The pairs separate and move to ends of the cell
11.1B: Meiosis I
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Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction. The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid.
Meiosis : First Look. The following images illustrate the key stages of meiosis. Clicking on each of the thumbnail images will bring up a larger, labeled version of the described scene. To see the Flash movie for the following sequence of images, click here. During prophase I, homologous chromosomes pair up and exchange sections of DNA. This is called recombination or crossing over. This is followed by metaphase I where the connected pairs of chromosomes align at the middle of the cell.
Mitosis is the normal type of cell division. Before the cells can divide, the chromosomes will have duplicated and the cell will have twice the normal set of genes. The first step of cell division is prophase , during which the nucleus dissolves and the chromosomes begin migration to the midline of the cell. Some biology textbooks insert a phase called "prometaphase" at this point. The second step, known as metaphase , occurs when all the chromosomes are aligned in pairs along the midline of the cell. As the cell enters anaphase , the chromatids, which form the chromosomes, will separate and drift toward opposite poles of the cell. As the separated chromatids, now termed chromosomes, reach the poles, the cell will enter telophase and nuclei will start to reform.
It contains one or more nucleoli. Just outside the nucleus are two centrosomes, formed duirng early interphase by replication of a single centrosome.
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In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells. Meiosis is preceded by an interphase consisting of three stages. The G 1 phase also called the first gap phase initiates this stage and is focused on cell growth. The S phase is next, during which the DNA of the chromosomes is replicated. This replication produces two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. Finally, during the G 2 phase also called the second gap phase , the cell undergoes the final preparations for meiosis.
Cell Biology/Cell division/Mitosis
M phase is relatively brief and consists of nuclear division mitosis and cytoplasmic division cytokinesis. In mitosis, replicated chromosomes divide, leading to identical daughter nuclei with the same number of chromosomes and the same genetic composition as the parents.