7.4: Nucleic Acids
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- Distinguish between nucleobases, nucleosides, nucleotides, RNA, and DNA
Nucleic acids store and process genetic information. They include DNA and RNA.
Building Blocks of Nucleic Acids
Just as amino acids with the same backbone but different side chains can be joined together in a different order to make each protein in an organism, different nucleotides also bond together to make nucleic acids. Like amino acids, all nucleotides have certain components in their backbones plus another structure that gives each nucleotide a unique identity.
Nucleobases
Nucleobases, frequently just referred to as bases, are the portion of a nucleotide with the most variation. They are analogous to the side chains in amino acids. Nucleobases consist of 1 - 2 rings each containing several nitrogen atoms.
Figure \(\PageIndex{1}\): Examples of nucleobases. This is not a complete list, but these are the three that are found in both RNA and DNA.
The bases adenine (A), cytosine (C), and guanine (G) are found in both RNA and DNA. An additional bases, uracil (U), is found in RNA while thymine (T) is found in DNA. Therefore, each nucleic acid contains four types of bases: A, C, G, and U in RNA and A, C, G, and T in DNA.
Notice that the nucleobases have several N-H bonds. This helps them to hydrogen bond to one another in base pairs. The structure of each base determines which other base it can partner with. Guanine (G) can only hydrogen bond with cytosine (C). Adenine (A) can hydrogen bond with either uracil (U) in RNA or thymine (T) in DNA.
Nucleosides
Each nucleobase can bond to a monosaccharide to make a nucleoside. The two possible monosaccharides are ribose and deoxyribose.
Figure \(\PageIndex{2}\): Ribose is the monosaccharide found in RNA (and in nucleosides and nucleotides associated with RNA) whereas deoxyribose is found in DNA and its nucleosides and nucleotides. The only difference is at the second carbon atom clockwise from the O in the ring. If an oxygen atom is present at the second carbon, the monosaccharide is ribose. If it is absent, it is deoxyribose.
Figure \(\PageIndex{3}\): Examples of nucleosides. Nucleosides consist of a nucleobase (either 1 or 2 rings, each with several N atoms) and a monosaccharide. The structure on the left has ribose as its monosaccharide; the structure on the right contains deoxyribose.
Nucleotides
RNA and DNA are polymers where the monomers are nucleic acids. In other words, nucleotides are the building blocks of nucleic acids. A nucleotide consists of a nucleobase, a monosaccharide, and a phosphate group. That means that a nucleotide is equivalent to a nucleoside bonded to a phosphate group.
Figure \(\PageIndex{4}\): Two nucleotides. The base is bonded to the first carbon of the monosaccharide and the phosphate is bonded to either the third or fifth carbon of the monosaccharide.
Identify each of the following as a nucleobase, nucleoside, or nucleotide.
Structure 1 image credit: NEUROtiker - Own work, Public Domain. Structure 3 image credit: By Hbf878 - Own work, CC0.
- Answer
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- Structure 1 is a nucleoside. It contains a nucleobase (the two rings with N atoms) and a monosaccharide (ribose in this case).
- Structure 2 is a nucleobase. It is a ring with two N atoms. It is not bonded to a monosaccharide or phosphate.
- Structure 3 is a nucleotide. It contains a nucleobase with two N atoms, a monosaccharide (deoxyribose in this case), and the monosaccharide is bonded to a phosphate group.
Structure of RNA and DNA
DNA is an acronym (rather than a chemical formula) for deoxyribonucleic acid. It stores the genetic information for an organism and also passes that information to the next generation. RNA is ribonucleic acid and it is used as a template for the production of proteins based on the genetic information. The similarity between the names ribonucleic acid and deoxyribonucleic acid reveals a similarity of their structures. We will first examine the similarities, followed by the differences.
Both DNA and RNA have backbones made of alternating sugar and phosphate groups. In the diagram of DNA below, the phosphate groups are highlighted in yellow and they are bonded to the sugar groups highlighted in orange. Notice that these form a chain or polymer down the left and right sides of the image.
Credit: DNA chemical structure by Madeleine Price Ball.
Connected to the backbone at each sugar group are bases. The four bases in the picture above can be grouped into two categories: adenine (A) and guanine (G) have a double ring structure whereas thymine (T) and cytosine (C) have a single ring structure. All of the bases have several N - H bonds and can participate in hydrogen bonding (there are also polar C=O groups on some of the bases). This hydrogen bonding holds the two strands of DNA together. Specifically, A and T always H-bond with one another (sharing two H-bonds) and G and C always H-bond with one another (3 H-bonds). Because of differences in their structures, other pairings of these four bases are not possible.
There are three differences between the structures of DNA and RNA:
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The sugars in the backbone of DNA and RNA differ by one oxygen atom. The deoxy- prefix in DNA indicates that the sugar is like ribose, but with one less oxygen atom.
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DNA is double stranded but RNA contains only a single strand
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In addition to the bases adenine (A), guanine (G), and cytosine (C) which are found in both DNA and RNA, DNA contains the base thymine (T) whereas RNA contains uracil (U).
Processes Involving DNA and RNA
DNA replicates itself within cells (makes another copy of itself). In addition, DNA is used as a template for the synthesis of mRNA which is used as a template for the creation of proteins. So, the main purpose of DNA in your body is to store the information that is ultimately used to make the proteins that serve a wide variety of functions throughout the body.
First classify the following relevant to carbohydrates, lipids, nucleic acids, or proteins. Then try to describe the structure more specifically.
Image credit: Boumphreyfr, CC BY-SA 3.0, via Wikimedia Commons
- Answer
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This molecule belongs to the carbohydrate class of biomolecules. Five monosaccharides are shown, with the structure unfinished in four places indicating that the pattern continues. Therefore it is a polysaccharide.
First classify the following relevant to carbohydrates, lipids, nucleic acids, or proteins. Then try to describe the structure more specifically.
- Answer
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This molecule belongs to the protein class of biomolecules. It contains the amino acid backbone -N-C-C- where there is a double bond to O on the second carbon and the R group on the first carbon varies. In this molecule the -N-C-C- pattern repeats indicating that this is a peptide rather than a single amino acids. This peptide contains seven amino acids because the -N-C-C- repeats seven times.
Summary
- Nucleobases, a component of nucleic acids, consist of 1 - 2 rings containing several N atoms. This can be helpful in identifying the structures of nucleic acids and their component parts.
- Nucleosides are made of a nucleobase and a monosaccharide (either ribose or deoxyribose)
- Nucelotides are made of a nucleobase, a monosaccharide, and a phosphate group (PO4)
- RNA and DNA are both nucleic acids made of many nucleotides. The monosaccharides and phosphate groups of the nucleotides form the backbone of these structures. RNA has one backbone whereas DNA has two. The monosaccharide in RNA is ribose (RNA stands for ribonucleic acid); the monosaccharide in DNA (deoxyribonucleic acid) is deoxyribose. Deoxyribose has one fewer O atom than ribose. Both DNA and RNA can contain the G, C, and A nucleobases. Only RNA contains U and only DNA contains T.