RNA: what it is, what it does and why it’s one of the most important molecules of life

What is RNA composed of?

Like its more famous chemical cousin, deoxyribonucleic acid (DNA), ribonucleic acid (RNA) is a nucleic acid – a molecule assembled from a chain of building blocks called nucleotides. In terms of composition, each nucleotide is accompanied by a chemical letter or “base”:

  • Adenine (A)
  • Cytosine (C)
  • Guanine (G)
  • Thymine (T)
  • Uracil (U)

Although both forms of nucleic acid use a four-letter alphabet that includes A, C, and G, DNA uses T, while RNA uses U.

However, this is a relatively trivial difference, as T and U are read as equivalent bases, as upper and lower case versions of the same letter.

RNA is often a self-contained chain. However, in complex cells, the DNA strands are arranged like chromosomes: they are wrapped around large histone proteins, like a thread wrapped around a bobbin. This arrangement is possible because, with millions of letters, DNA is usually several orders of magnitude longer than RNA.

The shortest RNAs are microRNAs and small interfering RNAonly 20-30 nucleotides in length, and the longest RNA in humans is a copy of the titin gene, which is 109,224 letters long.

What does RNA do?

RNA carries biological information. The function of DNA is to store information in the genome – the book of life or the instruction manual for creating an organism.

RNA, on the other hand, plays various roles that influence how genes (DNA instructions) are read, which determines characteristics such as observable characteristics and physiology.

The human genome contains over 6 billion letters, but only about 1% of DNA codes for the genes needed to make proteins – the molecules that perform the majority of tasks, such as working as enzymes (biological catalysts) that trigger the chemical reactions that sustain life.

The remaining 99% are non-coding: most are harmless “junk DNA,” but some are genes whose end product is RNA itself.

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What is the difference between RNA and DNA?

RNA is “ribonucleic acid” and DNA is “deoxyribonucleic acid”.

The two forms of nucleic acid have similar skeletons, consisting of two alternating chemical groups: phosphate and a sugar, ribose. The DNA backbone sugars lack a hydroxyl group (hydrogen plus oxygen), turning ribose into deoxyribose.

This small difference has a huge impact on chemistry: with its more reactive ribose sugar, RNA is more likely to react with other molecules – and even with itself.

The two nucleic acids also differ in their structure. While RNA is typically a single-stranded molecule, DNA forms the iconic double helix – two strands that wrap around each other like a twisted ladder.

The rungs of the ladder represent hydrogen bonds – electrostatic attractions that connect complementary letters on opposite strands in pairs: A with T, C with G. This base pairing occurs throughout DNA, maintaining its strands together, like teeth on the two halves of a zipper. .

Unlike DNA, RNA usually does not exist as a long double-stranded molecule, but as a lonely strand. And yet, complementary letters from separate sections of an RNA strand can pair together (“AAAA” matching “UUUU,” for example), forming double-stranded regions called stems.

However, such pairing does not occur at every point in the molecule and many sections remain single-stranded. Overall, an RNA will often include structures known as ‘hairpin curls’. RNA can sometimes pair with DNA, but the difference in size means that complete DNA-RNA hybrids do not occur.

An infographic showing the difference between the structure of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) © Getty images

Why is RNA relatively unstable?

DNA is organized into chromosomes, which helps protect its structural integrity. The double helix is ​​also more stable than RNA due to the bonding between base pairs along its entire length. In contrast, RNA molecules have short (if any) double-stranded rods that hold them together, making them more susceptible to falling apart. Because the single-stranded loops of RNA are not engaged in base pairing, they are also exposed and vulnerable to enzymes that break down nucleic acids.

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Stability explains why RNA and DNA tend to have distinct functions, even though they are both capable of carrying genes. Being more stable means that DNA is a better molecule for the permanent storage of genetic instructions, which should be inherited virtually unchanged from one generation to the next.

But RNA instability has its advantages, because an RNA copy of a gene lets the instructions have a temporary effect: for example, if the DNA directly instructed cells in the pancreas to produce insulin, the body would continue convert glucose into glycogen or fat – prevent it from being used as fuel.

Copying DNA onto RNA is therefore useful for making proteins.

How is RNA used to make proteins?

RNA is an integral part of gene expression, the process of synthesizing a protein by following instructions encoded in DNA. The first step involves RNA polymerase, an enzyme that unwinds and unwinds the double helix to access a protein-coding gene. The DNA sequence of the gene is used as a template to transcribe an RNA copy of that gene, which the polymerase assembles from a string of nucleotides.

The transcription step is similar to DNA replication, the process that duplicates a cell’s genome before it divides. In organisms like bacteria, this cell division involves binary fission, whereas eukaryotes – organisms where DNA is contained in a cell nucleus – will duplicate their DNA during division by mitosis.

In eukaryotic cells, the newly transcribed copy of a gene is exported to the cytoplasm as “precursor messenger RNA” (pre-mRNA). Protein-coding genes are rarely one continuous DNA sequence, but are usually split into segments whose RNA is spliced ​​together in alternate combinations – such as a regular and director’s cut from a movie – so that a gene can make many different proteins.

The splicing process produces an mRNA molecule whose nucleotide alphabet (ACGU) must then be translated into amino acids, building blocks and protein language.

The translation step involves the ribosome, a protein-making factory that uses another type of RNA – a cloverleaf-shaped adapter called transfer RNA (tRNA), which corresponds to a three-letter word (codon ) in mRNA and also carries a specific amino acid (such as “AUG” for tRNA carrying methionine).

Like paper passing through a printer, a strand of mRNA is pulled through a ribosome as one tRNA both binds to its corresponding word and its amino acid is added to a growing chain. This chain is folded into a 3D structure during the last step of protein synthesis.

A simplified diagram showing how gene expression works © Getty images

What is the world of RNA?

The RNA world is a theory related to the origin of life. Specifically, it is a hypothesis about how genes replicate and adapt on prebiotic Earth; evolution before the appearance of life. It requires a genetic system.

Cells today use a system in which DNA instructions make proteins while proteins replicate DNA, creating a chicken-or-egg paradox. Which came first?

A solution is neither.

According to the RNA world hypothesis, the first systems revolved around RNA. The strongest evidence to support this idea comes from the cell’s protein-making factory, the ribosome: instead of being built from protein, the core component of a ribosome is actually a enzyme-like molecule made of ribosomal RNA (rRNA): a ribozyme.

Scientists have since discovered other ribozymes and engineered self-replicating RNAs as proof of concept for what might have existed on an RNA world.

Although the hypothesis is not correct, it explains the versatility of the molecule: RNA has the ability not only to transport genes but, like proteins and contrary to DNA, the chemistry of its single strand allows RNA to fold into an enzyme-like structure that can modify other molecules.

10 milestones in RNA research

  • 1868: Friedrich Miescher isolates the material ‘nuclein’ from the nucleus
  • 1953: James Watson and Francis Crick discover the double helix structure of DNA
  • 1961: François Jacob and Jacques Monod suggest that DNA is transcribed into RNA
  • 1962: Alexander Rich proposes a hypothesis, later popularized as “RNA World”
  • 1965: Robert Holley reveals the stem-loop structure of a transfer RNA adapter
  • 1966: Scientists crack the genetic code to translate RNA into protein
  • 1977: Phillip Sharp and Richard Roberts discover split genes and RNA splicing
  • 1982: Thomas Cech discovers autonomous ribozymes that serve as catalysts
  • 2002: Gerald Joyce creates self-replicating RNA molecules that can evolve
  • 2012: Emmanuelle Charpentier and Jennifer Doudna invent CRISPR guide RNA

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