Mitosis and Meiosis Reproduction in Eukaryotic Cells

Mitosis and meiosis are two different forms of reproduction in eukaryotic cells. These two processes are similar in some aspects while different in others. Both result in the creation of new cells, but through different methods. Mitosis is a type of asexual reproduction, while meiosis is a type of sexual reproduction. These two processes are vital for the survival of cells and organisms. Mitosis is defined as a process of nuclear division in eukaryotic cells in which two daughter cells are formed that are genetically identical to the parent cell. It preserves the chromosome number by giving an equal amount of replicated chromosomes to each of the daughter nuclei.

The overall purpose of mitosis is asexual reproduction, growth, and cell regeneration in the somatic cells, or the body cells. Mitosis is broken down into five stages: prophase, prometaphase, metaphase, anaphase and telophase. In prophase, tightly coiled chromatin condense into chromosomes, the mitotic spindle begins to form and the nucleolus disappears, with the nucleus remaining intact. Next, prometaphase will occur, in which the nuclear envelope fragments and the spindle microtubules will attach to the kinetochores of chromosomes. In metaphase, the mitotic spindle completes its formation and the chromosomes, attached to microtubules at the kinetochores, will align at the metaphase plate. Anaphase consists of the chromatids of each chromosome separating and the daughter chromosomes moving to opposite poles of the cell. The mitotic phase ends with telophase, where two daughter nuclei form.

Cytokinesis typically overlaps with late telophase. Mitosis ends with the outcome of two diploid daughter cells that are genetically identical to their parent cell. The genetic material in the daughter cells remains constant thus there is no genetic variation that takes place in mitosis. On the other hand, meiosis is defined as a modified type of cell division in sexually reproducing organisms. It consists of two rounds of cell division but only one round of DNA replication. The cells end up with half the number of chromosomes as the original cell. The purpose of meiosis is to provide genetic diversity through sexual reproduction and to ensure that organisms that reproduce via sexual reproduction contain the correct number of chromosomes. The chromosomal reduction takes place in eukaryotic cells, such as plants, animals and fungi, and it leads to the production of sex cells, or gametes. Without the reduction in chromosome number, the merging of two gametes during fertilization would end in offspring with twice the number of chromosomes necessary. Meiosis consists of two sub-divisions- meiosis I and meiosis II- unlike the single step of mitosis.

Meiosis I focuses on the reduction of chromosomes, going from a diploid cell to a haploid cell, while meiosis II focuses on the separation of sister chromatids. Meiosis I begins with prophase I, which in itself contains five stages: leptotene, zygotene, pachytene, diplotene and diakinesis. This phase consists of the condensation of chromatin into chromosomes, the breaking down of the nuclear envelope, synapsis of chromosomes in each homologous pair and the crossing over between those synapsed chromosomes. Crossing over produces recombinant chromosomes, which contain combined genes that are inherited from each parent. Metaphase I continues with the random alignment of tetrads at the metaphase plate, rather than individual chromosomes, as in mitosis. Next, in anaphase I, the homologous chromosome move to the opposite poles of the cell, with the sister chromatids of the duplicated chromosome remained attached. Telophase I follows with the spindle fibers still moving to the opposite poles. Once this is complete, each pole consists of a haploid number of chromosomes.

Cytokinesis overlaps with this stage and two daughter cells are produced that contain half the number of chromosomes as the parent cell. The cell will then enter Meiosis II, starting with prophase II. This phase is defined by the formation of the spindle apparatus and the shifting of chromosomes toward the metaphase II plate. Metaphase II continues with the alignment of chromosomes at the metaphase plate and the kinetochores of sister chromatids are attached to microtubules, which extend from opposite poles. In anaphase II, the sister chromatids separate and move towards opposite poles through the elongation of unconnected spindle fibers. These separated sister chromatids are now considered daughter chromosomes. Meiosis II ends with telophase II, in which nuclei form and chromosomes begin to decondense. Cytokinesis follows, resulting in four haploid daughter cells that are genetically distinct from the parent cell. This genetic variation is caused by independent assortment of homologous chromosomes and non identical sister chromatids, crossing over between the homologous chromosomes and the random fertilization of an ovum by a sperm within meiosis.

As outlined above, mitosis and meiosis are two distinct processes, but are similar in some ways. Before either process can begin, interphase must occur. Interphases is defined by the G1, G2, and S phases. In the G1 and G2 phase, cell growth takes place, while in the S phase, DNA replication occurs. These steps must take place for the cell to either undergo mitosis or meiosis. Both of these processes begin with a diploid parent cell, even though the outcome of each will differ. Mitosis and meiosis both consist of the same multiple stages that are: prophase, metaphase, anaphase, telophase and cytokinesis. These stages will remain constant regardless of the differences in each specific step. Another similarity between mitosis and meiosis is the condensation of chromatin into chromosomes. In prophase, this is the first step to take place and allows the chromosomes to become visible.