Cytosine

Definition

Cytosine is one of the four main nitrogenous bases found in nucleotides, which are the building blocks of DNA and RNA. It is a pyrimidine base that pairs with guanine through three hydrogen bonds, playing a crucial role in the structure of nucleic acids and the encoding of genetic information. The presence of cytosine in a nucleotide affects the stability of DNA and RNA structures and influences various biochemical processes.

5 Must Know Facts For Your Next Test

1. Cytosine has a chemical formula of C4H5N3O, and its structure includes an amine group and a keto group, making it a pyrimidine base.
2. In DNA, cytosine pairs with guanine through three hydrogen bonds, contributing to the stability and integrity of the double helix structure.
3. Cytosine can undergo deamination, converting it into uracil, which can lead to mutations if not repaired during DNA replication.
4. The ratio of cytosine to guanine in DNA is often equal (Chargaff's rules), influencing the overall stability and properties of the DNA molecule.
5. Cytosine methylation is an important epigenetic modification that can regulate gene expression without changing the underlying DNA sequence.

Review Questions

• How does cytosine contribute to the structure and function of nucleic acids?
  • Cytosine contributes to the structure of nucleic acids by pairing with guanine through three hydrogen bonds, which is essential for maintaining the stability of the DNA double helix. This specific base pairing allows for accurate replication and transcription processes. Additionally, cytosine's presence affects how genetic information is encoded and expressed within organisms.
• Discuss the implications of cytosine deamination on genetic fidelity during DNA replication.
  • Cytosine deamination results in its conversion to uracil, which can lead to mispairing during DNA replication if not corrected. When DNA polymerase encounters uracil instead of cytosine, it may incorporate adenine opposite the uracil in the new strand. This mutation can become permanent if not repaired by mechanisms like base excision repair, potentially leading to significant changes in gene function and expression.
• Evaluate the role of cytosine methylation in gene regulation and its impact on cellular processes.
  • Cytosine methylation plays a critical role in gene regulation by adding a methyl group to cytosine residues within gene promoter regions. This modification typically represses gene expression by preventing transcription factor binding or recruiting repressive proteins. Consequently, cytosine methylation is crucial for processes such as cellular differentiation and development, influencing how cells respond to environmental signals and maintain their identity.

Related terms

Nucleotide: A nucleotide is the basic structural unit of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base, which can be adenine, guanine, cytosine, or thymine (in DNA) or uracil (in RNA).

Base Pairing: Base pairing refers to the specific hydrogen bonding between nitrogenous bases in nucleic acids, where adenine pairs with thymine (or uracil in RNA) and cytosine pairs with guanine.

DNA Replication:

DNA replication is the biological process by which a cell makes an identical copy of its DNA, ensuring that each new cell receives a complete set of genetic information, involving base pairing where cytosine plays an essential role.