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What events occur during meiosis I and meiosis II? Meiosis I and meiosis II are two important phases of cell division. Meiosis I is the first phase, during which the cell divides into two daughter cells. Meiosis II is the second phase, during which the two daughter cells divide into four gametes.
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Meiosis I
Meiosis I, also known as reduction division, is the first stage of meiosis. This stage begins with a diploid cell, which contains two copies of each chromosome. The diploid cell then goes through replication, so that each chromosome is duplicated. After replication, the cell undergoes division to produce four haploid cells, each containing one copy of each chromosome.
Prophase I
Meiosis I, the first step in creating sperm or eggs, is a reduction division that halves the chromosome number. The stage is composed of five substages: prophase I, prometaphase I, metaphase I, anaphase I and telophase I. Let’s take a look at each substage and the events that occur during them.
ProphaseI: This is the longest stage of meiosis I. In this substage, the following events occur:
-The chromatin—the material that makes up chromosomes—coils to form visible chromosomes.
-The nucleolus disappears as chromatin condenses.
-The nuclear envelope breaks down.
-Mesenchymal cells surrounding the developing gonads produce a network of tubes called the synaptonemal complex, which helps to align homologous chromosomes during meiosis I.
Metaphase I
During metaphase I of meiosis, the chromatids line up in the center of the cell on the metaphase plate, with one member of each homologous pair facing each other. This is accomplished by the spindle fibers pulling one homologous chromosome to each pole.
Anaphase I
Meiosis I, or reductional division, halves the chromosome number. This occurs during the production of sperm or eggs (gametes). During meiosis II, or equational division, the chromatids are separated to produce four genetically diverse daughter cells.
Anaphase I is characterized by the separation of homologous chromosomes (i.e. each chromosome is now paired with only one chromosome of the same type). This process is completed during meiosis I and precedes the start of anaphase II.
Telophase I
Telophase I is the final stage of meiosis I, where the daughter cells begin to form. During telophase I, the chromosomes line up in the center of the cell, and the cell begins to divide. The two new cells that are formed are called poles. The two poles are identical to each other and have half the number of chromosomes as the original cell.
Meiosis II
Meiosis II is the second part of meiosis. In meiosis II, the sister chromatids separate and move to opposite poles. This results in four genetically diverse daughter cells, each with half the number of chromosomes as the parent cell. Let’s take a closer look at what occurs during meiosis II.
Prophase II
In prophase II, the chromosomes arrive at the metaphase plate in random orientations. During this time, the crossing over of genetic material between homologous chromosomes (which began during prophase I) is completed. Crossing over is important because it further increases genetic variation by shuffling the alleles on each chromosome. This process is known as independent assortment.
Metaphase II
In metaphase II, the chromosomes line up in the middle of the cell and are pulled apart by the spindle fibers. This stage is similar to metaphase I, except that in meiosis II, there are four haploid cells instead of two. During anaphase II, the chromosomes are separated and moved to opposite poles of the cell. The cells then begin to divide in telophase II, and four new cells are formed.
Anaphase II
Anaphase II is initiated when the chromosomes reach the poles and the centromeres divide. This stage is marked by the separation of sister chromatids, which are now considered to be independent chromosomes. The centromeres will remain intact until the very end of mitosis, when they finally divide and the chromosomes are released from the spindle apparatus.
During anaphase II, the chromosomes will migrate to opposite sides of the cell, and each cell will receive an equal number of chromosomes. When meiosis II is complete, each cell will have half the number of chromosomes as the original parent cell.
Telophase II
Telophase II is the final stage of meiosis, where the two daughter cells from meiosis I divide to produce four haploid gametes. This process is similar to mitosis, with the chromosomes aligning in the middle of the cell and then splitting apart. However, there are some key differences between mitosis and telophase II.
First, during telophase II, the chromosomes do not decondense and form chromatids as they do in mitosis. Instead, they remain in their condensed state as they are pulled to opposite sides of the cell. Second, in mitosis, cleavage furrows form along the cell equator and pinch the cell into two equal halves. In telophase II, this does not happen. The cell simply divides in half without any noticeable furrowing.
Once telophase II is complete, four genetically diverse haploid cells have been produced. These cells will go on to become gametes that can be used in sexual reproduction.
