Main Difference – Cytosine vs Thymine
Cytosine and thymine are two types of nitrogenous bases in nucleotides, which build nucleic acids. The other nitrogenous bases found in nucleic acids are adenine, guanine, and uracil. Uracil is only found in RNA and is involved in protein synthesis. Both cytosine and thymine are pyrimidines, containing a six-membered ring made up of carbon and nitrogen atoms (heterocyclic aromatic ring). The main difference between cytosine and thymine is that cytosine is found in both DNA and RNA, complementary base pairing with guanine whereas thymine is only found in DNA, complementary base pairing with adenine.
This article explains,
1. What is Cytosine
– Definition, Structure, Characteristics
2. What is Thymine
– Definition, Structure, Characteristics
3. What is the difference between Cytosine and Thymine
What is Cytosine
Cytosine is one of the three pyrimidine bases found in nucleic acids. Other two pyrimidine bases are thymine and uracil. A keto group at the C-2 and an amine group at the C-4 occur in the heterocyclic aromatic ring of cytosine. Cytosine is found in both DNA and RNA as a part of a nucleotide. Cytosine binds to deoxyribose, forming deoxycytidine nucleoside. It also binds to ribose, forming cytidine nucleoside. Deoxycytidine and cytidine bind with three phosphate groups, forming their nucleotides, deoxycytidine triphosphate (dCTP) and cytidine triphosphate (CTP), building DNA and RNA, respectively. In the DNA double helix, cytosine complementary base pairs with guanine by forming three hydrogen bonds. The enzyme, DNA methyltransferase, methylates cytosine into 5-methylcytosine. This DNA methylation is an epigenetic mechanism, which controls the gene expression. The base pairing complex of cytosine with guanine is not stable, and cytosine can be altered into uracil by spontaneous deamination. This alteration is restored by the DNA repair enzymes like uracil glycosylase. If not, it leads to a point mutation. The nitrogenous base, cytosine is shown in figure 1.
Figure 1: Cytosine
What is Thymine
Thymine is another type of pyrimidine base found only in DNA. The heterocyclic aromatic ring of thymine contains two keto groups at C-2 and C-4 as well as a methyl group at C-5. Thymine forms a glycosidic bond with deoxyribose, producing deoxythymidine. Deoxythymidine is phosphorylated into deoxythymidine triphosphate (dTTP), which serves as one of the four building blocks of DNA. In the DNA double-helix, thymine complementary base pairs with adenine via two hydrogen bonds. In RNA, uracil pairs with adenine, replacing thymine. Thymine can be derived by the methylation of uracil at C-5. Hence, it is called 5-methyluracil. In the presence of UV, thymine forms dimers with adjacent thymine or cytosine bases, which causes kinks in the DNA double-helix. In cancer treatment, 5-fluorouracil (5-fU) is used to substitute thymine during DNA replication. This inhibits the DNA synthesis in all actively dividing cells.
Figure 2: Thymine
Difference Between Cytosine and Thymine
Definition
Cytosine: Cytosine is a pyrimidine base that is an essential constituent of RNA and DNA.
Thymine: Thymine is a pyrimidine base, which is paired with adenine in double-stranded DNA.
Presence
Cytosine: Cytosine occurs in both DNA and RNA.
Thymine: Thymine only occurs in DNA.
Functional Groups
Cytosine: The heterocyclic aromatic ring of cytosine contains a keto group at the C-2 and an amine group at the C-4.
Thymine: The heterocyclic aromatic ring of thymine contains two keto groups at C-2 and C-4 as well as a methyl group at C-5.
Molecular Formula
Cytosine: Molecular formula of cytosine is C4H5N3O.
Thymine: Molecular formula of thymine is C5H6N2O2.
Molar Mass
Cytosine: Molar mass of cytosine is 111.1 g/mol.
Thymine: Molar mass of thymine is 126.1133 g/mol.
Complementary Base
Cytosine: Cytosine complementary base pairs with guanine.
Thymine: Thymine complementary base pairs with adenine.
Number of Hydrogen Bonds in the Pair
Cytosine: Cytosine forms three hydrogen bonds with guanine.
Thymine: Thymine forms two hydrogen bonds with adenine.
Methylation
Cytosine: Methylation of cytosine into 5-methylcytosine regulate the gene expression.
Thymine: Thymine can be derived by the methylation of uracil at its C-5.
Significance
Cytosine: Cytosine in DNA can be altered into uracil by spontaneous deamination.
Thymine: The 5-fU can be used as a base substituting agent during cancer treatment.
Conclusion
Cytosine and thymine are two of the three pyrimidine nucleobases found in nucleic acids. Cytosine occurs in both DNA and RNA, complementary base pairing with guanine in the double-stranded structure. In contrast, thymine is only found in DNA, complementary base pairing with adenine. In RNA, thymine is replaced by uracil. Cytosine is involved in the gene regulation. Thymine is a targeted nucleobase during cancer treatment. The main difference between cytosine and thymine is in the occurrence in nucleic acids.
Reference:
1. Blackburn, Victoria, and Bronwyn Harris. “What is Cytosine?” WiseGEEK. Conjecture Corporation, 08 Apr. 2017. Web. 15 May 2017. <http://www.wisegeek.com/what-is-cytosine.htm>.
2. Smith, BPharm Yolanda. “What is Thymine?” News-Medical.net. N.p., 04 June 2015. Web. 15 May 2017. <http://www.news-medical.net/health/What-is-Thymine.aspx>.
Image Courtesy:
1. “Cytosine chemical structure” (CC BY-SA 3.0) via Commons Wikimedia
2. “Thymine chemical structure” (CC BY-SA 3.0) via Commons Wikimedia
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