Nucleic acids are the “genetic software” of the cell, allowing organisms to pass on their complex components to the next generation. You might know them better as DNA (deoxyribonucleic acid) or RNA (ribonucleic acid): the macromolecules responsible for storing and transmitting hereditary information. They’re the reason you have blonde hair, or long fingers, or a gigantic nose. Without them, no organism could produce offspring, so they’re essential for all life.
Nucleic acids are made up of a chain of monomers called nucleotides, which are in turn made up of five-carbon sugars, a nitrogenous base, and one or more phosphate groups. That might seem like an extra level of complication, but we need to know this to understand their structure.
Consider, for example, a DNA molecule:
The two strands running down on either side are called the molecule’s “sugar phosphate backbone”, which are connected in the middle by nitrogenous bases that pair up to the adjacent strand.There are four kinds of bases: Adenine and Guanine, which are purines, and Cytosine and Thymine, which are pyrimidines. In RNA, Thymine is replaced with Uracil (a pyrimidine). Purines have two rings and pyrimidines have one ring, so the groupings just refer to structure.
The bases are almost always shortened to A, G, C, T & U. Their order determines how life is built—they encode a sequence of amino acids, which instruct how proteins are built. We’ll learn more about later.
Nitrogenous bases are hydrophobic, meaning they hate water. This is crucial to the structure of the DNA, because the strands are oriented so the bases face each other rather than the outside world, protecting them from water. The bases pair together using hydrogen bonds—purines always pair with pyrimidines, so A pairs with T (U in RNA), and C with G. In any given DNA molecule, the amount of A equals the amount of T, and the amount of C equals the amount of G. This is important because it maintains a uniform diameter for the helix of DNA.
The person who realised this equality was Austrian biochemist Erwin Chargaff, and he was a contemporary of American biologist James Watson and English physicist Francis Crick, who you might have heard of. In 1953, they were the first to publish the spiralling, double-helix structure of DNA. (See my article on Rosalind Franklin for the reason I’m not a fan of Watson and Crick.)
DNA strands have a polarity, meaning they have a direction—strands are always synthesised from the 5’ (said “five prime”) end to the 3’ end. I’ll talk a whole lot more about how this happens later on. The important thing to know now is that when two strands are connected in a DNA molecule, they run antiparallel, like this:
So, what about an RNA molecule?
For starters, DNA is located in a different place to RNA:
RNA only has a single strand, and it’s made from a different sugar: ribose instead of deoxyribose. Basically, this means it has one more OH group). RNA also has a completely different function. While DNA is the blueprint for life, RNA is the guy who actually gets things done. Different types of RNA are specialised for different functons: mRNA (messenger RNA) carries the blueprints between DNA and ribosomes in order to make proteins; rRNA (ribosomal RNA) essentially makes up the ribosomes; and tRNA (transfer RNA) carries amino acids into the ribosome for synthesis into proteins.
In summary: nucleic acids are made up sugars, phosphate groups, and nitrogenous bases, and their function is to encode, transmit, and express hereditary information. Next article, we’ll take a look at how scientists learned that nucleic acids are the genetic material of life.
Body images sourced from Wikimedia Commons
Further resources: Structure of Nucleic Acids at Educationportal