How Does This Animation Accurately Depict The Process Of Cell Division?

Chapter half dozen: Introduction to Reproduction at the Cellular Level

half dozen.two The Cell Bicycle

Learning Objectives

By the finish of this section, you will exist able to:

• Describe the three stages of interphase
• Discuss the behavior of chromosomes during mitosis and how the cytoplasmic content divides during cytokinesis
• Define the quiescent 1000₀ phase
• Explain how the three internal command checkpoints occur at the end of G₁, at the K_two–M transition, and during metaphase

The cell wheel is an ordered series of events involving jail cell growth and cell division that produces two new daughter cells. Cells on the path to jail cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells. The cell cycle has ii major phases: interphase and the mitotic phase (Figure 6.3). During interphase, the jail cell grows and DNA is replicated. During the mitotic stage, the replicated DNA and cytoplasmic contents are separated and the cell divides.

Watch this video about the cell cycle: https://www.youtube.com/watch?v=Wy3N5NCZBHQ

Effigy 6.3 A cell moves through a series of phases in an orderly manner. During interphase, G1 involves cell growth and protein synthesis, the S phase involves DNA replication and the replication of the centrosome, and G2 involves farther growth and protein synthesis. The mitotic phase follows interphase. Mitosis is nuclear division during which duplicated chromosomes are segregated and distributed into daughter nuclei. Usually the prison cell will divide afterward mitosis in a process chosen cytokinesis in which the cytoplasm is divided and two daughter cells are formed.

Interphase

During interphase, the prison cell undergoes normal processes while also preparing for prison cell division. For a jail cell to movement from interphase to the mitotic phase, many internal and external conditions must be met. The three stages of interphase are chosen G₁, Due south, and G₂.

G₁ Phase

The first stage of interphase is called the G_i stage, or first gap, because little change is visible. Yet, during the G₁ phase, 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 plenty free energy reserves to complete the job of replicating each chromosome in the nucleus.

S Stage

Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the Due south phase (synthesis phase), DNA replication results in the formation of ii identical copies of each chromosome—sister chromatids—that are firmly attached at the centromere region. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the appliance that orchestrates the movement of chromosomes during mitosis. The centrosome consists of a pair of rod-similar centrioles at right angles to each other. Centrioles help organize cell sectionalization. Centrioles are non present in the centrosomes of many eukaryotic species, such every bit plants and nigh fungi.

G₂ Phase

In the G₂ 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 G₂. The final preparations for the mitotic phase must be completed before the prison cell is able to enter the first phase of mitosis.

The Mitotic Stage

To make two girl 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 2 new identical girl cells. The start portion of the mitotic phase, mitosis, is composed of five stages, which accomplish nuclear division. The 2d portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into ii girl cells.

Mitosis

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

Figure half-dozen.4 Brute jail cell mitosis is divided into five stages—prophase, prometaphase, metaphase, anaphase, and telophase—visualized here past light microscopy with fluorescence. Mitosis is usually accompanied by cytokinesis, shown here by a manual electron microscope. (credit "diagrams": modification of work past Mariana Ruiz Villareal; credit "mitosis micrographs": modification of piece of work by Roy van Heesbeen; credit "cytokinesis micrograph": modification of work past the Wadsworth Eye, NY State Section of Wellness; donated to the Wikimedia foundation; scale-bar data from Matt Russell)

Which of the following is the correct order of events in mitosis?

1. Sis chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus re-forms and the cell divides. The sister chromatids separate.
2. The kinetochore becomes fastened to the mitotic spindle. The sister chromatids dissever. Sister chromatids line upward at the metaphase plate. The nucleus re-forms and the cell divides.
3. The kinetochore becomes attached to metaphase plate. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus re-forms and the jail cell divides.
4. The kinetochore becomes fastened to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus re-forms and the cell divides.

During prophase, the "outset phase," several events must occur to provide access to the chromosomes in the nucleus. 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 brainstorm to move to contrary poles of the cell. The microtubules that course the ground of the mitotic spindle extend betwixt the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more than tightly and go visible nether 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 beyond the length of the former nuclear area. Chromosomes become more condensed and visually detached. 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 separate apart at the centromere. Each chromatid, now called a chromosome, is pulled apace toward the centrosome to which its microtubule was attached. The cell becomes visibly elongated as the non-kinetochore microtubules slide confronting 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 3 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.

Concept in Action

This page of movies illustrates different aspects of mitosis. Watch the movie entitled "DIC microscopy of jail cell division in a newt lung prison cell" and identify the phases of mitosis.

Cytokinesis

Cytokinesis is the 2nd part of the mitotic phase during which cell sectionalization is completed by the physical separation of the cytoplasmic components into two daughter cells. Although the stages of mitosis are similar for nearly eukaryotes, the procedure of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.

In cells such as animal cells that lack jail 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 inwards, forming a fissure. This fissure, or "crack," is called the cleavage furrow. The furrow deepens as the actin ring contracts, and eventually the membrane and cell are cleaved in two (Figure 6.5).

In establish cells, a cleavage furrow is not possible because of the rigid jail cell walls surrounding the plasma membrane. A new cell wall must course betwixt the girl cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking up into vesicles and dispersing throughout the dividing jail cell. During telophase, these Golgi vesicles move on microtubules to collect at the metaphase plate. There, the vesicles fuse from the center toward the prison cell walls; this structure is called a prison cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell wall at the periphery of the cell. Enzymes employ the glucose that has accumulated between the membrane layers to build a new prison cell wall of cellulose. The Golgi membranes go the plasma membrane on either side of the new jail cell wall (Figure 6.5).

Figure vi.5 In part (a), a cleavage furrow forms at the erstwhile metaphase plate in the animate being prison cell. The plasma membrane is drawn in by a ring of actin fibers contracting just within the membrane. The cleavage furrow deepens until the cells are pinched in two. In part (b), Golgi vesicles coagulate at the former metaphase plate in a plant prison cell. The vesicles fuse and grade the jail cell plate. The prison cell plate grows from the center toward the cell walls. New cell walls are fabricated from the vesicle contents.

Chiliad₀ Stage

Not all cells adhere to the classic cell-cycle pattern in which a newly formed girl cell immediately enters interphase, closely followed by the mitotic stage. Cells in the G₀ phase are not actively preparing to split up. The cell is in a quiescent (inactive) phase, having exited the cell cycle. Some cells enter Thou₀ temporarily until an external betoken triggers the onset of 1000_i. Other cells that never or rarely carve up, such equally mature cardiac musculus and nervus cells, remain in One thousand₀ permanently (Figure vi.half-dozen).

Figure vi.6 Cells that are not actively preparing to divide enter an alternate phase chosen G0. In some cases, this is a temporary condition until triggered to enter G1. In other cases, the cell will remain in G0 permanently.

Control of the Cell Cycle

The length of the jail cell cycle is highly variable even within the cells of an individual organism. In humans, the frequency of jail cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or to an entire human lifetime spent in G₀ by specialized cells such every bit cortical neurons or cardiac muscle cells. There is likewise variation in the fourth dimension that a cell spends in each phase of the cell wheel. When fast-dividing mammalian cells are grown in culture (exterior the body under optimal growing weather), the length of the cycle is approximately 24 hours. In rapidly dividing human being cells with a 24-hour prison cell cycle, the Thousand_one phase lasts approximately 11 hours. The timing of events in the cell cycle is controlled past mechanisms that are both internal and external to the cell.

Regulation at Internal Checkpoints

Information technology is essential that daughter cells be exact duplicates of the parent cell. Mistakes in the duplication or distribution of the chromosomes pb to mutations that may be passed forward to every new cell produced from the abnormal cell. To preclude a compromised cell from continuing to split up, at that place are internal control mechanisms that operate at iii principal cell bike checkpoints at which the cell cycle can exist stopped until atmospheric condition are favorable. These checkpoints occur virtually the end of Yard₁, at the G₂–Chiliad transition, and during metaphase (Figure 6.7).

Figure six.7 The cell cycle is controlled at three checkpoints. Integrity of the DNA is assessed at the G1 checkpoint. Proper chromosome duplication is assessed at the G2 checkpoint. Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint.

The G₁ Checkpoint

The G_i checkpoint determines whether all weather are favorable for prison cell division to continue. The Chiliad₁ checkpoint, besides called the restriction point, is the signal at which the jail cell irreversibly commits to the cell-partition process. In addition to acceptable reserves and prison cell size, there is a bank check for impairment to the genomic DNA at the G₁ checkpoint. A cell that does non come across all the requirements will not exist released into the S phase.

The K_ii Checkpoint

The Thou_two checkpoint bars the entry to the mitotic stage if certain conditions are not met. As in the Yard₁ checkpoint, cell size and protein reserves are assessed. Withal, the most important role of the G₂ checkpoint is to ensure that all of the chromosomes have been replicated and that the replicated Dna is not damaged.

The G Checkpoint

The M checkpoint occurs near the end of the metaphase stage of mitosis. The Thousand checkpoint is also known as the spindle checkpoint because it determines if all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible stride, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to spindle fibers arising from contrary poles of the cell.

Concept in Action

Watch what occurs at the G_i, G₂, and Grand checkpoints past visiting this animation of the cell bicycle.

Section Summary

The cell cycle is an orderly sequence of events. Cells on the path to prison cell division keep through a serial of precisely timed and carefully regulated stages. In eukaryotes, the prison cell cycle consists of a long preparatory period, called interphase. Interphase is divided into Yard_one, Southward, and Grand₂ phases. Mitosis consists of 5 stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis is unremarkably accompanied by cytokinesis, during which the cytoplasmic components of the daughter cells are separated either by an actin band (brute cells) or past prison cell plate germination (found cells).

Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell bike: i near the finish of Thou₁, a second at the 1000_two–G transition, and the third during metaphase.

Glossary

anaphase : the stage of mitosis during which sis chromatids are separated from each other

cell cycle : the ordered sequence of events that a cell passes through between ane jail cell division and the next

cell bicycle checkpoints: mechanisms that monitor the preparedness of a eukaryotic jail cell to advance through the diverse cell wheel stages

cell plate: a structure formed during plant-cell cytokinesis by Golgi vesicles fusing at the metaphase plate; volition ultimately lead to germination of a cell wall to separate the two girl cells

centriole: a paired rod-like structure constructed of microtubules at the center of each animate being prison cell centrosome

cleavage furrow: a constriction formed by the actin ring during animal-cell cytokinesis that leads to cytoplasmic division

cytokinesis: the division of the cytoplasm post-obit mitosis to form two daughter cells

G₀ stage: a jail cell-cycle stage singled-out from the G_ane stage of interphase; a jail cell in 1000₀ is not preparing to divide

Grand₁ stage : (as well, start gap) a cell-bicycle phase; first stage of interphase centered on jail cell growth during mitosis

G₂ phase: (as well, 2d gap) a cell-cycle stage; 3rd phase of interphase where the cell undergoes the final preparations for mitosis

interphase: the period of the cell bike leading upwardly to mitosis; includes One thousand_one, S, and Thousand₂ phases; the interim between two consecutive jail cell divisions

kinetochore: a protein construction in the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase

metaphase plate: the equatorial plane midway between two poles of a cell where the chromosomes marshal during metaphase

metaphase : the stage of mitosis during which chromosomes are lined upward at the metaphase plate

mitosis: the period of the cell bike at which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase

mitotic phase: the period of the prison cell cycle when duplicated chromosomes are distributed into two nuclei and the cytoplasmic contents are divided; includes mitosis and cytokinesis

mitotic spindle: the microtubule apparatus that orchestrates the movement of chromosomes during mitosis

prometaphase : the phase of mitosis during which mitotic spindle fibers attach to kinetochores

prophase: the stage of mitosis during which chromosomes condense and the mitotic spindle begins to grade

quiescent: describes a cell that is performing normal jail cell functions and has not initiated preparations for cell division

S stage: the second, or synthesis phase, of interphase during which DNA replication occurs

telophase: the phase of mitosis during which chromosomes go far at reverse poles, decondense, and are surrounded past new nuclear envelopes

Source: https://opentextbc.ca/biology/chapter/6-2-the-cell-cycle/

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