The key difference between cytosine and cysteine is that cytosine is one of the five nitrogenous bases found in DNA and RNA while cysteine is a non-essential amino acid.

Cytosine and cysteine are two types of important molecules. Cytosine is a nitrogenous base which is a pyrimidine derivative. It is needed for the formation of cytosine nucleotides, which are the building blocks of DNA and RNA. Cysteine, on the other hand, is a non-essential amino acid which has sulfur. Cysteine is needed for protein synthesis, detoxification, and diverse metabolic functions.

CONTENTS

1. Overview and Key Difference
2. What is Cytosine 
3. What is Cysteine
4. Similarities Between Cytosine and Cysteine
5. Side by Side Comparison – Cytosine vs Cysteine in Tabular Form
6. Summary

What is Cytosine?

Cytosine is a nitrogenous base found in both DNA and RNA. It is a pyrimidine base which has only one six-membered nitrogen-containing ring, similar to thymine and uracil. There are two substituents attached to the ring of cytosine. Therefore, it has an amine group at C4 and a keto group at C2. Cytosine forms three hydrogen bonds with guanine in the complementary strand of DNA double helix. The chemical formula of cytosine is C4H5N3O. Its molecular weight is 111.1 g/mol.

Figure 01: Cytosine

Cytosine binds with ribose in order to form nucleoside cytidine and with deoxyribose to form deoxycytidine. The nucleotide of cytosine in DNA has three components: cytosine base, deoxyribose and a phosphate group. Cytosine can alter into uracil, making a point mutation, since it is inherently unstable. It can also be methylated into 5-methylcytosine by an enzyme called DNA methyltransferase.

Cytosine is a part of the nucleotide called cytidine triphosphate (CTP) which can act as a co-factor to enzymes and can transfer a phosphate to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP).

What is Cysteine?

Cysteine is a non-essential amino acid which is also a hydrophilic amino acid. The chemical formula of cysteine is C3H7NO2S, and its molecular weight is 121.15 g/mol. Cysteine contains sulfur. Cysteine is coded in mRNA by the UGU and UGC codons. If a sufficient amount of methionine is available, the human body can synthesize cysteine under normal physiological conditions. Since the human body can make cysteine, it is classified as a non-essential amino acid. The body uses cysteine to make other amino acids.

Figure 02: Cysteine

Cysteine is present in high protein food. Cysteine is a precursor to the antioxidant glutathione. Moreover, it is a precursor to iron-sulfur clusters. In pharmaceutical and personal-care industries, cysteine is used as a precursor. Furthermore, cysteine is useful when reducing the toxic effects of alcohol in order to minimize liver damage and hangover. Not only that, cysteine is important in collagen production, as well as in skin elasticity and texture.

What are the Similarities Between Cytosine and Cysteine?

• Both cytosine and cysteine are made in the human body.
• They are important and carry out many functions in the human body.

What is the Difference Between Cytosine and Cysteine?

Cytosine is a nitrogenous base or a precursor of cytosine nucleotides in DNA and RNA while cysteine is a sulfur-containing amino acid which is a non-essential type. So, this is the key difference between cytosine and cysteine. C4H5N3O is the chemical formula of cytosine while C3H7NO2S is the chemical formula of cysteine.

The following infographic shows more differences between cytosine and cysteine.

Summary – Cytosine vs Cysteine

Cytosine is a nitrogenous base found in both DNA and RNA.  It forms three hydrogen bonds with guanine in the complementary strand of DNA double helix. Cysteine is a sulfur-containing amino acid which is classified as non-essential. Therefore, the human body can synthesize cysteine if methionine is available. Cysteine is important for protein synthesis, detoxification and other various metabolic functions. Thus, this summarizes the difference between cytosine and cysteine.

Reference:

1. “L-Cysteine.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, Available here.
2. “Cytosine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.

Image Courtesy:

1. “Cytosine chemical structure” By Engineer gena – Own work (Public Domain) via Commons Wikimedia
2. “L-Cystein – L-Cysteine” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia

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