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The two macromolecules that are essential for heredity in all living cells are DNA, deoxyribonucleic acid and RNA, ribonucleic acid. Both DNA and RNA are nucleic acids, and they perform vital functions in the continuation of life.
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Team DNA and RNA Teachers
The chief function of DNA is to store genetic information in structures called chromosomes. In eukaryotic cells, DNA can be found in the nucleus, the mitochondria and the chloroplast (in plants only). Meanwhile, prokaryotes carry DNA in the nucleoid, which is a region in the cytoplasm, and plasmids.
RNA transfers genetic information from the DNA found in the nucleus to the ribosomes, specialized organelles comprised of RNA and proteins. The ribosomes are especially important as translation (the final stage of protein synthesis) occurs here. There are different types of RNA, such as messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) , each with its specific function.
mRNA is the primary molecule responsible for carrying genetic information to the ribosomes for translation, tRNA is responsible for carrying the correct amino acid to the ribosomes and rRNA forms ribosomes. Overall, RNA is vital in the creation of proteins, such as enzymes.
In eukaryotes, RNA is found in the nucleolus, an organelle within the nucleus, and ribosomes. In prokaryotes, RNA can be found in the nucleoid, plasmids and ribosomes.
DNA and RNA are polynucleotides , meaning they are polymers made of monomers. These monomers are called nucleotides. Here, we will explore their structures and how they differ.
A single DNA nucleotide is comprised of 3 components:
Fig. 1 - The diagram shows the structure of a DNA nucleotide
Above, you'll see how these different components are organized within a single nucleotide. There are four different types of DNA nucleotides as there are four different types of nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). These four different bases can be further divided into two groups: pyrimidine and purine.
Pyrimidine bases are the smaller bases as these are composed of a 1 carbon ring structure. The pyrimidine bases are thymine and cytosine. Purine bases are the larger bases as these are 2 carbon ring structures. The purine bases are adenine and guanine.
An RNA nucleotide has a very similar structure to a DNA nucleotide and like DNA, it is comprised of three components:
Fig. 2 - The diagram shows the structure of an RNA nucleotide
You'll see the structure of a single RNA nucleotide above. An RNA nucleotide can contain four different types of nitrogenous bases: adenine, uracil, cytosine or guanine. Uracil, a pyrimidine base, is a nitrogenous base that is exclusive to RNA and cannot be found in DNA nucleotides.
The main differences between DNA and RNA nucleotides are:
The main similarities between DNA and RNA nucleotides are:
Both nucleotides contain a phosphate group
Both nucleotides contain a pentose sugar
Both nucleotides contain a nitrogenous base
DNA and RNA polynucleotides are formed from condensation reactions between individual nucleotides. A phosphodiester bond is formed between the phosphate group of one nucleotide and the hydroxyl (OH) group at the 3 'pentose sugar of another nucleotide. A dinucleotide is created when two nucleotides are joined together by a phosphodiester bond. A DNA or RNA polynucleotide occurs when many nucleotides are joined together by phosphodiester bonds. The diagram below shows where the phosphodiester bond is positioned between 2 nucleotides. A hydrolysis reaction must take place to break phosphodiester bonds.
A dinucleotide is built of only 2 nucleotides, whereas a polynucleotide consists of MANY nucleotides!
Fig. 3 - The diagram illustrates the phosphodiester bond
The DNA molecule is an anti-parallel double helix formed of two polynucleotide strands. It is anti-parallel as the DNA strands run in opposite directions to each other. The two polynucleotide strands are joined together by hydrogen bonds between complementary base pairs, which we will explore later. The DNA molecule is also described as having a deoxyribose-phosphate backbone - some textbooks may also call this a sugar-phosphate backbone.
The RNA molecule is a little different to DNA in that it is made of only one polynucleotide which is shorter than DNA. This helps it carry out one of its primary functions, which is to transfer genetic information from the nucleus to the ribosomes - the nucleus contains pores that mRNA can pass through due to its small size, unlike DNA, a larger molecule. Below, you can visually see how DNA and RNA differ from each other, both in size and the number of polynucleotide strands.
Fig. 4 - The diagram shows the structure of DNA and RNA
The bases can pair up together by forming hydrogen bonds and this is termed complementary base pairing . This keeps the 2 polynucleotide molecules in DNA together and is essential in DNA replication and protein synthesis.
Complementary base pairing requires the joining of a pyrimidine base to a purine base via hydrogen bonds. In DNA, this means
Adenine pairs with thymine with 2 hydrogen bonds
Cytosine pairs with guanine with 3 hydrogen bonds
In RNA, this means
Adenine pairs with uracil with 2 hydrogen bonds
Cytosine pairs with guanine with 3 hydrogen bonds
Fig. 5 - The diagram shows complementary base pairing
The diagram above helps you to visualize the number of hydrogen bonds formed in complementary base pairing. Although you do not need to know the chemical structure of the bases, you will need to know the number of hydrogen bonds formed.
Due to complementary base pairing, there are equal quantities of each base in a base pair. For example, if there are approximately 23% guanine bases in a DNA molecule, there will also be approximately 23% cytosine.
As cytosine and guanine form 3 hydrogen bonds, this pair is stronger than adenine and thymine which only form 2 hydrogen bonds. This contributes to the stability of DNA. DNA molecules with a high proportion of cytosine-guanine bonds are more stable than DNA molecules with a lower proportion of these bonds.
Another factor that stabilizes DNA is the deoxyribose-phosphate backbone. This keeps the base pairs inside the double helix, and this orientation protects these bases which are highly reactive.
It's important to know that while DNA and RNA work closely together, they also differ. Use the table below to see how these nucleic acids are different and similar.
| DNA | RNA | |
| Function | Stores genetic information | Protein synthesis - transfers genetic information to the ribosomes (transcription) and translation |
| Size | 2 large polynucleotide strands | 1 polynucleotide strand, relatively shorter than DNA |
| Structure | Anti-parallel double helix | Single-stranded chain |
| Location in cell (eukaryotes) | Nucleus, mitochondria, chloroplast (in plants) | Nucleolus, ribosomes |
| Location in cell (prokaryotes) | Nucleoid, plasmid | Nucleoid, plasmid, ribosomes |
| Bases | Adenine, thymine, cytosine, guanine | Adenine, uracil, cytosine, guanine |
| Pentose sugar | Deoxyribose | Ribose |
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How do RNA and DNA work together?
DNA and RNA work together because DNA stores genetic information in structures called chromosomes while RNA transfers this genetic information in the form of messenger RNA (mRNA) to the ribosomes for protein synthesis.
What are the main differences between DNA and RNA?
DNA nucleotides contain deoxyribose sugar, while RNA nucleotides contain ribose sugar. Only DNA nucleotides can contain thymine, while only RNA nucleotides can contain uracil. DNA is an anti-parallel double helix made of 2 polynucleotide molecules while RNA is a single-stranded molecule made of only 1 polynucleotide molecule. DNA functions to store genetic information, while RNA functions to transfer this genetic information for protein synthesis.
What are the three basic parts of DNA and RNA?
The three basic parts of DNA and RNA are: a phosphate group, a pentose sugar and an organic nitrogenous base.
What is the basic structure of DNA?
A DNA molecule is made of 2 polynucleotide strands that run in opposite directions (anti-parallel) to form a double helix. The 2 polynucleotide strands are kept together by hydrogen bonds found between complementary base pairs. DNA has a deoxyribose-phosphate backbone which is kept together by phosphodiester bonds between individual nucleotides.
Why can DNA be described as a polynucleotide?
DNA is described as a polynucleotide as it is a polymer made of many monomers, called nucleotides.
What are the three types of RNA and their functions?
The three different types of RNA are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). mRNA carries genetic information from the DNA in the nucleus to the ribosomes. tRNA brings the correct amino acid to the ribosomes during translation. rRNA forms the ribosomes.
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