the kinetochore-microtubule interface. Nature Reviews Molecular Cell Biology ix, 33–46 (2008) doi:10.1038/nrm2310 (link to article)

Cremer, T., & Cremer, C. Chromosome territories, nuclear architecture and gene regulation in mammalian cells. Nature Reviews Genetics 2, 292–301 (2001) doi:10.1038/35066075 (link to article)

Hagstrom, K. A., & Meyer, B. J. Condensin and cohesin: More than chromosome compactor and glue. Nature Reviews Genetics 4, 520–534 (2003) doi:ten.1038/nrg1110 (link to article)

Hirano, T. At the heart of the chromosome: SMC proteins in action. Nature Reviews Molecular Cell Biological science vii, 311–322 (2002) doi:ten.1038/nrm1909 (link to article)

Mitchison, T. J., & Salmon, E. D. Mitosis: A history of partitioning. Nature Cell Biological science 3, E17–E21 (2001) doi:10.1038/35050656 (link to article)

Paweletz, N. Walther Flemming: Pioneer of mitosis inquiry. Nature Reviews Molecular Cell Biology two, 72–75 (2001) doi:x.1038/35048077 (link to commodity)

Satzinger, H. Theodor and Marcella Boveri: Chromosomes and cytoplasm in heredity and development. Nature Reviews Genetics 9, 231–238 (2008) doi:10.1038.nrg2311 (link to article)