[Music] many organisms pass their genes to their offspring through sexual reproduction this begins when two gametes unite to form an embryo that is genetically unique from the parent organisms the embryo then grows into an adult who in turn passes their genetic information on to their own offspring gametes are formed through a process called meiosis the cells that undergo meiosis to produce the gametes are called germline cells in diploid organisms germline cells have two copies of each chromosome germline cells undergo meiosis to produce haploid gametes which only have one copy of each chromosome these haploid gametes fuse to form a diploid embryo that grows into the adult meiosis is just one step in the life cycle of a germline cell similar to mitosis the cells also pass through the interphase g1 s and g2 stages before they enter meiosis the DNA inside a germline cell is duplicated before meiosis begins during the S phase the duplicated germline chromosomes are called sister chromatids these chromatids remain attached to each other until the second cell division event in meiosis there are two cell division events during meiosis the first division meiosis one results in two unique daughter cells that have half the amount of DNA as the parent germline cell the second division meiosis two results in four unique haploid cells that only have one copy of each chromosome these haploid cells are the gametes that could go on to produce an offspring through sexual reproduction let's look more closely at each of the division events meiosis begins with prophase one in this stage the DNA condenses to form chromosomes here we see the duplicated sister chromatids joined together at the centromere they stay fused at the centromere throughout meiosis 1 next each pair of homologous chromosomes undergoes synapses to form a complex involving two pairs of sister chromatids chromosomal material is exchanged between the two pairs of sister chromatids this event is called a recombination or more commonly crossing over after crossing over the sister chromatids for each chromosome are no longer identical to one another this is one of the reasons why no two siblings aside from twins are genetically identical there are several more key steps in prophase one the nuclear membrane begins to break down then the two centrosomes migrate to opposite ends of the cell and microtubules appear the microtubules then attach to the chromosomes the next phase of meiosis one is called metaphase 1 here the synapsed chromosomes align at the equator of a cell the chromosomes align randomly which results in different combinations each time meiosis occurs the next phase is anaphase 1 during this phase homologous chromosomes separate and migrate to the two poles of the cell importantly the sister chromatids remain attached at their centromeres the final steps of meiosis 1 are telophase 1 and cytokinesis here the cell divides into two daughter cells each of these two cells now undergo meiosis 2 meiosis 2 is similar to mitosis the first stage of meiosis 2 is prophase 2 again chromosomes condense the nuclear envelope breaks down and the spindle apparatus forms the major difference between prophase 2 and prophase 1 is the fact that the daughter cells have only one copy of each homologous chromosome so in prophase 2 there is no synapses of homologous chromosomes or crossing over in metaphase to the chromosomes align at the equator of the cell again the alignment is random since the sister chromatids are no longer identical there will be many different possible ways for these chromosomes to align in anaphase to the sister chromatids are pulled apart as the microtubules shorten also the ends of the cell are pushed farther apart as microtubules elongate in telophase to the nuclear membrane reforms and the cytoplasm is divided into the two haploid daughter cells this division is called cytokinesis since meiosis 2 began with two cells and each of those cells were split into two cells we now have four unique haploid cells at the end of meiosis these cells are gametes two gametes one from a father and one from a mother may fuse to produce a diploid embryo the resulting embryo then grows through many cycles of mitosis you