meiosis is the process that results in the formation of sperm cells and egg cells the cells that will undergo meiosis are typically found in the testes and ovaries of males and females respectively these germline cells are diploid having two sets of chromosomes but will undergo meiosis to produce haploid cells having one set of chromosomes during fertilization these haploid cells fuse to form a diploid offspring like mitosis meiosis is preceded by an

interphase during this time dna replicates and each chromosome becomes doubled consisting of two identical strands of dna meiosis involves two divisions the first division meiosis one halves the number of chromosomes and in the second division meiosis ii the sister chromatids are split the end result is four haploid cells now let's look at each of these divisions in more detail

meiosis one begins with prophase one individual replicated chromosomes become visible and each chromosome consists of identical sister chromatids joined at their centromeres the spindle fibers start to form and the nuclear envelope disintegrates in addition homologous chromosomes line up next to each other and an event unique to meiosis occurs during this unique event called crossing over two homologous chromosomes which are lined up next to each other

exchange dna between adjacent homologous chromatids after crossing over has occurred the sister chromatids of one chromosome are no longer identical to one another in metaphase one homologous chromosomes line up along the equator of the cell for each pair of homologs the orientation on the equator is random currently the large purple chromosome and the small green chromosome are on top however the small purple chromosome could just have easily have ended on top

each different orientation results in gametes with different combinations of parental chromosomes this process is called independent assortment once the chromosomes are lined up the ends of the spindle fibers attach to each centromere in anaphase one the chromosomes move apart from one another along the spindle fiber to the opposite ends of the cell at this point each chromosome is still double stranded and has two sister chromatids

however we are separating homologous chromosomes into two different cells so that each new cell will have only one member of the homologous pair and will be haploid in telophase 1 the spindle fiber disintegrates and cytokinesis cell division begins thus in meiosis 1 which progresses from prophase one to telophase one two cells are created and each cell has half the number of chromosomes compared to the original cell

after cytokinesis is complete a second division or meiosis ii starts this division is identical to mitosis and the resulting cells have the exact same number of chromosomes as the original cell at the end of meiosis one in prophase two we now have two cells each with two chromosomes in this stage the spindle fibers again start to form at the poles of the cell in metaphase two the chromosomes line up along the equator

this is different from metaphase 1 where the homologous chromosomes were lined up in metaphase 2 each cell has only one of each homologous chromosome in anaphase ii the sister chromatids move away from each other along the spindle fiber and in telophase ii along with cytokinesis we see the formation of four genetically different haploid cells you