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Organelles are a category of structures that constitute the cell within the organism. The most prominent one in eukaryotic cells is the nucleus, which contains the DNA and controls the cells operations. The mitochondria are primarily responsible for energy conversion, and the ribosomes produce proteins that are necessary for operations. The endoplasmic reticulum, which consists of rough (RER) and smooth (SER) areas, manufactures carbohydrates, hormones, lipids and other items, and the Golgi complex stores them. The lysosomes digest nucleic acids, polysaccharides, proteins and fats, and the peroxisomes break down fats. There is also a multitude of other organelles, many of which are only present in specific organism types, but they are beyond the scope of this discussion.
Nucleic acids are responsible for encoding the cells genetic information, which is copied and passed on to its children. They are separated into two general types: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). RNA is separated into multiple different types, of which this essay will mention messenger (mRNA), which is the copy produced during DNA transcription, and transfer (tRNA), which assists in translating mRNA into proteins. The DNA stores the information long-term within the nucleus and consists of chromosomes formed of four bases: adenine, guanine, cytosine and thymine. Combined, the two types of acid form the basis for the processes of transcription and translation, which are central to cell division.
To form proteins, it is necessary to retrieve the genetic information contained in the DNA. However, to avoid contamination, the cell first forms an RNA copy of that information in a process known as transcription. An enzyme known as RNA polymerase attaches to DNA, transcribes a strand into mRNA and detaches from it. After that, the DNA segments that do not contribute to protein formation (introns) are removed in a process known as mRNA splicing. The resulting copy is then moved to the ribosome, where it is translated into a polypeptide chain with the help of tRNA. Said chain then finally becomes a protein through post-translational modification, becoming usable by the cell.
The translation process uses a specific system, which is known as a genetic code. It is expressed via the aforementioned adenine (A), guanine (G), cytosine (C) and thymine (T). When transcribing DNA to RNA, the cell replaces thymine with uracil (U), a highly similar compound, in the copy. Combinations of these letters are used to encode information such as where the transcription should begin or end or the structure of the protein that is to be created. These instructions are read in three-element blocks known as codons, and each one describes either a single instruction or part of an instruction. These blocks are the same for every living organism, showing their shared ancestry.
Cells are typically separated into two different types: prokaryotes and eukaryotes. The former are earlier and less advanced, lacking membranes that separate organelles, including the nucleus, from each other. Instead, all of the cells components, including the DNA, float freely within it. Eukaryotes, on the other hand, evolved later and have the membranes in question. The formation of the nucleus is generally considered their most significant differentiating factor, as it protects the DNA from interference. Another notable difference is that eukaryotic cells have numerous chromosomes, as opposed to a minuscule number for prokaryotes. Most multicellular organisms are eukaryotic, indicating the relatively advanced level of evolution that the approach represents.
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