What is RNA and What are its Types?

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RNA is the second most well-known ribonucleic acid after DNA. When it comes to structure, it is highly similar to DNA, but has some important differences. For example, RNA has one distinct nucleotide called uracil (or U) instead of thymin (or T). Furthermore, RNA commonly exists in single-strand format, while DNA is almost always double-stranded.

Human DNA GraphicAs we discussed in the previous post, all human cells contain genetic information for protein expression in the form of DNA. Our DNA is tightly packed in the cell nucleus to ensure protection from outside factors. However, almost all processes that are required for protein generation are taking place outside the nucleus. For this reason, there are mechanisms in place for creating copies of genetic information that can traverse from the nucleus to other locations. This is where RNA comes into play, since it can be synthesized in a complementary manner to DNA and contain the same type of genetic information. Furthermore, RNA can interact with proteins and create necessary complexes for various cell processes.

The main types of RNA are the following:

  • mRNA: Also known as messenger RNA, mRNA is a single-stranded RNA molecule that contains information for generating a protein sequence. It is created in the nucleus through the process of transcription, during which the two DNA strands are temporarily separated to copy the information of one gene onto the new mRNA molecule. Then, mRNA can exit the nucleus and go to the ribosomes, where proteins are generated using its information.

  • tRNA: tRNA, or transfer RNA, is a small RNA molecule that is essential for protein synthesis. Proteins consist of amino acids, and each amino acid has a 3-letter code (or codon) in the DNA or mRNA that corresponds to it. This way a DNA or RNA sequence can be translated to a protein sequence. Codons are examined in series, and tRNA is responsible for bringing the correct amino acid depending on each codon. This is done by utilizing the complementary structure of one tRNA part to the codon triplet.

  • rRNA: Ribosome RNA (or rRNA) is an RNA molecule that is a fundamental component of ribosomes. As mentioned before, ribosomes are the machinery for protein synthesis in the cell. There are two rRNA subunits in each ribosome, a large one and a small one. Along with proteins and enzymes, they facilitate the process of translating an mRNA sequence into protein.

Honorable Mentions

There are some other RNA types in human cells that are not commonly known but are often used in molecular biology research. For example, micro RNA or miRNA is a small single-strand RNA molecule that can silence other types of RNA (such as mRNA) by binding to them using the rules of complementarity. Small interfering RNA or siRNA is a double-stranded RNA molecule that has a similar goal, but acts by triggering an mRNA degradation response called RNA interference. Overall, these molecules can affect protein expression levels by effectively silencing certain genes without affecting the DNA itself.

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— Vicky Kanta