What Are The Three Phases Of The Cell Cycle – The cell cycle is an ordered series of events involving cell growth and cell division that produce two new daughter cells. Cells on the path to cell division proceed through a series of timed and carefully controlled phases of growth, DNA replication, and division that produce two genetically identical cells. The cell cycle has two main phases: interphase and mitotic phase (Figure 6.3). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

Figure 6.3 A cell moves through a series of phases in an orderly fashion. During interphase, G1 involves cell growth and protein synthesis, S phase involves DNA replication and centrosome replication, and G2 involves further growth and protein synthesis. The mitotic phase follows the interphase. Mitosis is nuclear division in which duplicated chromosomes are separated and distributed to daughter nuclei. Normally the cell divides after mitosis in a process called cytokinesis in which the cytoplasm divides and two daughter cells are formed.

What Are The Three Phases Of The Cell Cycle

During interphase, the cell undergoes normal processes while also preparing for cell division. For a cell to transition from interphase to mitotic phase, several internal and external conditions must be met. The three stages of interphase are called G

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Stage, the cell is relatively active at the biochemical level. The cell assembles the building blocks of chromosomal DNA and the associated proteins, as well as accumulates enough energy reserves to complete the task of copying each chromosome in the nucleus.

Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome—sister chromatids—that are tightly attached to the centromere region. At this stage, each chromosome is made of two sister chromatids and one duplicated chromosome. The centrosome is duplicated during S phase. The two centrosomes provide the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. The centrosome consists of a pair of rod-like centrioles at right angles to each other. Centrioles help organize cell division. Centrioles are absent from the centrosomes of many eukaryotic species, such as plants and most fungi.

Phase, or second gap, the cell replenishes its energy stores and synthesizes proteins needed 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

. The final preparations for the mitotic phase must be completed before the cell enters the first stage of mitosis.

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To produce two daughter cells, the contents of the nucleus and cytoplasm must be divided. The mitotic phase is a multistep process in which the duplicated chromosomes are aligned, separated, and moved to opposite poles of the cell, and then the cell divides into two new identical daughter cells. The first part of the mitotic phase, mitosis, consists of five stages, which carry out nuclear division. The second part of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic parts into two daughter cells.

Mitosis is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—resulting in the division of the cell nucleus (Figure 6.4).

Figure 6.4 Animal cell mitosis is divided into five phases—prophase, prometaphase, metaphase, anaphase, and telophase—visualized here by light microscopy with fluorescence. Mitosis is often accompanied by cytokinesis, shown here by a transmission electron microscope. (credit “diagrams”: modification of work by Mariana Ruiz Villareal; credit “mitosis micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: modification of work by the Wadsworth Center, NY State Department of Health; donated to the Wikimedia foundation; scale-bar data from Matt Russell)

During prophase, the “first phase,” several events must occur to give chromosomes access to the nucleus. The nuclear envelope begins to break down into small vesicles, and the Golgi apparatus and endoplasmic reticulum fragment and spread around the cell. The nucleolus is missing. The centrosomes begin to move to opposite poles of the cell. The microtubules that form the base of the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers elongate. The sister chromatids begin to twist more tightly and are visible under a light microscope.

Cell Cycle Tutorial Video And Questions

During prometaphase, many processes initiated in prophase continue to progress and culminate in the formation of a connection between chromosomes and the cytoskeleton. The remnants of the nuclear envelope are missing. The mitotic spindle continues to form as more microtubules accumulate and extend the length of the former nuclear area. Chromosomes become more condensed and visually discrete. Each sister chromatid is attached to the spindle microtubule at the centromere by a protein complex called a kinetochore.

During metaphase, all the chromosomes line up in a plane called the metaphase plate, or the equatorial plane, 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, sister chromatids in the equatorial plane divide at the centromere. Each chromatid, now called a chromosome, is rapidly pulled toward the centrosome where its microtubule is 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 the events that set up the duplicated chromosomes for mitosis in the first three phases are reversed. Chromosomes reach opposite poles and begin to decondense (unravel). Mitotic spindles are broken down into monomers that will be used to assemble parts of the cytoskeleton for each daughter cell. Nuclear envelopes form around the chromosomes.

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This page of movies illustrates different aspects of mitosis. Watch the movie titled “DIC microscopy of cell division in a newt lung cell” and identify the stages of mitosis.

Cytokinesis is the second part of the mitotic phase in which cell division is completed by physically separating the cytoplasmic parts 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 after the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. Actin filaments pull the cell’s equator inward, forming a fissure. This fissure, or “crack,” is called a cleavage furrow. The furrow deepens as the actin ring contracts, and eventually the membrane and cell split in two (Figure 6.5).

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 before breaking up into vesicles and spreading throughout the dividing cell. During telophase, these Golgi vesicles move along microtubules to collect at the metaphase plate. There, the vesicles fuse from the center toward the cell walls; this structure is called the cell plate. As more vesicles fuse, the cell plate grows until it fuses with the cell wall at the cell’s periphery. Enzymes use the glucose accumulated between the membrane layers to build a new cell wall of cellulose. Golgi membranes become plasma membranes on both sides of the new cell wall (Figure 6.5).

The Cell Cycle What Are The 3 Phases Of The Cell Cycle? What Are The 4 Phases Of Mitosis? What Is Differentiation?

Figure 6.5 In part (a), a cleavage furrow forms at the former metaphase plate in the animal cell. The plasma membrane is drawn by a ring of actin fibers that contract just inside the membrane. The cleavage furrow deepens until the cells are pinched in two. In part (b), Golgi vesicles fuse with the former metaphase plate in a plant cell. The vesicles fuse and form the cell plate. The cell plate grows from the center towards the cell walls. New cell walls are made from the contents of the vesicle.

Not all cells follow the classic cell-cycle pattern in which a newly formed daughter cell immediately enters interphase, closely followed by the mitotic phase. Cells in G

Phase is not actively preparing to split. The cell is in a quiescent (inactive) phase, exiting the cell cycle. Some cells enter the G

. Other cells that never or rarely divide, such as mature heart muscle and nerve cells, remain in G

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Figure 6.6 Cells that are not actively preparing to divide enter an alternative phase called G0. In some cases, this is a temporary condition until triggered to enter G1. In other cases, the cell stays in G0 permanently.

The length of the cell cycle is highly variable even within the cells of an individual organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or an entire human lifetime spent in G.

By specialized cells such as cortical neurons or cardiac

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