Question

The spontaneous cleavage of adenine or guanine from DNA, a process called spontaneous depurination, is a common form of DNA damage in mammalian cells. The rate of spontaneous depurination is estimated to be \(3 \times 10^{-9}\) per purine per minute. This damage must be repaired. A diploid human cell has \(6 \times 10^{9} \mathrm{bp.}\) How many spontaneous depurinations must be repaired per cell per day?

Short answer

Approximately 12,960 spontaneous depurinations must be repaired per cell per day.

Step-by-step solution

Calculate Daily Time in Minutes

First, we need to find out how many minutes are in a day. Since there are 60 minutes in an hour and 24 hours in a day, multiply these together to get the total minutes in a day:\[60 \text{ minutes/hour} \times 24 \text{ hours/day} = 1440 \text{ minutes/day}.\]

Estimate the Number of Purines in the Human Genome

A human diploid cell has \(6 \times 10^9\) base pairs. Since each base pair consists of a purine and a pyrimidine, approximately half of these base pairs, or \(3 \times 10^9\), are purines (adenines or guanines).

Calculate Spontaneous Depurination Per Minute

The rate of spontaneous depurination is \(3 \times 10^{-9}\) per purine per minute. Therefore, calculate the depurination events per minute by multiplying the number of purines by the depurination rate:\[3 \times 10^9 \text{ purines} \times 3 \times 10^{-9} \text{ depurations/purine/minute} = 9 \text{ depurations/minute}.\]

Calculate Daily Depurinations

Now, use the number of depurations per minute to find the total number of depurations per day by multiplying by the number of minutes in a day:\[9 \text{ depurations/minute} \times 1440 \text{ minutes/day} = 12960 \text{ depurations/day}.\]

Key concepts

Spontaneous Depurination

In the world of DNA, spontaneous depurination refers to the natural and non-enzymatic removal of purines—adenine (A) or guanine (G)—from the DNA strand. This process happens rather frequently and is a significant source of DNA damage. In mammals, including humans, cells continuously experience spontaneous depurination, but have developed robust repair mechanisms to counteract this damage.

Given its spontaneous nature, this process can occur randomly across the genome. Each purine depurination leaves behind an apurinic site (AP site), which can be problematic for DNA replication if not promptly repaired.

• Spontaneous depurination is estimated at a rate of 3 x 10^-9 per purine per minute in mammalian cells.
• This means that every day, vast numbers of DNA bases require repair to maintain the integrity of genetic information.

DNA Base Pairs

DNA is composed of two long polymer strands coiled around each other, forming a double helix. Each strand is made up of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). These nucleotides pair with each other to form the rungs of the DNA ladder.

Each base pair consists of one purine and one pyrimidine. Specifically, adenine pairs with thymine, and guanine pairs with cytosine.

• Purines (adenine and guanine) pair with pyrimidines (thymine and cytosine) via hydrogen bonds.
• This specific pairing ensures the DNA structure remains stable and allows it to encode genetic information properly.

Understanding the nature of these base pairs is crucial because any alteration, such as depurination, can disrupt genetic information.

Human Genome

The human genome contains the complete set of genetic instructions for human development, functioning, growth, and reproduction. It is made up of over 3 billion base pairs, organized into 23 pairs of chromosomes in each cell. In a diploid human cell, which contains two sets of chromosomes, there are approximately 6 x 10⁹ base pairs.

Each base pair is a combination of one purine and one pyrimidine, meaning the genome contains a significant number of each. Approximately half of these are purines (adenines or guanines), equating to roughly 3 x 10⁹ purines in a diploid cell.

• This organization allows the cell to efficiently manage and access large amounts of genetic data.
• The human genome's structure and function are highly significant in understanding diseases, especially genetic disorders.

Purines

Purines are a type of nitrogenous base found in nucleic acids, such as DNA and RNA. The two primary purines in DNA are adenine (A) and guanine (G). They play a crucial role in pairing with their respective pyrimidines (thymine and cytosine) to help form the double helix structure of DNA.

Purines are notable not only for their pairing properties but also because of their involvement in several cellular processes, such as coding, decoding, and expression of genes.

• In the context of DNA damage, purines are susceptible to loss through processes like spontaneous depurination, necessitating quick repair to prevent mutations.
• The ability of cells to repair and thus maintain purines is critical for preserving genetic stability and preventing disease.

Understanding purines helps us appreciate their pivotal role in maintaining cellular integrity.