![]() In metaphase II, the chromosomes align at the equator of the cell. So, in prophase II, there is no synapsis of homologous chromosomes or crossing over. The major difference between prophase II and prophase I is the fact that the daughter cells have only one copy of each homologous chromosome. Again, chromosomes condense, the nuclear envelop breaks down, and the spindle apparatus forms. The first stage of meiosis II is prophase II. Here the cell divides into two daughter cells.Įach of these two cells now undergo meiosis II. The final steps of meiosis I are telophase I and cytokinesis. Importantly, the sister chromatids remain attached at their centromeres. During this phase, homologous chromosomes separate and migrate to the two poles of the cell. The chromosomes align randomly which results in different combinations each time meiosis occurs. Here the synapsed chromosomes align at the equator of the cell. The next phase of meiosis I is called metaphase I. The microtubules then attach to the chromosomes. Then the two centrosomes migrate to opposite ends of the cell and microtubules appear. The nuclear membrane begins to break down. There are several more key steps in prophase I. This is one of the reasons why no two siblings (aside from twins) are genetically identical. After crossing over, the sister chromatids for each chromosome are no longer identical to one another. This event is called recombination or more commonly, crossing over. Chromosomal material is exchanged between the two pairs of sister chromatids. Next, each pair of homologous chromosomes undergoes synapsis to form a complex involving two pairs of sister chromatids. They stay fused at the centromere throughout Meiosis I. Here we see the duplicated sister chromatids joined together at the centromere. In this stage, the DNA condenses to form chromosomes. Let’s look more closely at each of the division events. These haploid cells are the gametes that could go on to produce an offspring through sexual reproduction. The second division, meiosis II, results in four unique haploid cells that only have one copy of each chromosome. The first division, meiosis I, results in two unique daughter cells that have half the amount of DNA as the parent germ-line cell. There are two cell division events during meiosis. ![]() These chromatids remain attached to each other until the second cell division event in meiosis. The duplicated germ-line chromosomes are called sister chromatids. The DNA inside a germ-line cell is duplicated before meiosis begins during the S phase. Similar to mitosis, the cells also pass through the interphase, G1, S, and G2 stages before they enter meiosis. ![]() Meiosis is just one step in the life cycle of a germ-line cell. These haploid gametes fuse to form a diploid embryo that grows into the adult. Germ-line cells undergo meiosis to produce haploid gametes which have only one copy of each chromosome. In diploid organisms, germ-line cells have two copies of each chromosome. The cells that undergo meiosis to produce the gametes are called germ-line cells. ![]() Gametes are formed through a process called meiosis. The embryo then grows into an adult who in turn passes their genetic information on to their offspring. This begins when two gametes unite to form an embryo that is genetically unique from the parent organisms. Many organisms pass their genes to their offspring through sexual reproduction.
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