DNA and RNA

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.

Functions of DNA

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.

Functions of RNA

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.

What are the nucleotide structures?

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.

DNA nucleotide structure

A single DNA nucleotide is comprised of 3 components:

• A phosphate group
• A pentose sugar (deoxyribose)
• An organic nitrogenous base

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:

• A phosphate group
• A pentose sugar (ribose)
• An organic nitrogenous base

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.

Comparing DNA and RNA nucleotides

The main differences between DNA and RNA nucleotides are:

• DNA nucleotides contain a deoxyribose sugar, while RNA nucleotides contain a ribose sugar
• Only DNA nucleotides can contain a thymine base, while only RNA nucleotides can contain a uracil base

The main similarities between DNA and RNA nucleotides are:

DNA and RNA structure

Fig. 3 - The diagram illustrates the phosphodiester bond

DNA structure

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.

RNA structure

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

What is base pairing?

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.

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.

Differences and similarities between DNA and RNA

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.