Essay Example on The cell cycle is a sequence of phases that cells Pass









The cell cycle is a sequence of phases that cells pass through between cell divisions and the next This is divided into interphase where cells spend approximately 90 percent of their time and the M phase where cell division occurs A Dictionary of Biology 2015 Cell D is in interphase as the genetic material is in the uncondensed state known as chromatin there fore chromosomes are not visible under light microscopes Interphase occurs prior to the M phase and consists of three stages including the G1 phase S phase and G2 phase Within interphase the cell carries out metabolic activities for the preparation of mitosis Biology arizona edu 2018 In the G1 phase there is a high rate of biosynthesis and growth A Dictionary of Biology 2015 Genes are transcribed and proteins required for mitosis are synthesised Centrioles organelles and the cytoplasmic content is duplicated The S phase stands for synthesis and in this phase DNA content doubles and chromosomes replicate A Dictionary of Biology 2015 by semiconservative replication 

Therefore after cell division each daughter cell has the same number of chromosomes as the parent cell The next stage is the second gap phase G2 in which the cell continues to grow in size and enzymes required for cell division are produced Between these stages are checkpoints which ensures successful completion of the previous stage This includes the restriction point in G1 where the cell commits to S phase When extracellular conditions are not optimal cells enter the G0 phase which is the resting or quiescent state where it does not divide Et Al 2008 AB 2018 Cells can remain in this state for an indefinite amount of time until conditions improve and stimulate growth via a signal sent that instructs the cell to re enter the cell cycle Hardin Bertoni and Kleinsmith 2012 In G2 the spindle checkpoint ensures chromosomes are attached to the spindle apparatus if unattached it prevents separation of the duplicated chromosomes A 2018 The postreplication checkpoint is a DNA damage checkpoint which ensures damaged DNA is repaired 

This provides time for the repair of daughter strand gaps generated by the eukaryotic replication fork at sites of DNA damage Callegari 2018 The M phase includes mitosis which consists of nuclear division and cytokinesis which consists of cytoplasmic division Mitosis consists of 5 stages including prophase prometaphase metaphase anaphase and telophase Cell B is in prophase as chromatin condenses into chromosomes appearing as sister chromatids attached by the centromere Cohesin a multiprotein complex A Dictionary of Biology 2015 binds to chromosomes before S phase and is converted into a physical linkage that binds sister chromatids together Ishiguro Watanabe 2007 The nucleolus breaks down and centrioles move towards opposite poles of the cell forming spindle fibres between them In the cytoplasm the mitotic spindle forms which is composed of composed of microtubules and proteins Thoughtco 2018 Following prophase is prometaphase late prophase in which the nuclear envelope breaks down releasing the chromosomes Proteins attach to the centromeres creating kinetochores to which microtubules attach to enabling chromosomes to begin active movement Biology arizona edu 2018 These kinetochores attach to a type of microtubule called kinetochore fibres which interact with spindle fibres 

Thoughtco 2018 Cell A is in metaphase in which spindle fibres align chromosomes at the equator of the cell known as the metaphase plate This ensures that the chromosomes are on one plane in the centre of the cell The spindle checkpoint confirms that chromosomes are correctly attached to the microtubule spindle apparatus by their kinetochores Lara Gonzalez 2012 The following stage anaphase is shown by cell E which involves the separation of sister chromatids This occurs as the proteins holding the sister chromatids together begin to degrade enabling them to move synchronously apart The chromatids appear as V shaped as they are pulled to opposite poles by their kinetochores shortening the kinetochore fibres Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules Biology arizona edu 2018 Microtubules attached to chromatids shorten while those that are unattached extend and push apart opposite poles elongating the cell Thoughtco 2018

The next stage is telophase as shown in cell C where chromosomes arrive at opposite poles of the cell and the spindle fibres break down A new nuclear envelope made from formed from remnant pieces of the parent cell's nuclear envelope and from pieces of the endomembrane system Thoughtco 2018 forms around each set of daughter chromosomes therefore forming two daughter nuclei The nucleoli reappear and chromosomes uncoil into chromatin Cytokinesis occurs at the end of the M phase where the cytoplasm divides A contractile ring composed of actin filaments forms a cleavage furrow Bms ed ac uk 2003 in the centre of the cell by tightening around the cytoplasm of the cell and drawing in the membrane between daughter nuclei When the contractile ring contracts the cleavage furrow deepens eventually resulting in the membrane breaking This gives rise to two genetically identical diploid cells Microtubules reorganise into a new cytoskeleton for the return to interphase Cellsalive com 2018

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