Cells undergoing meiosis do not usually have a G 2 phase. Interphase is as term used to include those phases of the cell cycle excluding mitosis and meiosis. Many variants of this generalized cell cycle also exist. Some cells never leave G 1 phase, and are said to enter a permanent, non-dividing stage called G 0. On the other hand, some cells undergo many rounds of DNA synthesis S without any mitosis or cell division, leading to endoreduplication. The cell grows and accumulates the building blocks of chromosomal DNA and the associated proteins as well as sufficient energy reserves to complete the task of replicating each chromosome in the nucleus.
The synthesis phase of interphase takes the longest because of the complexity of the genetic material being duplicated.
Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase.
The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis.
At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles, which are at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation.
Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G 2.
The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. During the multistep mitotic phase, the cell nucleus divides, and the cell components split into two identical daughter cells. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis or nuclear division.
The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells. Karyokinesis, also known as mitosis, is divided into a series of phases prophase, prometaphase, metaphase, anaphase, and telophase that result in the division of the cell nucleus.
Stages of the Cell Cycle : Karyokinesis or mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. The images at the bottom were taken by fluorescence microscopy hence, the black background of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA chromosomes and green fluorescence indicates microtubules spindle apparatus.
The membranous organelles such as the Golgi apparatus and endoplasmic reticulum fragment and disperse toward the periphery of the cell. The nucleolus disappears and the centrosomes begin to move to opposite poles of the cell. Microtubules that will eventually form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen.
The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope. The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region.
The proteins of the kinetochore attract and bind mitotic spindle microtubules. Kinetochore and Mitotic Spindle : During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore. In anaphase, the connection between the sister chromatids breaks down and the microtubules pull the chromosomes toward opposite poles.
The cytoplasm is usually divided as well, resulting in two daughter cells. During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G 1 , S, and G 2. The first stage of interphase is called the G 1 phase first gap because, from a microscopic aspect, little change is visible.
However, during the G 1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration.
In the S phase, DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles, which are at right angles to each other.
Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase.
There may be additional cell growth during G 2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells.
The first portion of the mitotic phase is called karyokinesis, or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells. Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus Figure 2.
Karyokinesis is also called mitosis. Figure 2. Karyokinesis or mitosis is divided into five stages—prophase, prometaphase, metaphase, anaphase, and telophase.
The pictures at the bottom were taken by fluorescence microscopy hence, the black background of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA chromosomes and green fluorescence indicates microtubules spindle apparatus.
The nucleolus disappears disperses. The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope.
Figure 3. During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore.
In anaphase, the connection between the sister chromatids breaks down, and the microtubules pull the chromosomes toward opposite poles. The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region Figure 3.
The proteins of the kinetochore attract and bind mitotic spindle microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles.
Spindle microtubules that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis. The sister chromatids are still tightly attached to each other by cohesin proteins.
At this time, the chromosomes are maximally condensed. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached.
The cell becomes visibly elongated oval shaped as the polar microtubules slide against each other at the metaphase plate where they overlap. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.
Figure 4. During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two.
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