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Meiosis Introduction

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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 haploid 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 6th full reduction in the number of chromosomes to occur in the new haploid 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 haploid 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 mitotic spindle sister Crow kids remain attached at the centromere and condense as the cell enters prophase 1 of meiosis up to this point the cell looks you turn 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 bivalent 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 non sister occurs increasing genetic diversity prophase 1 concludes with the fragmentation of the nuclear envelope as the duplicated Central pairs move to opposite poles of the cell as they moved the centrioles extends spindle fibers form of the mitotic spindle in prometaphase 1 the paired centrioles in place the mitotic spindle is fully formed the sister chromatids attach to the spindle fibers by their kinetochores here another key difference between mitosis and meiosis occurs because of synapsis and crossing over in meiosis homologous chromosomes remain aligned so the pair of sister chromatids is attached to only one pole by the kinetochore microtubules in mitosis each sister chromatid is attached to a spindle fiber the fibers originate from opposite poles during metaphase 1 by violence 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 wait and move toward opposite poles meiosis 1 and with telophase 1 when the chromosomes decondense and the nuclear envelope reform cytokinesis separates the cytoplasmic material and the two daughter cells are separated by a cleavage Furrow since the final set of meiosis 1 is two haploid cells meiosis 2 begins without the chromosomes going through another round of DNA replication those again duplicate and begin moving to opposite poles of each cell in prophase 2 the sister chromatids condense while the spindle starts to alarm as the nuclear envelope disappears in prometaphase to the sister chromatids attached to the spindle by kinetochore microtubules sister chromatids attached to opposite poles the spindle aligns the sister chromatids along the metaphase plate during metaphase 2 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 cytokinesis occurs and cleavage Furrow separate the two daughter cells four haploid daughter cells the haploid daughter cells will specialize in two gametes either sperm or egg these fuse in fertilization performa 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 and fertilization is random this contributes to genetic diversity this explains why a child isn't identical either parent but why aren't siblings identical the diversity comes from several sources in prophase 1 Nun Sister Chrome can exchange DNA through crossing over increasing the genetic diversity of individual chromatids in addition to crossing over in metaphase 1 the pairs of homologous chromosomes align independently along the metaphase plate and sort independently into the daughter cells this process call Independent Assortment produces four genetically distinct haploid gametes as the total number of chromosomes increases in an organism number of genetically distinct gametes increases by 2 to the nth power this means an organism with n equals 3 can produce eight unique gametes for humans where an equals 23 there are two to the 23rd power unique gametes form 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 why a child will not look exactly like his or her parents or siblings

Uploaded by D. Henriques.

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Meiosis Introduction

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