During what phase does the nuclear envelope break down and a spindle forms between the pairs of centrioles?
MITOSIS OR M PHASE MM phase is relatively brief and consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis). In mitosis, replicated chromosomes divide, leading to identical daughter nuclei with the same number of chromosomes and the same genetic composition as the parents. In human cells, which have 23 pairs of chromosomes, the number of chromosomes (2n = 46) remains unchanged from the beginning till the end of mitosis. Show
The phases are in the following sequence: prophase, metaphase, anaphase, and telophase (see figure below). The movement of chromosomes is facilitated by a structure called the mitotic spindle, which consists of microtubules and associated proteins. Spindles extend from centrioles on each of the two sides (or poles) of the cell, attach to the chromosomes and align them, and pull the sister chromatids apart. Chromosomes are usually visible under light microscope. We can determine the number of chromosomes [and chromatid(s) per chromosome] for each stage in mitosis. Prophase Chromatin in the nucleus begins to condense and becomes visible under the light microscope as chromosomes, each with two chromatids that are held together at the centromere. The nuclear envelope breaks down. Centrioles begin moving to opposite ends of the cell, and microtubules extend from the centrioles and begin to attach to the centromeres of chromosomes. Eventually, the microtubules extending from centrioles on opposite poles of the cell attach to every centromere and develop into spindle fibers. Metaphase By growing on one end and shrinking on the other, spindle fibers align the chromosomes along the middle of the cell nucleus, approximately equidistant from the spindle poles. This imaginary line where the chromosomes are aligned is called the metaphase plate and is equidistant from the centrioles, which are on opposite poles of the cell. Anaphase After all of the chromosomes are aligned on the metaphase plate, each pair of sister chromatids splits at the centromere, separates and moves along the shortening spindle fibers to opposite sides of the cell. Now, the number of centromeres and chromosomes within the cell is doubled. Note that each separated chromosome has only one chromatid. Telophase Chromosomes, each with one chromatid, arrive at opposite poles of the cell, and a new nuclear membrane forms around each of the two new daughter nuclei, which are identical to each other. The spindle fibers begin to disperse and the chromosomes decondense; chromosomes are no longer visible under the light microscope. Notice that the number of centromeres and chromosomes within each of the two new cells is identical to the parent cell before it underwent mitosis. Cytokinesis Although the nucleus divides during mitosis, the cytoplasm does not. Following mitosis, cytokinesis is the process where the cytoplasm of a eukaryotic cell is divided to form two identical daughter cells. In animal cells, the center of the cell contracts, pinching the cell into two daughter cells, each of which contains a nucleus with a complete genome.  Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.
MitosisMitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour. The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells. In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation. Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase. The phases of mitosisProphaseProphase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere. PrometaphaseThe chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense. MetaphaseThe chromosomes align themselves along the metaphase plate of the spindle apparatus. AnaphaseThe shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.) TelophaseThe final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear. CytokinesisThe final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions. MeiosisMeiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids. Meiosis IMeiosis I separates the pairs of homologous chromosomes. In Meiosis I a special cell division reduces the cell from diploid to haploid. Prophase IThe homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:
Prometaphase ISpindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores. Metaphase IHomologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.) Anaphase IThe homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell Telophase IThe chromosomes become diffuse and the nuclear membrane reforms. CytokinesisThe final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome. Meiosis IIMeiosis II separates each chromosome into two chromatids. The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes involved has been halved. Meiosis generates genetic diversity through:
In what phase does the nuclear envelope break down?During prophase, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down.
At what stage of the cell cycle does the nuclear envelope break down and spindle fibers start to form?Prophase is the first stage of mitosis, during which the cell begins to position itself in order to separate the chromatids and divide. During prophase, the nuclear envelope and nucleolus are dissolved and the chromosomes condense. The centrioles and spindle fibers begin to form at opposite poles of the cell.
What stages the nuclear membrane disintegrates and the spindle appears?Prometaphase. After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid.
During which phase of mitosis spindles are formed?The mitotic spindle also begins to develop during prophase. As the cell's two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.
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