Mitosis and Meiosis - Assignment Example

There are 3 aspects that are critical for the survival of all organisms and they are growth, repair and reproduction. Division of cells is very important for these events. Division of cells happens through 2 different types of processes, the mitosis and meiosis which constitute different phases of cell cycle (Sadava, 2013). As and when needed, different hormones in the body of the organism send signals to the cells for preparation for division. There are mainly 2 parts in the process of division.

The first phase is the interphase during which there is growth and preparation. This is followed by cell division which includes cytokinesis and mitosis. Interphase constitutes 90 percent of the cell cycle. During mitosis, the nucleus of the cell is replicated and divided into 2 nuclei that are similar and have identical genetic material. This type of cell division is mainly for growth of cells, repair and asexual reproduction. The end result of mitosis is somatic cells, each of which will have diploid number of chromosomes (Sadava, 2013).

There are basically four stages in mitosis. The first stage is known as prophase. Sister chromatids in the cell condense and become visible. The envelope of the nucleus breaks to expose the chromosomes. Formation of spindle fibers begins extending from the centrioles. The spindle fibers are made up of microtubules and they are attached to the centromere of sister chromatids. The centrioles migrate to opposite sides of the cells slowly. The next phase is the metaphase. There chromosomes form a line along the equatorial plate which is in the center of the cell.

From here, the chromosomes are moved with the help of centrioles and spindle fibers. The third phase is the anaphase. The centromeres are pulled apart and the sister chromatids separate. This is done by spindle fibers. The daughter chromosomes are also pulled to opposite poles. Thus each daughter cell will have identical set of chromosomes similar to the parent cell. In the telophase, new nuclei begin to form in each cell around chromosomes. The chromosomes also unravel into their loose form and spindle fibers disassemble.

Cytokinesis starts and new cells are towed (Sadava, 2013). In cytokinesis, the cytoplasm of the cell divides into two parts, each one having a new nucleus. Thus, the daughter cells are genetically identical to the parent cell. Meiosis is another type of cell division which occurs in reproductive cells or germ cells only. It involves two fissions of the nucleus and gives rise to four sex cells or gametes (Sadava, 2013). Each of the cells has half the number of chromosomes of the parent cell. Meiosis occurs in 2 stages, meiosis -1 and meiosis-2.

In meiosis-1, the pairs separate and in meiosis-2 sister chromatids separate as in mitosis. There are four steps in meiosis-1. The interphase is similar to mitosis during which the DNA replicates and forms sister chromatids connected at the centromere. However, homologous pairs of chromosomes pair up. In prophase-1, the chromosomes thicken and the homologous pairs of chromosomes tangle together and move towards the equatorial plate. Thus, 4 sister chromatids move together and this is known as tetrad.

Nuclear envelop disappears and spinal fibers start forming. Crossing-over also occurs in this phase. Swapping of parts of chromosomes also occurs due to tangling. This leads to genetic variation between individuals. In metaphase-1, homologous pair line up along the equatorial plate. In anaphase-1, separation of homologous pairs occurs due to pulling of spindle fibers from the centromere. Now, each chromosome has 2 sister chromatids. In telophase-1, cytokinesis occurs with or without formation of nuclear membrane and this leads to new cells with haploid set of chromosomes in each and resembles sister chromatids (Sadava, 2013).

In meiosis-2 that occurs after cytokinesis and telophase-1, division of cells occurs, but