Discuss in Detail the Cell Cycle, Mitosis and Meiosis Essay
Discuss in Detail the Cell Cycle, Mitosis and Meiosis
Cells have the ability to grow, have particular functions, and replicate during their life. Although cell enlargement is part of organismal growth, cell replication is also required and allows growth without each cell becoming too large. All of these activities are part of a repeating set of events known as the cell cycle. The major feature in the cell cycle is cellular replication and what enables for cellular replication is the process of mitosis. Mitosis is the only part of the cell cycle, and the remainder of the cycle consists of interphase, cytokinesis, gap 1, synthesis, and gap 2 phases. But cell division and reproduction can occur in two ways mitosis (which I just mentioned), and meiosis. Mitosis is used by single celled organisms to reproduce; it is also used for the organic growth of tissues, fibers, and membranes. While meiosis is useful for sexual reproduction of organisms; the male and female sex cells, the sperm and egg combined to create a new biological organism, in other words a baby.
Mitosis is the replication and division of the nucleus of a eukaryotic cell in preparation for cytokinesis. Cytokinesis is the division of the cell and follows immediately after mitosis. During mitosis, replicated chromosomes within the cell are separated into two identical sets. Each of the two new nuclei has a full set of chromosomes containing a copy of all the genetic material for the organism. Mind that only eukaryotic cells go through this process; prokaryotic cells lack a nucleus so they undergo binary fission in order to replicate their chromosome. Interphase basically starts off the cell cycle. Stages in interphase are G1, S, and G2.
The G1 stage of the cell cycle occurs after mitosis and cytokinesis, and is when the majority of cellular activity for the functions of the cell occurs. Many cell-specific proteins and other molecules are produced for the metabolism of the cell during this stage. During the S stage, the DNA making up the chromosomes is duplicated. At the end of the S stage each chromosome consists of an identical pair of chromosomal DNA strands known as sister chromatids and attached at the centromere. And during the G2 stage, molecules and structures necessary to undergo mitosis are synthesized.
The purpose of mitosis is to increase the number of cells without changing the genetic material. The daughter cells are identical to the parent cell in the number of chromosomes, the genes, and alleles. Mitosis can occur in haploid, diploid, or polyploidy cells. Mitosis is divided into four phases. The four phases are prophase, metaphase, anaphase, and telophase. During prophase, chromosomes become visible, centrioles move to opposite poles in a spindle formation, nucleolus becomes invisible, and nuclear membrane disappears.
At this stage, each chromosome consists of two identical sister chromatids, held together by a centromere. During metaphase, chromosomes move to the equator and spindle microtubules attach to the centromeres. During anaphase, centromeres separate, chromatids separate and move to opposite poles; this is where they are now called chromosomes. And lastly, during telophase, chromosomes have arrived at poles, spindle disappears, centrioles replicate, nuclear membrane reappears, nucleolus becomes visible, and the chromosomes become chromatin.
Meiosis is a type of cellular reproduction in which the number of chromosomes is reduced by half through the separation of homologous chromosomes in a diploid cell. Meiosis is sometimes also known as “reduction division”. Although meiosis and mitosis are similar, and their corresponding stages of prophase, metaphase, anaphase, and telophase have much in common; meiosis is a much lengthy process than mitosis. It is much longer because meiosis involves two processes of division instead of one. These two reductions are called meiosis I and meiosis II.
Meiosis I follow premeiotic interphase after the chromosomes of a diploid cell have replicated. DNA replication then occurs during the S phase. After the two successive divisions of meiosis, a single cell will produce four cells each having the haploid number of chromosomes. Prophase I of meiosis begins with the condensation of chromosomes, and the vesiculation of the nuclear membrane. Centrosomes, which duplicate at the beginning of meiosis, begin moving apart and the spindle fibers begin forming. Early in prophase, synaptonemal complexes form and the homologous chromosomes are seen to align next to each other to form a structure called a bivalent. A bivalent is composed of four sister chromatids and crossing will occasionally occur between homologous chromatids within a bivalent. During prometaphase I, the spindle fibers continue to form and bivalents become attached to kinetochore microtubules.
One pair of sister chromatids is connected to one pole while the homologous pair sister chromatids are connected to the opposite pole. The complete assembly of the spindle fibers occurs during prometaphase I. The chromosomes, bivalents, align at the metaphase plate at metaphase I. Then, anaphase I involves the separation or disjunction of homologous chromosomes with each pair of sister chromatids moving to the opposite pole of the cell. When the conjoined sister chromatids reach the poles of the cell, they detach from the spindle fibers and the nuclear membrane begins to form. A cleavage furrow forms and cytokinesis results in two cells having the haploid chromosome number with each chromosome consisting of a pair of sister chromatids. This completes Meiosis I, and after a period of interkinesis the two daughter cells start the second cell division cycle called Meiosis II.
At prophase II, the chromosomes condense again, the spindle begins to form, and the nuclear membrane begins to split. During prometaphase II, the sister chromatids begin to attach to the kinetochore microtubules. Here is the difference between prometaphase I, the pair of sister chromatids is attached to both poles in prometaphase II, and not just one. At metaphase II, sister chromatids align along the metaphase plate. In anaphase II, the sister chromatids separate and individual chromosomes move toward poles as kinetochore microtubules shorten. Polar microtubules lengthen and push poles apart. Telophase II, the nuclear membrane reforms and chromosomes decondense. And in animal cells, a cleavage furrow forms causing cytokinesis. When cytokinesis is completed, meiosis II results in the formation of four haploid cells.