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Most of them are normally microscopic and possess a cell wall or a cell membrane rather than an exoskeleton, thus, they do not have a fossil record. Single celled organisms belong to either prokaryotes including…
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mitive; karyon= nucleus) cells lack a true nucleolus, cellular organelles such as mitochondria, chloroplast, endoplasmic reticulum (ER), Golgi bodies etc. The DNA found in prokaryotes is circular, devoid of basic histones; they do not undergo normal cell division of mitosis and meiosis, they multiply by simple replication of DNA, followed by binary fission, thereby the two daughter cells are clones (exactly similar).
Single celled organisms do not possess defined nutrition and excretory mechanisms. Some single-celled organisms such as amoeba have simple cell structures known as vesicles that are responsible for excretion and osmoregulation. Moreover, they ventilate through simple diffusion process across the cell membrane. Single cellular organisms can be deemed as simple, as unlike eukaryotic cells where progeny is formed after the fusion of male and the female gametes, and the progeny possesses the characteristics of both the parents, while most single celled organism do not. As a matter of fact, they can divide a number of times within one hour.
Additionally, unlike most multicellular organisms, single-celled organisms can readily synthesize most of the essential substances that they need from simple nutrients. These processes are very simple and straight forward. Also, single-celled organisms can be considered as simple due to their limited capacity to occupy a wide range of habitats. This is attributed to the fact that they do not have division of labour necessary for the exploitation and subsequent utilization of a wide range of resources. On the contrary, they achieve division of labour by formation of colonies, which they achieve by having their daughter cells remaining attached after subsequent cell divisions. Variation has been estimated as a vital component in the sustenance of species. Most single-celled organisms tend to lack variation owing to their ability to reproduce asexually, implying that organisms retain the exact genetic composition ...
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