The centrosome consists of a pair of rod-like centrioles at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of many eukaryotic species, such as plants and most fungi. In the G 2 phase , or second gap, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic spindle.
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. To make two daughter cells, the contents of the nucleus and the cytoplasm must be divided. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and moved to opposite poles of the cell, and then the cell is divided into two new identical daughter cells.
The first portion of the mitotic phase, mitosis , is composed of five stages, which accomplish nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into two daughter cells.
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]. The nuclear envelope starts to break into small vesicles, and the Golgi apparatus and endoplasmic reticulum fragment and disperse to the periphery of the cell.
The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell. The microtubules that form the basis of the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly and become visible under a light microscope. During prometaphase , many processes that were begun in prophase continue to advance and culminate in the formation of a connection between the chromosomes and cytoskeleton.
The remnants of the nuclear envelope disappear. 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 visually discrete. Each sister chromatid attaches to spindle microtubules at the centromere via a protein complex called the kinetochore.
During metaphase , all of the chromosomes are aligned in a plane called the metaphase plate , or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other. At this time, the chromosomes are maximally condensed.
During anaphase , the sister chromatids at the equatorial plane are split apart at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule was attached. The cell becomes visibly elongated as the non-kinetochore microtubules slide against each other at the metaphase plate where they overlap. During telophase , all of the events that set up the duplicated chromosomes for mitosis during the first three phases are reversed.
The chromosomes reach the opposite poles and begin to decondense unravel. The mitotic spindles are broken down into monomers that will be used to assemble cytoskeleton components for each daughter cell. Nuclear envelopes form around chromosomes. This page of movies illustrates different aspects of mitosis.
Cytokinesis is the second part of the mitotic phase during which cell division is completed by the physical separation of the cytoplasmic components into two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells that lack cell walls, cytokinesis begins following the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. The furrow deepens as the actin ring contracts, and eventually the membrane and cell are cleaved in two [Figure 3]. In plant cells, a cleavage furrow is not possible because of the rigid cell walls surrounding the plasma membrane.
A new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking up into vesicles and dispersing throughout the dividing cell.
Cytokinesis : 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. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast. A cell plate formed by the fusion of the vesicles of the phragmoplast grows from the center toward the cell walls and the membranes of the vesicles fuse to form a plasma membrane that divides the cell in two.
In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell.
During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast a vesicular structure at the metaphase plate.
There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall.
The Golgi membranes become parts of the plasma membrane on either side of the new cell wall. Not all cells adhere to the classic cell cycle pattern in which a newly-formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase.
Cells in G 0 phase are not actively preparing to divide. The cell is in a quiescent inactive stage that occurs when cells exit the cell cycle.
Some cells enter G 0 temporarily until an external signal triggers the onset of G 1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G 0 permanently.
Privacy Policy. Skip to main content. Cell Reproduction. Search for:. The Cell Cycle. Interphase Cells must grow and duplicate their internal structures during interphase before they can divide during mitosis. Learning Objectives Describe the events that occur during Interphase.
Cells spend most of their lives in interphase, specifically in the S phase where genetic material must be copied.
The cell grows and carries out biochemical functions, such as protein synthesis, in the G 1 phase. During the S phase, DNA is duplicated into two sister chromatids, and centrosomes, which give rise to the mitotic spindle, are also replicated. In the G 2 phase, energy is replenished, new proteins are synthesized, the cytoskeleton is dismantled, and additional growth occurs. Key Terms interphase : the stage in the life cycle of a cell where the cell grows and DNA is replicated sister chromatid : either of the two identical strands of a chromosome DNA material that separate during mitosis mitotic spindle : the apparatus that orchestrates the movement of chromosomes during mitosis.
The Mitotic Phase and the G0 Phase During the multistep mitotic phase, the cell nucleus divides, and the cell components split into two identical daughter cells. Learning Objectives Describe the events that occur at the different stages of mitosis. Key Takeaways Key Points During prophase, the nucleus disappears, spindle fibers form, and DNA condenses into chromosomes sister chromatids.
This page has been archived and is no longer updated. Spindle fibers form a protein structure that divides the genetic material in a cell. The spindle is necessary to equally divide the chromosomes in a parental cell into two daughter cells during both types of nuclear division: mitosis and meiosis.
During mitosis, the spindle fibers are called the mitotic spindle. Meanwhile, during meiosis, the spindle fibers are referred to as the meiotic spindle. At the beginning of nuclear division, two wheel-shaped protein structures called centrioles position themselves at opposite ends of the cell forming cell poles.
Long protein fibers called microtubules extend from the centrioles in all possible directions, forming what is called a spindle.
0コメント