Biology

Meiosis 

"Meiosis is a process that includes two sequential nuclear divisions, producing haploid daughter nuclei that contain only one member of each pair of homologous chromosomes, thus reducing the number of chromosomes in half."

Explanation 

The production of offspring by sexual reproduction includes the union(fertilisation)of two cells,each with a haploid set of chromosomes.

The doubling of the chromosome number at fertilization is compensated by an equivalent reduction in chromosome number at a stage prior to formation of the gametes. This is accomplished by meiosis, a term coined in 1905 from the Greek word meaning “reduction.” Meiosis ensures production of a haploid phase in the life cycle, and fertilization ensures a diploid phase. Without meiosis, the chromosome number would double with each generation, and sexual reproduction would not be possible.

To compare the events of mitosis and meiosis, we need to examine the fate of the chromatids. Prior to both mitosis and meiosis, diploid G2 cells contain pairs of homologous chromosomes, with each chromosome consisting of two chromatids. During mitosis, the chromatids of each chromosome are split apart and separate into two daughter nuclei in a single division. As a result, cells produced by mitosis contain pairs feat by incorporating two sequential divisions without an intervening round of DNA replication (Figure below).

Meiosis consist of two consecutive divisions,meiosis I and meiosis II.

Meiosis I

In the first meiotic division (meiosis I) , each chromosome (consisting of two chromatids) is separated from its homologue. As a result, each daughter cell contains only one member of each pair of homologous chromosomes. For this to occur, homologous chromosomes are paired during prophase of the first meiotic division (prophase I, Figure below) by an elaborate process that has no counterpart in mitosis.

As they are paired, homologous chromosomes engage in a process of genetic recombination that produces chromosomes with new combinations of maternal and paternal alleles (see metaphase I, Figure above).

In the second meiotic division(meiosis-II) ,the two chromatids of each chromosome are separated from one another (anaphase II,Figure above).

A survey of various eukaryotes reveals marked differences with respect to the stage within the life cycle at which meiosis occurs and the duration of the haploid phase. The following three groups (Figure below) can be identified on these bases:

Figure:A comparison of three major groups of organisms based on the stage within the life cycle at which meiosis occurs and the duration of the haploid phase.

1. Gametic or terminal meiosis. In this group, which includes all multicellular animals and many protists, the meiotic divisions are closely linked to the formation of the gametes (above Figure, left). In male vertebrates (Figure below),

Figure:The stages of gametogenesis in vertebrates: a comparison between the formation of sperm and eggs. In both sexes, a relatively small population of primordial germ cells present in the embryo proliferates by mitosis to form a population of gonial cells(spermatogonia or oogonia) from which the gametes differentiate. In the male, meiosis occurs before differentiation.

for example, meiosis occurs just prior to the differentiation of the spermatozoa. Spermatogonia that are committed to undergo meiosis become primary spermatocytes, which then undergo the two divisions of meiosis to produce four relatively undifferentiated spermatids. Each spermatid undergoes a complex differentiation to become the highly specialized sperm cell (spermatozoon). In female vertebrates (Figure below),

Figure:Both meiotic divisions occur after differentiation. Each primary spermatocyte generally gives rise to four viable gametes,whereas each primary oocyte forms only one fertilizable egg and two or three polar bodies.

Oogonia become primary oocytes, which then enter a greatly extended meiotic prophase. During this prophase, the primary oocyte grows and becomes filled with yolk and other materials. It is only after differentiation of the oocyte is complete (i.e.,the oocyte has reached essentially the same state as when it is fertilized) that the meiotic divisions occur. Vertebrate eggs are typically fertilized at a stage before the completion of meiosis (usually at metaphase II). Meiosis is completed after fertilization, while the sperm resides in the egg cytoplasm.

2. Zygotic or initial meiosis. In this group, which includes only protists and fungi, the meiotic divisions occur just after fertilization (main above Figure, right) to produce haploid spores. The spores divide by mitosis to produce a haploid adult generation. Consequently, the diploid stage of the life cycle is restricted to a brief period after fertilization when the individual is still a zygote.

3. Sporic or intermediate meiosis. In this group, which includes plants and some algae, the meiotic divisions take place at a stage unrelated to either gamete formation or fertilization (main above Figure, center). If we begin the life cycle with the union of a male gamete (the pollen grain) and a female gamete (the egg), the diploid zygote undergoes mitosis and develops into a diploid sporophyte. At some stage in the development of the sporophyte, sporogenesis (which includes meiosis) occurs, producing spores that germinate directly into a haploid gametophyte. The gametophyte can be either an independent stage or, as in the case of seed plants, a tiny structure retained within the ovules. In either case, the gametes are produced from the haploid gametophyte by mitosis.

"Meiotic cells have an interphase period that is similar to mitosis,with G1, S, and G2 phases. After interphase, germ-line cells enter meiosis I."

Stages of Meiosis-I (Reduction Division)
There are following stages of meiosis I.

Prophase I,Sets the Stage for the Reductive Division
During prophase I, homologous chromosomes pair.(see figure).

In prophase I, the DNA coils tighter, and individual chromosomes first become visible under the light microscope as a matrix of fine threads. Because DNA has already replicated before the onset of meiosis, each of these threads actually consists of two sister chromatids joined at their centromeres.
In prophase I, homologous chromosomes become closely associated, exchange segments by crossing over, and later separate.

The Prophase I of meoisis I  is divided into following stages.

Leptotene
The first stage of prophase I is leptotene, during which the chromosomes become compacted and visible in the light microscope. Although the chromosomes have replicated at an earlier stage, there is no indication that each chromosome is actually composed of a pair of identical chromatids. In the electron microscope, however, the chromosomes are revealed to be composed of paired chromatids.

Zygotene
The second stage of prophase I, which is called zygotene,is marked by the visible association of homologues with one another. This process of chromosome pairing is called synapsis and is an intriguing event with important unanswered questions: On what basis do the homologues recognize one  another? How does the pair become so perfectly aligned?When does recognition between homologues first occur? Recent studies have shed considerable light on these questions. It had been assumed for years that interaction between homologous chromosomes first begins as chromosomes initiate synapsis. However, studies on yeast cells by Nancy Kleckner and her colleagues at Harvard University demonstrated that homologous regions of DNA from homologous chromosomes are already associated with one another during leptotene.

Chromosome compaction and synapsis during zygotene simply make this arrangement visible under the microscope. As will be discussed below, the first step in genetic recombination is the deliberate introduction of double-stranded breaks in aligned DNA molecules. Studies in both yeast and mice suggest the DNA breaks occur in leptotene, well before the chromosomes are visibly paired.

These findings are supported by studies aimed at locating particular DNA sequences within the nuclei of premeiotic and meiotic cells.The individual chromosomes occupy discrete regions within nuclei rather than being randomly dispersed throughout the nuclear space. When yeast cells about to enter meiotic prophase are examined, each pair of homologous chromosomes is found to share a joint territory distinct from the territories shared by other pairs of homologues. This finding suggests that homologous chromosomes are paired to some extent before meiotic prophase begins. The telomeres (terminal segments) of leptotene chromosomes are distributed throughout the nucleus. Then, near the end of leptotene, there is a dramatic reorganization of chromosomes in many species so that the telomeres become localized at the inner surface of the nuclear envelope at one side of the nucleus.

Clustering of telomeres at one end of the nuclear envelope occurs in a wide variety of eukaryotes and causes the chromosomes to resemble the clustered stems of a bouquet of flowers. Mice carrying mutations that prevent the association of chromosomes with the nuclear envelope exhibit defects in synapsis, genetic recombination, and gamete formation. These experimental results suggest that the nuclear envelope plays an important role in the interaction between homologous chromosomes during meiosis.

Electron micrographs indicate that chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The synaptonemal complex (SC) is a ladder-like structure with transverse protein filaments
connecting the two lateral elements (see figure below).

The chromatin of each homologue is organized into loops that extend from one of the lateral elements of the SC (above Figure b).
The lateral elements are composed primarily of cohesin, which presumably binds together the chromatin of the sister chromatids. For many years, the SC was thought to hold each pair of homologous chromosomes in the proper position to initiate genetic recombination between strands of homologous DNA. It is now evident that the SC is not required for genetic recombination. Not only does the SC form after genetic recombination has been initiated, but mutant yeast cells unable to assemble an SC can still engage in the exchange of genetic information between homologues. It is currently thought that the SC functions primarily as a scaffold to allow
interacting chromatids to complete their crossover activities,as described below.

The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. The former term reflects the fact that the complex contains two homologues,whereas the latter term calls attention to the presence of four chromatids. The end of synapsis marks the end of zygotene and the beginning of the next stage of prophase I, called pachytene.

Pachytene 
Pachytene  is characterized by a fully formed synaptonemal complex. During pachytene, the homologues are held closely together along their length by the SC.see figure

The DNA of sister chromatids is extended into parallel loops(above Figure b). Under the electron microscope, a number of electron-dense bodies about 100 nm in diameter are seen within the center of the SC. These structures have been named recombination nodules because they correspond to the sites where crossing-over is taking place, as evidenced by the associated synthesis of DNA that occurs during intermediate steps of recombination. Recombination nodules contain the enzymatic machinery that facilitates genetic recombination, which is completed by the end of pachytene.
Crossing Over
Along with the synaptonemal complex that forms during prophase I,another kind of structure appears at the same time that recombination occurs. These are called recombination nodules, and they are thought to contain the enzymatic machinery necessary to break and rejoin chromatids of homologous chromosomes.

Crossing over involves a complex series of events in which DNA segments are exchanged between nonsister chromatids.

Reciprocal crossovers between nonsister chromatids are controlled such that each chromosome arm has one or a few crossovers per meiosis, no matter what the size of the chromosome. Human chromosomes, for example, typically have two or three.
When crossing over is complete, the synaptonemal complex breaks down, and the homologous chromosomes become less tightly associated but remain attached by chiasmata. At this point,for each chromosome, there are two homologues, each of which consists of two sister chromatids joined at the centromere.
The four chromatids are held together in two ways: (1) The two sister chromatids of each homologue, the products of DNA replication, are held together by cohesin proteins (sister chromatid cohesion); and (2) exchange of material by crossing over between homologues locks all four chromatids together.
While this elaborate behavior of chromosome pairing is taking place during prophase I, other key events also occur. The
nuclear envelope is dispersed, along with the interphase structure of microtubules. These microtubules then re-form into a spindle,just as in mitosis.

Diplotene
The beginning of diplotene, the next stage of meiotic prophase I (Figure),

is recognized by the dissolution of the SC, which leaves the chromosomes attached to one another at specific points by X-shaped structures, termed chiasmata (singular chiasma) (see figure ).

Chiasmata are located at sites on the chromosomes where crossing-over between DNA molecules from the two chromosomes had previously occurred. Chiasmata are formed by covalent junctions between a chromatid from one homologue and a nonsister chromatid from the other homologue. These points of attachment provide a striking visual portrayal of the extent of genetic recombination. The chiasmata are made more visible by a tendency for the homologues to separate from one another at the diplotene stage.In vertebrates, diplotene can be an extremely extended phase of oogenesis during which the bulk of oocyte growth occurs. Thus diplotene can be a period of intense metabolic activity. Transcription during diplotene in the oocyte provides the RNA utilized for protein synthesis during both oogenesis and early embryonic development following fertilization.

Diakinesis
During the final stage of meiotic prophase I, called diakinesis, the meiotic spindle is assembled and the chromosomes are prepared for separation. In those species in which the chromosomes become highly dispersed during diplotene, the chromosomes become recompacted during diakinesis. Diakinesis ends with the disappearance of the nucleolus, the breakdown of the nuclear envelope, and the movement of the tetrads to the metaphase plate.
In vertebrate oocytes, these events are triggered by an increase in the level of the protein kinase activity of MPF (maturation-promoting factor).MPF was first identified by its ability to initiate these events, which represent the maturation of the oocyte.

At metaphase I, the two homologous chromosomes of each bivalent are connected to the spindle fibers from opposite poles (Figure).

In contrast, sister chromatids are connected to microtubules from the same spindle pole, which is made possible by the side-by-side arrangement of their kinetochores as seen in the inset of above  figure.The orientation of the maternal and paternal chromosomes of each bivalent on the metaphase I plate is random; the maternal member of a particular bivalent has an equal likelihood of facing either pole.

when homologous chromosomes separate during anaphase I, each pole receives a random assortment of maternal and paternal chromosomes . Thus,anaphase I is the cytological event that corresponds to Mendel’s law of independent assortment. As a result of independent assortment, organisms are capable of generating a nearly unlimited variety of gametes.
Separation of homologous chromosomes at anaphase I requires the dissolution of the chiasmata that hold the bivalents together. The chiasmata are maintained by cohesion between sister chromatids in regions that flank these sites of recombination (Figure above). The chiasmata disappear at the metaphase I–anaphase I transition, as the arms of the chromatids of each bivalent lose cohesion (Figure).

Loss of cohesion between the arms is accomplished by proteolytic cleavage of the cohesin molecules in those regions of the chromosome. In contrast, cohesion between the joined centromeres of sister chromatids remains strong, because the cohesin situated there is protected from proteolytic attack (Figure above). As a result, sister chromatids remain firmly attached to one another as they move together toward a spindle pole during anaphase I.

Telophase I, Completes Meiosis I
Telophase I of meiosis I produces less dramatic changes than telophase of mitosis. Although chromosomes often undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The nuclear envelope
may or may not reform during telophase I. The stage between the two meiotic divisions is called interkinesis and is generally short-lived. In animals, cells in this fleeting stage are referred to as secondary spermatocytes or secondary oocytes. These cells are characterized as being haploid because they contain only one member of each pair of homologous chromosomes. Even though they are haploid, they have twice as much DNA as a haploid gamete because each chromosome is still represented by a pair of attached chromatids. Secondary spermatocytes are said to have a 2C amount of DNA, half as much as a primary spermatocyte, which has a 4C DNA content, and twice as much as a sperm cell, which has a 1C DNA content.

Meiosis II

Meiosis II is like a mitotic division without DNA replication.Typically, the period between meiosis I and meiosis II is brief and critically, does not include an S phase. It is often called interkinesis instead of interphase. Meiosis II resembles a normal mitotic division with prophase II, metaphase II, anaphase II, and telophase II (see figure).

Prophase II
Interkinesis is followed by prophase II, a much simpler prophase than its predecessor.At the two poles of the cell, the clusters of chromosomes enter a brief prophase II, if the nuclear envelope had reformed in telophase I, it is broken down again. In some species the nuclear envelope does not re-form in telophase I, obviating the need for nuclear envelope breakdown. During prophase II,a new spindle apparatus forms in each cell.

Metaphase II
In metaphase II, spindle fibers from opposite poles bind to kinetochores of each sister chromatid, allowing each chromosome to migrate to the metaphase plate as a result of tension on the chromosomes from polar microtubules pulling on sister centromeres. This process is the same as metaphase during a mitotic division.The chromosomes become recompacted and line up at the metaphaseplate. Unlike metaphase I, the kinetochores of sister chromatids of metaphase II face opposite poles and become attached to opposing sets of chromosomal spindle fibers (see figure).

The progression of meiosis in vertebrate oocytes stops at metaphase II. The arrest of meiosis at metaphase II is brought about by factors that inhibit APCCdc20 activation,thereby preventing cyclin B degradation. As long as cyclin B levels remain high within the oocyte, Cdk activity is maintained, and the cells cannot progress to the next meiotic stage.Metaphase II arrest is released only when the oocyte (now called an egg) is fertilized. Fertilization leads to a rapid influx of Ca2 ions, the activation of APCCdc20, and the destruction of cyclin B. The fertilized egg responds to these changes by completing the second meiotic division. Anaphase II begins with the synchronous splitting of the centromeres,which had held the sister chromatids together, allowing them to move toward opposite poles of the cell (see figure ).

Anaphase II
The spindle fibers contract, and the cohesin
complex joining the centromeres of sister chromatids is destroyed, splitting the centromeres and pulling the sister chromatids to opposite poles. This process is also the same as anaphase during a mitotic division.

Telophase II
Finally, the nuclear envelope re-forms around the four sets of daughter chromosomes.Meiosis II ends with telophase II, in which the chromosomes are once again enclosed by a nuclear envelope. The products of meiosis are haploid cells with a 1C amount of nuclear DNA.No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I.Cytokinesis then follows.

Sexual Reproduction

Most animals and plants reproduce sexually. In humans, gametes of opposite sex unite to form a cell that, dividing repeatedly by mitosis, eventually gives rise to an adult body with some 100 trillion cells. The gametes that form the initial cell are the products of a special form of cell division called meiosis. Meiosis is far more intricate than mitosis, and the details behind it are not as well understood. The basic process,however, is clear. Also clear are the profound consequences of sexual reproduction: It plays a key role in generating the tremendous genetic diversity that is the raw material of evolution.

The essence of sexual reproduction is the merging of the genetic contribution of two cells from different individuals. This mode of reproduction results in evolutionary advantages that biologists have long recognized. However, we are only recently making progress on understanding the underlying mechanism that produces the elaborate behavior of chromosomes that occurs during meiosis, the process that underlies sexual reproduction. To begin, we briefly consider the history of meiosis and its relationship to sexual reproduction.

Fertilization in Sexual Reproduction 

Only a few years after Walther Flemming’s discovery of chromosomes in 1879, Belgian cytologist Edouard van Beneden was surprised to find different numbers of chromosomes in different types of cells in the roundworm Ascaris. Specifically, he observed that the gametes (eggs and sperm) each contained two chromosomes, but all of the nonreproductive cells, or somatic cells, of embryos and mature individuals each contained four.

From his observations, van Beneden proposed in 1883 that an egg and a sperm, each containing half the complement of chromosomes found in other cells, fuse to produce a single cell called a zygote. The zygote, like all of the cells ultimately derived from it, contains two copies of each chromosome. The fusion of gametes to form a new cell is called fertilization, or syngamy.

It was clear even to early investigators that gamete formation must involve some mechanism that reduces the number of chromosomes to half the number found in other cells. If it did not, the chromosome number would double with each fertilization, and after only a few generations the number of chromosomes in each cell would become impossibly large. For example, in just 10 generations, the 46 chromosomes present in human cells would increase to over 47,000 (46 × 2^10).

The number of chromosomes does not explode in this way because of a special reduction division, meiosis. Meiosis occurs during gamete formation, producing haploid cells—cells with half the normal number of chromosomes. The subsequent fusion of two of these cells to form a diploid cell—a cell with twice as many chromosomes as haploid cells—ensures a consistent chromosome number from one generation to the next. This reduction division process, the subject of this chapter, lies at the heart of sexual reproduction.

Fusion of Two Haploid Cells (Sperm and Egg)

Meiosis and fertilization together constitute a cycle of reproduction. Below Figure illustrates how two haploid cells, a sperm cell containing three chromosomes contributed by the father and an egg cell containing three chromosomes contributed by the mother, fuse to form a diploid zygote with six chromosomes.
Figure:Diploid cells carry chromosomes from two parents. A diploid cell contains two versions of each chromosome, a maternal homologue contributed by the haploid egg of the mother, and a paternal homologue contributed by the haploid sperm of the father.

Reproduction that involves this alternation of meiosis and fertilization is called sexual reproduction. Its outstanding characteristic is that offspring inherit chromosomes from two parents, as you saw in above figure. You, for example, inherited 23 chromosomes from your mother (maternal homologue) and 23 from your father (paternal homologue).
The life cycles of all sexually reproducing organisms follow a pattern of alternation between diploid and haploid chromosome numbers, but there is variation in the pattern’s timing.

Many types of algae, for example, spend the majority of their life cycle in a haploid state. Most plants and some algae alternate
between a multicellular haploid phase and a multicellular diploid phase. In most animals, by contrast, the diploid state dominates. The zygote first undergoes mitosis to produce diploid cells. Then, later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes (see figure)
Figure:The sexual life cycle in animals. In animals, the zygote undergoes mitotic divisions and gives rise to all the cells of the adult body. Germ-line cells are set aside early in development and undergo meiosis to form the haploid gametes (eggs or sperm). The rest of the body cells are called somatic cells.

Somatic and Germ Cells

In animals, the single diploid zygote undergoes mitosis to give rise to somatic cells that form all of the cells in the adult body. 
The cells that will eventually undergo meiosis to produce gametes are set aside from somatic cells early in the course of development. These cells are referred to as germ-line cells.  Both somatic cells and germ-line cells are diploid, but somatic cells undergo mitosis to form genetically identical, diploid daughter cells, and germ-line cells undergo meiosis to produce haploid gametes (see figure above).
Some organisms do not reproduce sexually and never produce gametes. Reproduction in these organisms is referred to as asexual reproduction. The cell division of yeasts  is an example of asexual reproduction, and some plants can reproduce asexually.

Next

Meiosis

Mitosis is one of the most dramatic and beautiful biological processes that we can easily observe. To better illustrate what happens, biologists have traditionally divided mitosis into five arbitrary phases ,namely Prophase, prometaphase, metphase, anaphase, and telophase.  However, the actual process is dynamic and continuous, and not broken into discrete steps. 

Prophase

The first stage of mitosis, prophase, is said to begin when the  chromosome condensation initiated in G2 phase reaches the  point at which individual condensed chromosomes first become visible with the light microscope. Because the condensation process begun in G2 continues throughout prophase, chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bearing the rRNA genes becomes condensed.

Spindle Apparatus and Centrioles 

The assembly of the spindle apparatus that will later separate the  sister chromatids occurs during prophase, replacing the normal microtubule structure of the cell that was disassembled in the  G2 phase. In animal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers. By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules, called the spindle apparatus, between them.

In plant cells, a similar bridge of microtubular fibers forms between opposite poles of the cell, although centrioles are absent in plant cells.

In animal cell mitosis, the centrioles extend a radial array of microtubules toward the nearby plasma membrane when they reach the poles of the cell. This arrangement of microtubules is called an aster. Although the aster’s function is not fully understood, it probably braces the centrioles against the membrane and stiffens the point of microtubular attachment during the retraction of the spindle. Plant cells, which have rigid cell walls, do not form asters.

Nuclear Envelope Breakdown 

During the formation of the spindle apparatus, the nuclear envelope breaks down, and the endoplasmic reticulum reabsorbs its components. At this point, the microtubular spindle fibers extend completely across the cell, from one pole to the other. 

Their orientation determines the plane in which the cell will subsequently divide, through the center of the cell at right angles to the spindle apparatus.

Prometaphase

Interphase Includes the Synthesis and Gap Phases of the Cell Cycle. During Interphase, Cells Grow and Prepare for Mitosis.

Interphase is the portion of the cell cycle between two consecutive cell divisions. The events that occur during interphase—the G1, S, and G2 phases—involve very important preparations for the successful completion of mitosis. During G1, cells undergo the major portion of their growth.During the S phase, each chromosome replicates to produce two sister chromatids, which remain attached to each other at the centromere. In the G2 phase, the chromosomes coil even more tightly.

Centromere and Kinetochore 

The centromere is a point of constriction on the chromosome containing repeated DNA sequences that bind specific proteins. These proteins make up a disklike structure called the kinetochore. This disk functions as an attachment site for microtubules necessary to separate the chromosomes during cell division (figure below). Each chromosome’s centromere is located at a characteristic site along the length of the chromosome.

Figure:Kinetochores. Separation of sister chromatids during mitosis depends on microtubules attaching to proteins found in the kinetochore. These kinetochore proteins are assembled on the centromere of chromosomes. The centromeres of the two sister chromatids are held together by cohesin proteins.

After the S phase, the newly synthesized sister chromatids appear to share a common centromere, but at the molecular level the DNA of the centromere has already replicated, so there are two complete DNA molecules. This means that you have two chromatids held together by cohesin proteins at the centromere, and each chromatid has its own set of kinetochore proteins.

In multicellular animals, most of the cohesin that holds sister chromatids together after replication is replaced by condensin as the chromosomes are condensed. This leaves the chromosomes still attached tightly at the centromere, but only loosely attached elsewhere.

Cell Growth 

A eukaryotic cell typically grows throughout interphase. The G1 and G2 segments of interphase are periods of active growth, during which proteins are synthesized and cell organelles are produced. However, the cell’s DNA replicates only during the S phase of the cell cycle.

After the chromosomes have replicated in S phase, they remain fully extended and uncoiled, although cohesin proteins are associated with their centromeres at this stage. In G2 phase, they begin the process of condensation, coiling ever more tightly. Special motor proteins are involved in the rapid final condensation of the chromosomes that occurs early in mitosis. 

Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. In animal cells, a pair of barrel-shaped organelles called centrioles replicate, producing one for each pole. These act as microtubule-organizing centers: Surrounding each centriole is pericentriolar material, ring-shaped structures composed of tubulin that can nucleate the assembly of microtubules. Plants and fungi lack centrioles but still contain microtubule-organizing centers. All eukaryotic cells undertake an extensive synthesis of tubulin, the protein that forms microtubules.