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You are watching: Why does cell division take place in single-celled organisms

Griffiths AJF, Gelbart WM, Miller JH, et al. Modern Genetic Analysis. New York: W. H. Freeman; 1999.

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Cell division is the basis for all forms of organismal remanufacturing. Single-celled organismsdivide to recreate. Cell department in multicellular organisms produces specialized reproductivecells, such as egg and sperm, and is also responsible for the breakthrough of a many-celledorganism from a single fertilized egg cell. In order for a cell to divide, the genome must alsodivide, so, in all forms of cell department in all organisms, DNA replication comes before celldepartment.

The major types of cell department are shown in Figure 4-6on the adhering to page. Broadly they deserve to be grouped into asex-related and sexual cell department.


Figure 4-6

The different kinds of cell departments and also linked departments of genomes. M = mitosis;Mei = meiosis.

Asexual Cell Division

In prokaryotes tright here is only one quite basic type of cell division, which produces twoidentical daughter cells from one progenitor cell. This is asexual cell division,because it involves no sex-related union of various people. Prior to bacterial cell division,DNA replicates and also two complete circular genomes outcome. As the cell divides, among the daughterDNA molecules passes right into each of the daughter cells. The specific mechanism of DNA movement isnot well-known, but in one model the DNA is anchored to the cell membrane, and as the membranebroadens to make two cells, the DNA is drawn together with it.

Eukaryotes likewise display asexual cell department, likewise making two identical daughter cells from oneprogenitor cell. This is the form of cell department that converts a solitary fertilized egg cell,a zygote, into two cells, then four, then eight,and also so on until an organism composed of many cells is produced. It is also the type of celldepartment that geneprices a populace of single-celled organisms from one, for instance, inyeasts and also protozoans. When a eukaryotic cell divides asexually, the nucleus and also its geneticcontents need to divide as well, in a procedure referred to as mitosis. Both haploid (n) and diploid (2n) cells can divide asexually, at which timetheir nuclei divide mitotically. Figure 4-7 shows whereasex-related (mitotic) and also sex-related (meiotic) cell division happen in the life cycles of people,plants, and also fungi. At mitosis the programmed movements of the chromosomes guarantee that eachdaughter cell has chromosomes and also genes similar with those of the other daughter cell and also theoriginal cell.


Figure 4-7

Life cycles of human beings, plants, and fungi, showing the divisions in which mitosis andmeiosis take place. Keep in mind that in human beings and also many kind of plants, three cells of the meiotic tetrad abort.The abbreviation n shows a haploid cell, 2n a diploidcell; gp stands (more...)

From any particular phase in a progenitor cell to the very same stage in a daughter cell is calledone cell division cycle. It is written of four stages: S (DNAsynthesis), M (mitosis), and also G1and G2 (gaps, or intermediate stages), as was displayed in Figure 4-6. Passage of the cell right into each of these sequentialstperiods is a precisely regulated process, overwatched by a battery of diverse genes whose job is toencertain that this sequence is carried out correctly.

During S phase the DNA of each chromosome replicates semiconservatively. The chromosomesplits longitudinally to create a pair of sister chromatids, each of which contains one of thereplicated DNA molecules. The chromatids end up being visible microscopically only during mitosis,once they shorten and thicken as a result of increased coiling of the DNA and its associatedhistones. However, it is important to remember that the replication right into chromatids took placein the time of premitotic S phase. Chromatids and also replicated DNA are illustrated in Figure 4-8 on the following web page. As examples, the number provides diploid cellsof genoform b+/b+,b+/b, and b/b, and haploid cellsof genoform b+ and b. It is assumed that themutant allele b has been created by replacement of a G·C base pair in thewild-type allele via an A·T base pair in the mutant allele (this is a prevalent form ofmutational event).


Figure 4-8

Chromatid development and underlying DNA replication. (Left) Eachchromosome divides longitudinally into two chromatids; (right) at themolecular level, the single DNA molecule of each chromosome replicates, creating 2 DNAmolecules, one for each chromatid. (more...)

During mitosis, a netjob-related of proteinaceous microtubules called the spindleapparatus forms parallel to the cell axis, connecting the poles of the cell. One tomany microtubules from one pole attach to one chromatid, and also a comparable number from the oppositepole affix to the various other chromatid of a chromosome. The attachment point on the chromosome isthe centromere, a specific DNA sequence vital for chromatid movement in the time of mitosis. Thecentromere is replicated throughout the formation of sister chromatids, and each sister centromereacts as a binding website for a multiprotein complicated referred to as the kinetochore. The kinetochores consequently act as the sites for attaching tomicrotubules. The spindle fibers (microtubules) then pull sister chromatids to opposite poles.Thus each pole receives a copy of each chromosome from the parent cell. The sets of sisterchromatids at each pole end up being incorporated into the nuclei of the two daughter cells. Thesedaughter nuclei are similar via each various other and also via the nucleus from which they wereacquired. In the daughter cells, chromatids are aobtain referred to as chromosomes. Notethat it is the spindle apparatus and the kinetochore-centromere complex that determine thefidelity of nuclear department.

The major genetic events of the S phase and also mitosis are diagrammed in the left and centralcolumns in Figure 4-9 on web page 97. The figure provides ahaploid of genotype A and also a diploid of genoform A/a to shownot just that mitosis geneprices two chromosomally the same cells but also that the specificallelic constitution is additionally the same. A thorough account of the stperiods of mitosis is provided inGenetics in Process 4-2.


Figure 4-9

DNA and also gene transmission during asexual and sex-related cell departments in eukaryotes. S phaseand the primary stperiods of mitosis and meiosis are presented. The diagrams emphadimension the DNA contentof each cell and chromosome. The first 2 panels in each column present (even more...)

Sexual Cell Division

Although some bacteria have a simple create of sex-related reproduction (check out Chapter 9), there is no linked specialized cell department. However, the majority of eukaryotic organisms deserve to reproduce sexually, and also during the sexual partof the life cycle specialized sexual cell divisions take place. In plants and also animals, the sexualcell divisions cause eggs and sperm (gametes); in fungi, sexual cell divisions outcome insex-related spores such as ascospores. The founding suggest for sexual cell division is always adiploid cell referred to as a meiocyte (Figure 4-6). In the majority of complex organisms, such as animals andflowering plants, the cells of the organism are typically diploid and the meiocytes are simply asubpopulation of cells that are set aside for sexual division—for instance, those found intestes and ovaries in animals. In haploid organisms a transient diploid meiocyte is constructedas component of the normal refertile cycle (Figure 4-7).In all eukaryotes the meiocyte divides twice, bring about 4 haploid cells called a tetrad. The 2 nuclear divisions that acagency thetwo sex-related cell divisions are referred to as meiosis.

In contrast to mitosis, whose aim is a conservative propagation of onegenoform, meiosis is a diversity-generating process. It shuffles alleliccombinations so that if the meiocyte has heterozygous pairs of al-leles (and a lot of do), thecells that recurrent the products of meiosis (that is, the cells of the tetrads)will contain many type of various combicountries of these alleles. The main sites of meiosis aredepicted in the life cycle diagrams in Figure 4-7.

Meiosis is preyielded by the DNA synthesis phase in the diploid meiocyte. This achieves thesame outcome as the S phase before a diploid mitosis—each chromosome in the 2 sets isreplicated right into a pair of sister chromatids, specifically as shown in Figure 4-8. Just before the first nuclear department, homologous chromosomespair alengthy their lengths, so that now for each chromosomal kind tright here are two pairs of sisterchromatids juxtaposed, making a bundle of 4 likewise referred to as a tetrad. Figure 4-9 mirrors a meiotictetrad of homologous chromosomes. (Contrast this stacking of homologs with metaphase inmitosis, in which chromosomes line up side by side, rather.) At the tetrad stage a remarkableprocedure occurs: paired nonsister chromatids exchang homologous sections of DNAthrough breakage and reunion of their arms at points called crossovers.

At the first division of meiosis, centromeres act as though they are still unseparated,although it is recognized that replication of the centromeric DNA has actually occurred. Spindle fibers fromeach pole affix and also pull homologous centromeres, in addition to their sister chromatids, toopposite poles (Figure 4-9). As such in the two cellsdeveloped by the first division the variety of centromeres is halved. At the second department ofmeiosis the centromeres divide, and spindle fibers affix and also pull one sister chromatid to eachpole (Figure 4-9). Hence in its entirety the 2 divisions ofmeiosis produce 4 cells, each of which includes the haploid variety of chromosomes. This isinescapable because tright here is just one doubling of hereditary product (premeiotic S phase) and also twohalvings of the genetic material, arising throughout the 2 cell divisions. As we have seen,this team of four haploid cells (which constitute the assets of a single meiosis) is alsoreferred to as a tetrad; the bundle of 4 chromatids that constitutes a tetrad ispartitioned at meiosis right into these 4 cells. In the assets of meiosis, chromatids are onceagain called chromosomes.Figure 4-9 shows that if a diploid meiocyte isheterozygous (for instance A/a) then half the haploid commodities of meiosis willlug the A allele and fifty percent will certainly bring a. These alleles areshelp to segregate at meiosis because they sepaprice into different haploidcells.

Some essential attributes of meiosis are as follows:


Tbelow is one round of DNA replication but two rounds of nuclear department (generally tworounds of cell division as well). Thus, at the end of meiosis, the number of chromosomes percell is halved.


At the first meiotic division, homologous centromeres are pulbrought about oppowebsite poles of thecell by spindle fibers. Since spindle fibers from one pole affix randomly to either of apair of homologous chromosomes (through their centromeres), heterologous centromeres andtheir attached chromosomes integrate randomly. This independent assortment outcomes in manydifferent genotypes and is one resource of the diversity produced by meiosis.


Early in the first meiotic department, tright here is exreadjust of chromosomal product betweenhomologous chromatids as an outcome of crossing-over. Crossovers serve 2 duties. First, theyare the various other excellent source of genotypic diversity, considering that no two crossed-over chromatids areprecisely the exact same. Second, the exchange events that occur in between homologous chromatids in thetetrad serve to organize the tetrad together until the homologs pull acomponent at the finish of thefirst division. This ensures appropriate chromosome segregation and also stays clear of the development ofproducts of meiosis bearing abnormal chromosome numbers.


The process of meiosis starts via diploid meiocytes in the refertile tconcern andproduces an array of haploid cells through varied genoforms.

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The distinctions between meiosis and mitosis are summarized in Figure 4-10. Meiosis itself is defined in even more detail in Genetics in Process 4-3.