an adult human contains an estimated 100 trillion cells and yet we start Life as a single cell to grow develop and repair tissue damage we rely on cell division in eukaryotic cells this process is accomplished by a series of well orchestrated steps called mitosis every day our bodies must produce millions of skin cells to replace those lost through normal activity each of these cells must have a complete complement of the genetic material prior to cell division where the cell divides into two identical cells the DNA needs to be replicated so that each daughter cell receives an exact copy following DNA replication the chromosomes condense in the nucleus of the cell DNA condenses by wrapping around cores of histone proteins forming nucleosomes this beads on a string structure is called chromatin as a cell prepares to divide chromatin coils up further shortening and condensing the chromosome the replicated chromosomes are called sister chromatids the replication of DNA and the formation of sister chromatids is one part of the entire cell cycle to prepare for cell division the cell goes through interphase which can be divided into three distinct phases during G1 or Gap 1 phase all the organel and cytoplasmic components including the centrioles in animal cells replicate then during s or synthesis phase the DNA replicates finally during G2 or Gap 2 phase all the enzymes needed to Aid in the process of cell division are produced most eukariotic cells spend a great deal of time in interphase and a very short period of time dividing a process called mitosis the cell is now ready to go through mitosis which consists of prophase metaphase anaphase and telophase during prophase the chromosomes condense and become visible appearing as two sister chromatids held together at the centromere the cytoskeleton disassembles as the spindle begins to form in animal cells cental play an important role in the distribution of the chromosomes in the dividing cell the centrioles migrate to opposite poles establishing a bridge of microtubules called the spindle apparatus and the nuclear envelope breaks down towards the end of prophase chromosomes attach by proteins in their centromeres called kinetic cores to microtubules from each pole moving the chromosomes toward the equator of the cell during metaphase all chromosomes are aligned at the equator of the cell called the metaphase plate anaphase begins with the degradation of proteins that hold sister chromatids together freeing individual chromosomes the free chromosomes are then pulled by their kinetic or to opposite poles at telophase a cleavage Furrow forms in the center of the cell this indentation is made from a constricting belt of actin Phil M surrounding the inside of the cell's circumference chromosomes cluster at opposite poles and begin decondensing as the nuclear envelope reforms around them the spindle apparatus disassembles as the microtubules are broken down into tubulin monomers that can be used to form the cytoskeleton of the daughter cells in animal cells cyto canis completes cell division by extending the cleavage Furrow to complet separate the newly formed daughter cells since plant cell walls cannot be constricted by actin fibers vesicles form an expanding membrane partition called the cell plate like animal cells plant cells use cyto canis to finish the division of the contents of the cytoplasm between the two identical daughter cells during the cell cycle certain checkpoints are encountered to make sure the process is occurring accurately and if it is not the cell cycle will stop at the checkpoint and correct or possibly inhibit that cell from dividing the first checkpoint is the g1s checkpoint and is considered the primary point at which the cell cycle continues or stops external signals and growth factors can influence the cell cycle and affect the progress at or before this critical checkpoint the g2m checkpoint allows cells that have successfully completed all three phases of interphase to begin mitosis the last checkpoint is the spindle checkpoint ensuring that all chromosomes have attached to the spindle in preparation for anaphase growth factors the size of the cell and the nutritional state of the cell are all contributing factors in cell cycle regulation ensuring that only certain cells divide at appropriate times once all the checkpoints in interphase are cleared mitosis can occur from interphase to cyto canis the entire process of cell division can take on average 10 to 20 hours in a typical plant or animal cell depending on the nature and use of the cell the process can happen at different frequencies as well in humans our skin cells have a high turnover rate due to wear and tear and go through mitosis very frequently whereas other cells such as adult neurons and muscle cells rarely divide the accuracy of mitosis as well as the consistency of the checkpoints during interphase ensure that most cells in a eukaryotic organism can produce identical copies of themselves this process allows for growth and repair to prolong overall physiology as well as life itself have you ever wondered why children do not look exactly like their parents or why some siblings look so different from each other the answer lies in the process of meiosis which generates the gametes for reproduction meiosis is the process by which haid cells are produced from a diploid cell for this to occur the chromosomes must be correctly sorted and distributed in a manner to create genetically unique cells with half the number of chromosomes as the original cell meiosis occurs in special cells called germ cells within the gonads of males and females for the successful reduction in the number of chrom omes to occur in the new haid daughter cells two rounds of divisions are necessary these divisions are termed meiosis 1 and meiosis 2 although the processes of mitosis and meiosis look similar there are a few major differences one obvious difference is that mitosis results in two diploid daughter cells and meiosis results in four haid cells as in mitosis meiosis begins after a cell has successfully completed the G1 s and G2 stages of interphase during the S phase of interphase the DNA is replicated producing two copies of each chromosome called sister chromatids the paired centrioles in the cytoplasm duplicate and begin extending microtubules that will form the myotic spindle sister chromatids remain attached at the centromere and condense as the cell enters prophase 1 of meiosis up to this point the cell looks similar to mitosis but two events occur in meiosis that do not occur in mitosis and lead to genetic diversity the first event occurs during prophase 1 when homologous pairs of sister chromatids lie side by side in a process called synapsis forming a tetrad or balent the homologous chromosomes share similar but not necessarily identical genes once this structure is formed the second event called crossing over can occur during crossing over a physical exchange between chromosome segments of nonsister chromatids occurs increasing genetic diversity prophase 1 concludes with the fragmentation of the nuclear envelope as the duplicated cental pairs move to opposite poles of the cell as they move the centrioles extend spindle fibers forming the myotic spindle in prometaphase 1 with the paired centrioles in place the myotic spindle is fully formed the sister chromatids attach to the spindle fibers by their kinetic cores here another key difference between mitosis and meiosis occurs because of synapsis and crossing over in meiosis homologous chromosomes remain aligned so that a pair of sister chromatids is attached attached to only one pole by the kineticore microtubules in mitosis each cister chromatid is attached to a spindle fiber the fibers originate from opposite poles during metaphase 1 B veence randomly align along the metaphase plate due to independent assortment this alignment is random and adds to genetic diversity in anaphase 1 the homologous chromosomes separate and move toward opposite poles meiosis 1 ends with telophase 1 when the chromosomes decondense and the nuclear envelope reforms cytokinesis separates the cytoplasmic material and the two daughter cells are separated by a cleavage Furrow since the final product of meiosis 1 is two haid cells meiosis 2 begins without the chromosomes going through another round of DNA replication centrioles again duplicate and and begin moving to opposite poles of each cell in prophase 2 the sister chromatids condense while the spindle starts to form as the nuclear envelope disappears in prometaphase 2 the sister chromatids attach to the spindle by kineticore microtubules with sister chromatids attached to opposite poles the spindle aligns the sister chromatids along the metaphase plate during metaphase 2 during anaphase 2 sister chromatids separate and individual chromosomes move toward the poles the entire process ends with telophase 2 as the chromosomes decondense and the nuclear envelope reforms cyto canis occurs and cleavage furo separate the two daughter cells into four haid daughter cells the haid daughter cells will specialize into gametes either sperm or egg these fuse in fertilization to form a zygote which will grow into a child the child receives half its chromosomes from its mother and half from its father because men and women produce millions of gametes and the selection of gametes in fertilization is random this contributes to genetic diversity this explains why a child isn't identical to either parent but why aren't siblings identical the diversity comes from several sources in prophase 1 nonsister chromatids can exchange DNA through crossing over increasing the genetic diversity of individual chromatids in addition to crossing over in metaphase one the pairs of homologous chromosomes align independently along the metaphase plate and sort independently into the daughter cells this process called independent assortment produces four genetically distinct haid gametes as the total number of chromosomes increases in an organism the number of genetically distinct gtes increases by 2 to the nth power this means an organism with Nal 3 can produce eight unique gametes for humans where n equal 23 there are 2 to the 23rd power unique gamt formed or over a million different possible combinations the combination of Independent Assortment crossing over and the random pairing of gametes during sexual reproduction increases genetic diversity and explains why a child will not look exactly like his or her parents or siblings