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Cell reproduction is required in the human body for growth, reproduction and for essential repairs in the body. The reproduction of cells occurs with the help of cell division. Cell division occurs when a parent cell divides into two new daughter cells. There are four parts of cell division, first a reproductive signal, starts cell division. [Sadava et al. 2008]
Cell reproduction is different in prokaryotic and eukaryotic cells. A prokaryotic cell, a cell which does not have a nucleus, reproduces via binary fission. Their genetic material, which would normally be found in the nucleus, exists as a simple Deoxyribonucleic acid (DNA) molecule. A eukaryotic cell, a cell which has a nucleus, reproduces when the genetic material in the nucleus replicates and then the cell is divided by cytokinesis. [Sadava et al. 2008]
Prokaryotic cells reproduce differently to eukaryotes due to the absence of a nucleus. First the reproduction needs to be initiated using a signal, The DNA molecule replicates and gets longer to an extent that it is twice as long, and then divides in the middle, into two separate molecules. The cell then undergoes cytokinesis, and the two DNA strands move to the opposite ends of the now dividing cell, then once cytokinesis is complete, two new cells have been made. [Angert. 2005]
The reproduction of a eukaryotic cell mostly takes place in the nucleus. Reproduction in the eukaryotes are of two types, mitosis and meiosis. There are significant differences between these two types, as mitosis involves the replication of one parent cell into two daughter cells, the parent and daughter cells are all diploid, all having 46 chromosomes.
Reproduction of a eukaryotic cell by mitosis can be explained by using â€œThe Cell Cycle.â€ In the cell cycle there are four phases, the M-phase, which is mitosis the S-phase, where DNA synthesis takes place, the G1 phase, which is the gap between the M and S phases, and the G2 phase, which is where preparation for cell reproduction takes place. The S, G1 and G2 phase are collectively known as the interphase, where the cell is not reproducing. The process of reproduction is firstly the cell is in G1, which will prepare it for the S-phase, but it is important to know the time it will remain in G1. Some cells move from G1 to S-phase straight away, but others may lie dormant for a while, which is a phase called G0. While in G0, the cell is â€œresting.â€ The cell may move out of the phase G0 to G1, with the help of a growth factor. [Israels and Israels. 2000] Once the cell moves to S from G1, DNA replication takes place. DNA replication is semi conservative, where the existing strands of DNA act as a template, and when reproduction takes place, each of the new strands of DNA are exactly half of new material and half of old. [Sadava et al. 2008] Once the replication has taken place, the cell moves to G2 phase, which is another interval where the cell is being prepared for mitosis. [Israels and Israels. 2000] On reaching the M-phase the cell has been prepared for reproduction and starts to reproduce. During the G2 to M transition, the centrosomes move to the opposite ends of the nucleus, and forms spindle poles. Mitosis consists of five stages, prophase, pro-metaphase, metaphase, anaphase and telophase. In prophase, the chromatids (chromatids are parts of a pair of new chromosomes formed from DNA replication) are visible. In prometaphase, the nuclear envelope disappears and the chromosomes are pushed to the equator of the cell. In metaphase the centromeres (the region in the chromosome where the two chromatids join) have lined up at the equatorial place. In anaphase the chromatids separate and gradually move away, to the opposite poles, the spindles. Finally in telophase the spindles break down and nuclear envelopes form, two nucleuses form. Then finally in cytokinesis the cytoplasm divides into two new cells each with one nucleus. Mitosis produces two daughter cells which are exactly the same as the parent cell, having 46 chromosomes. This form of reproduction is useful for growth. [Sadava et al. 2008]
Meiosis, on the other hand, is the reproduction involving one diploid parental cell, reproducing into four haploid cells, the parental diploid cell having 46 chromosomes, while each daughter cell, being haploid, and having only 23 chromosomes. [Sadava et al. 2008]
Meiosis is not to be confused with mitosis as two nuclear divisions take place. There are more stages to meiosis compared to mitosis. In prophase I, the chromatids coil up and shorten. Then in metaphase the chromosomes line up on the equatorial plate. In Anaphase I the chromosomes move to the opposite ends of the cell. In telophase I the chromosomes form nuclei and the cell divides into two cells in cytokinesis. Then the second division takes place. [Sadava et al. 2008]
Meiosis II involves the reproduction of the two daughter nuclei into a further four nuclei. The four stages are similar to those of meiosis 1. In prophase II the chromosomes coil up gain, DNA does not replicate, metaphase II involves the centromeres lining up at the equator of the cell, then anaphase II is where the chromosomes separate, and telophase involves the formation of nuclear envelope and cytokinesis splits the two cells into four. [Sadava et al. 2008]. Meiosis is very important for the reproductive organs. The reason why the cells of meiosis are haploid (23 chromosomes) because two gametes form a zygote (a fertilised organism), which is fertilisation. In humans, sperm and eggs both have 23 chromosomes, they are formed via meiosis. When a sperm and egg fertilise the zygote becomes diploid with 46 chromosomes.
Therefore in conclusion, prokaryotes reproduce only by binary fission and eukaryotes reproduce by mitosis and meiosis. Prokaryotic cells do not have a nucleus, so they do not reproduce in the same way as eukaryotes, which have a nucleus and reproduce very differently. Eukaryotes reproduce by mitosis and meiosis, which depend on the function of the cell. Most cells, for growth and repair, reproduce by mitosis forming two identical diploid cells, whereas in meiosis there are only 23 chromosomes in each of the four cells it produces, which is required for reproduction.