Question

One of the most common spontaneous lesions that occurs in DNA under physiological conditions is the hydrolysis of the amino group of cytosine, converting it to uracil. What would be the effect on DNA structure if a uracil group replaced cytosine?

Short answer

Answer: If uracil were to replace cytosine in DNA, it would disrupt the normal base pairing, lead to errors in the transcription and translation processes, and ultimately affect the structure and function of proteins. This highlights the importance of maintaining the correct sequence of bases in DNA for proper genetic information storage and expression.

Step-by-step solution

Understand DNA structure

DNA, or deoxyribonucleic acid, is composed of four types of nucleotide bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These bases pair up to form the double helix structure of DNA, with A pairing with T and G pairing with C. This is known as complementary base pairing.

Know the role of each base

In DNA, the bases serve as a template for the synthesis of RNA and, ultimately, proteins. Adenine and guanine are purine bases, while cytosine and thymine are pyrimidine bases. Both purine and pyrimidine bases have unique shapes and hydrogen-bonding patterns that allow them to pair specifically with their complementary bases (A with T and G with C) in a process called base pairing.

Understand the difference between cytosine and uracil

Cytosine and uracil are both pyrimidine bases, meaning they have a similar structure, but they differ in one important aspect: cytosine has an amino group at the 4th position, while uracil has an oxygen atom at the same position. In RNA (ribonucleic acid), uracil replaces thymine and forms base pairs with adenine.

Hypothesize the effect of uracil replacing cytosine

If uracil were to replace cytosine in DNA, it would disrupt the normal base pairing, as uracil pairs with adenine, not guanine. This would result in a change in the DNA structure and could potentially lead to errors in the transcription process (the synthesis of RNA from DNA) and, therefore, errors in the translation process (the synthesis of proteins from RNA).

Conclusion

In summary, if a uracil group replaced cytosine in DNA, it would disrupt the normal base pairing and potentially lead to errors in the transcription and translation processes, which could ultimately affect the structure and function of proteins. This highlights the importance of maintaining the correct sequence of bases in DNA for proper genetic information storage and expression.

Key concepts

DNA structure

DNA, short for deoxyribonucleic acid, is the hereditary material found in all living organisms. It carries the genetic instructions necessary for the development, functioning, growth, and reproduction of all known organisms and many viruses. At its core, DNA is comprised of a long chain of nucleotides. Each nucleotide holds three components:

• A phosphate group
• A sugar molecule called deoxyribose
• One of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T)

The structure of DNA is a double helix, which looks like a twisted ladder. The sides of this ladder are made up of the sugar-phosphate backbone, while the rungs are formed by pairs of these nitrogenous bases connected by hydrogen bonds. This iconic shape allows DNA to be tightly packed into chromosomes, which ensures the efficient storage and transfer of genetic information.
DNA's double helix is stabilized by the specific pairing of the bases, which is critical for accurate replication and transcription. This precision in structure enables the DNA to maintain its integrity and function over time.

base pairing

Base pairing is a fundamental concept that determines how DNA strands come together in the double helix. This occurs through hydrogen bonds between complementary nucleotides. There are specific pairing rules: adenine (A) pairs exclusively with thymine (T), and guanine (G) pairs with cytosine (C). These pairs are held together by hydrogen bonds—A and T by two bonds, and G and C by three.
The specificity of these pairings is crucial for DNA's ability to replicate and function as a template for RNA during transcription. When DNA is replicated, the pairs serve as a blueprint for creating a new complementary strand. This ensures the genetic information is passed down accurately during cell division.
Errors in base pairing can lead to mutations or changes in the DNA sequence, potentially affecting gene function. Hence, the integrity of base pairing is key to the organism's survival, facilitating proper genetic expression and preventing errors that lead to diseases.

transcription errors

Transcription is the first step in the process by which genetic information in DNA is copied into messenger RNA (mRNA). This mRNA then carries the genetic message to be translated into a protein. Transcription involves the enzyme RNA polymerase, which reads the DNA strand and synthesizes RNA by linking the correct nucleotides together.
During this process, errors can occur, leading to mutations. Transcription errors occur when RNA polymerase inserts an incorrect base into the growing mRNA strand. This can result from spontaneous mutations, such as when a uracil replaces a cytosine. Normally, in RNA, uracil pairs with adenine, replacing thymine. However, if uracil appears in DNA instead of cytosine, it can mistakenly pair with adenine during transcription, leading to a mismatched or faulty protein product.
The result of such transcription errors can be significant, possibly affecting the structure and function of the resulting proteins. This can lead to improper protein folding, loss of function, or diseases such as genetic disorders or cancer. Regular cellular mechanisms aim to correct these errors but are not always successful, which emphasizes the importance of accuracy in transcription.