RNA

RNA, or ribonucleic acid, is a nucleic acid responsible for various cellular functions, differing from its more well-known counterpart, DNA. RNA molecules are distinct in both structure and function and come in various forms with unique roles in the cell.

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Overview

Unlike DNA, which consists of two intertwined strands forming a double helix, RNA is typically single-stranded. It incorporates the following bases:

• :(A) Adenine
• :(G) Guanine
• :(C) Cytosine
• :(U) Uracil

These bases allow for specific pairings: adenine (A) with uracil (U), and guanine (G) with cytosine (C). Notably, RNA uses uracil in place of the thymine found in DNA.

Another significant difference between RNA and DNA is the sugar component. RNA molecules contain ribose, whereas DNA contains deoxyribose. The presence of ribose renders RNA more chemically reactive than DNA, equipping it to participate actively in cellular reactions.

In specific viruses, particularly retroviruses like HIV, RNA carries genetic information, deviating from the common rule where DNA is the primary genetic material.

Protein synthesis RNAs

RNA's primary role is facilitating protein synthesis. It conveys the amino acid sequence information from genes to ribosomes in the cytoplasm, where proteins are assembled.

Messenger RNA

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The structure of a mature eukaryotic mRNA. A fully processed mRNA includes a 5' cap, 5' UTR, coding region, 3' UTR, and poly(A) tail. UTR = untranslated region

messenger RNA (mRNA) is responsible for this information transfer. A DNA strand serves as a template for mRNA transcription, which is facilitated by the enzyme RNA polymerase. Once transcribed, the mRNA moves from the nucleus to the ribosomes in the cytoplasm. Here, mRNA's sequence of bases dictates the sequence of amino acids in the protein synthesis, a process called translation.

While DNA remains in the nucleus due to its considerable size and its association with proteins like histones in chromosomes, mRNA is mobile and can interact with various cellular enzymes.

Non-coding RNAs, including transfer RNA (tRNA) and ribosomal RNA (rRNA), assist in protein synthesis.

tRNA

Transfer RNA (tRNA) is approximately 80 nucleotides in length and delivers specific amino acids to the growing polypeptide chain on a ribosome. Different tRNAs correspond to different amino acids, each having a unique attachment site for its respective amino acid and an anti-codon matching the mRNA's codon. For instance, the codons UUU or UUC denote the amino acid phenylalanine.

rRNA

Ribosomal RNA (rRNA) forms the primary component of ribosomes, cellular machinery where protein synthesis takes place. In eukaryotes, there are four rRNA molecules: 18S, 5.8S, 28S, and 5S rRNA. Three of these are synthesized in the nucleolus, while the fourth is created elsewhere. In the cytoplasm, the rRNA and proteins combine to form ribosomes, which, by binding mRNA, facilitate protein synthesis. Several ribosomes can attach to a single mRNA simultaneously. rRNA is abundant and constitutes about 80% of the 10 mg/ml RNA in a typical eukaryotic cytoplasm.

snRNAs

Small nuclear RNAs (snRNA) combine with proteins to form spliceosomes, which control alternative splicing. Genes encode proteins in segments known as exons. These can be combined in various ways to produce different mRNAs, allowing multiple proteins to originate from a single gene. Unwanted protein versions are degraded by proteases, with their components recycled.

Regulatory RNAs

Some RNAs play a role in gene regulation, modulating the rate of gene transcription or translation. MicroRNAs and small interfering RNAs are examples. These molecules are instrumental in RNA interference (RNAi), a mechanism responsible for silencing gene expression.

RNA Processing

After transcription, eukaryotic mRNAs undergo several modifications before they can function in the cytoplasm. These processes, collectively termed RNA processing, include 5' capping, 3' polyadenylation, and RNA splicing.

RNA viruses

Certain viruses utilize RNA instead of DNA as their genetic material. These RNA viruses include HIV, Ebola, and the coronavirus family, like SARS-CoV-2. These viruses can be further classified based on their genomic structure and replication mechanisms.

Applications and relevance

Research into RNA's structure and function has led to crucial developments in biotechnology and medicine. RNA interference technology, for instance, shows promise in gene regulation and therapeutic applications.

RNA vaccines, which use a small piece of an RNA molecule to trigger an immune response, have become particularly relevant in the face of the COVID-19 pandemic, with the approval and deployment of the Pfizer-BioNTech COVID-19 vaccine and the Moderna COVID-19 vaccine.

See also

• DNA
• Protein synthesis
• RNA world hypothesis
• RNA interference
• RNA splicing
• Transcription (genetics)
• Translation (genetics)

External links

• The RNA Society
• RNA - The Versatile Molecule

RNA Resources
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