Nucleic acids are one of the four main types of biomolecules. They are heteropolymers with a complex structure. Nucleic acids mostly carry out the role of genetic information storage and expression, though sometimes they do have other functions. There are two types of nucleic acids: DNA and RNA, which differ in structure and functions.
The main focus of this topic will be the chemical structure of nucleic acids and their monomers, nucleotides.
Nucleotides
Let's get started with nucleotides: the small molecules that make up nucleic acids. A nucleotide itself consists of three parts: a nitrogenous base, a five-carbon cyclical sugar, and a phosphoric acid residue. Look at the picture below to see a structure of a nucleotide.
In sugars, the carbon atoms are numbered. The enumeration comes in handy if we need to point to a certain carbon; without a numbering system, it would take too long to explain which one we mean. The prime (') is added to a carbon number. For example, 3' stands for "three prime".
As we see, carbons from 1' to 4' form a ring with the oxygen atom, while the 5' is not in the ring. The 5' carbon connects a hydroxyl group( an oxygen-hydrogen pair) to the ring, and this hydroxyl group connects the cyclical sugar to a phosphoric acid residue — a phosphate group. The 1' carbon bears the distinctive feature of a nucleotide — a nitrogenous base, also known as a nucleobase.
Nucleobases are subdivided into two groups: pyrimidines, the smaller ones with one carbon-nitrogen ring, and purines, the bigger ones with two side-by-side carbon-nitrogen rings. There are five types of primary nucleobases: 3 pyrimidines — thymine (T), cytosine (C) and uracil (U), and 2 purines — adenine (A) and guanine (G). Nucleotides within the two groups differ from each other by only a few atoms, but these subtle modifications change the properties of the nucleobase drastically. These differences determine the electron density distribution, which, in turn, determines the specific pattern of hydrogen bonds that the nucleobase can form.
The patterns of electron density distribution in different nucleobases are such that the bases can form pairs: guanine with cytosine, and adenine with either thymine or uracil. The G/C pair has 3 hydrogen bonds and its connection is stronger than the bonding in A/T and A/U pairs, which both only have 2 bonds. Nitrogenous bases in the pair fit as a lock and a key: where one has a lack of electron density, the other has an excess. This is called a principle of complementarity. The nucleotides don't form bonds in other combinations.
Nucleic acid macromolecules
To perform their functions, nucleotides are polymerized and form nucleic acids. They form linear polymers that don't branch – meaning the nucleotides are connected one after another and none of the nucleotides connect to more than one other nucleotide. Two adjacent nucleotides connect through their phosphate and sugar: the hydroxyl group on the 3' carbon of one nucleotide connects to the phosphate of the next nucleotide, forming a phosphate-sugar backbone. Nucleobases don't participate in this process.
The resulting nucleotide chain, or strand, has two different ends that are identified by the name of the carbon atom left unbonded on them: the one with the phosphoric acid residue is the 5' end, and the one with the sugar is the 3' end. This is called directionality. The chains are only synthesized by adding new nucleotides to the 3' end of the existing chain, and the chains can be billions of nucleotides in length.
Because nitrogenous bases can form hydrogen bonds in their specific pairings, two nucleic chains can stick to each other if they are bound by matching complementary bases. The more bases that match, the stronger the bond between chains. Like the nitrogenous base pairs, matching strands are also called complementary.
If all of the nucleobases in two chains match, the chains form a double helix structure – the most common way DNA is shown in videos or pictures. In a double-stranded nucleic acid, the polymer strands are antiparallel to each other: the 3' end of one lines up with the 5' end of the other. For example, if we're looking for a complementary strand for 3'-ATGC-5', it will be 5'-TACG-3', but if we switch the direction to 3'-TACG-5', it will not fit.
DNA and RNA
There are two types of nucleic acids: RNA, which stands for ribonucleic acid, and DNA — deoxyribonucleic acid. These two are different in both their structure and function, but we'll cover their functions in later topics.
As for structure, the feature that differentiates RNA and DNA also gives them their names: the sugars. RNA, the ribo-nucleic acid, has ribose, while DNA, the deoxy-ribo-nucleic acid, has deoxyribose. These sugars are almost identical, but ribose has a hydroxyl group on the 2' carbon. It is absent in deoxyribose (de = no—oxy = hydroxyl—ribose).
DNA and RNA also differ in the nucleobases that they contain: they each contain 4 specific types. Guanine (G), cytosine (C), and adenine (A) are found in both, but DNA contains thymine (T) and not uracil (U), while in RNA, there is uracil but not thymine. The complementary pairs in DNA are G/C and A/T, and in RNA it is G/C and A/U, so each nucleobase only has one possible pair.
Finally, DNA commonly forms double-stranded structures (the double helix), while RNA remains a single strand. This difference also makes DNA more stable than RNA.
Conclusion
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Nucleic acids are made up of nucleotides, which are, in turn, made up of a sugar, a phosphate and a nucleobase.
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There are 5 types of nucleobases that can form hydrogen bonds if paired in certain combinations: G/C, and A/T or A/U, which are called complementary pairs.
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Nucleotides form strands that are directional — have 3' and 5' ends. Strands can connect to each other if their antiparallel nucleotide sequences match, and form a double helix.
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The two types of nucleic acids are DNA and RNA. They differ in sugar structure, incorporated nucleobases (T is specific for DNA, U for RNA) and dimerization (DNA forms double strands while RNA forms single).