Another of the basic and universal properties of cells, Alberts et al. tell us, is that:
Though I have talked a lot about DNA copying itself, this is not all that is required for DNA to carry out its function as an information-bearing molecule. DNA must also express the information that it carries, allowing the information to guide the synthesis of other molecules in the cell. In all living organisms, the method by which this expression occurs is the same, and first leads to the production of RNAs and proteins, two other classes of polymers that are essential. The process begins with transcription, which is a templated polymerization. In transcription, segments of the DNA sequence serve as templates for the creation of shorter molecules related closely to RNA, or ribonucleic acid. Translation, a more complex process, occurs later on and involves the RNA molecules directing the creation of proteins, another type of polymer of a largely different chemical class (Alberts et al. 2014).
There are a few slightly differences between RNA and DNA that are important to understand. First, the sugar-phosphate backbone of DNA and RNA differ in that, in RNA, the backbone is made of ribose rather than the deoxyribose that makes up the backbone of DNA. Secondly, the bases differ slightly between RNA and DNA. RNA, rather than the thymine (T) that relates to DNA, uses uracil (U) instead. However, only uracil is different, and the other bases are the same including adenine, guanine, and cytosine. And in RNA, all four bases pair with their complementary counterparts in DNA: A, U, C, and G of RNA pair with the T, A, G, and C of DNA, respectively. And during the process of transcription, RNA monomers are lined up on a template strand of DNA and selected for polymerization. This is the same for DNA monomers, when they are selected during replication. What this results in is a polymer molecule with a sequence of nucleotides that successfully represents a piece of the cell’s genetic information. This is despite the fact that it is written using a slightly different alphabet (A, U, C, and G rather than T, A, G, and C) that consists of RNA monomers rather than DNA monomers (Alberts et al. 2014).
Many identical RNA molecules can be synthesized using the same segment of DNA repeatedly. So, while the storage of the cell’s genetic information in the form of DNA is fixed and not to be altered in any way, RNA transcripts are different in that they are produced on a massive scale and are disposable. What ends up happening is that these RNA transcripts serve was intermediates in the transfer of genetic information. They serve most notably as messenger RNA (mRNA) molecules, which according to the genetic instructions within DNA, guide the synthesis of proteins (Alberts et al. 2014).
The structures of RNA molecules are also specialized such that they can have other distinct chemical capabilities. RNA backbones are flexible because they are single-stranded. So, the polymer chain is allowed to bend back on itself, which allows parts of the molecule to bind to other parts along itself, forming weak bonds. This is the result of segments of the sequence being locally complementary, such as a segment consisting of “GGGG” associating with a segment of “CCCC.” So, the RNA chain may fold up into a specific shape that is dictated by its sequence. The specificity of the shape formed by the RNA molecule can promote the recognition of other molecules by selectively binding to them. In special cases, chemical changes in the bound molecules may be catalyzed by the RNA molecules, and some of the chemical reactions used in this process have been shown to be around since time immemorial, making it the case that this extensive catalysis by RNA might have played a large role in life in its early evolutionary stages (Alberts et al. 2014).
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