EFFECT OF ENVIRONMENTAL FACTORS ON DNA Treatment 2SEX Mean Percent Of DNA With Double-Strand Breaks 9.8 | Untreated Control 2.4 Bap Only 9.7 3.2 10.1 3.4 UVA Only UVA And Bar 35.1 5.0 Mean Percent Of DNA With Double Strand Breaks Untreated BaP Only UVA Only UVA And Bap Treatment
Environmental mutagens can affect DNA in cells. Benzo[a]pyrene (BaP) is a mutagen that is commonly found in urban air pollution. Researchers claim that the effect of UVA radiation (UVA), another known mutagen, is amplified by the presence of BaP. To test their claim, the researchers exposed cultured eukaryotic cells to either BaP, UVA radiation, or both mutagens. The researchers then determined the percent of chromosomal DNA that contained damage in the form of double-strand breaks. The results are shown in the table.
(a) Using the results from all treatments, describe the effect of BaP alone and UVA alone compared with the effect of the combined treatment of BaP and UVA on DNA. [4]
(b) Predict the most likely effect on cell division for a cell containing DNA with double-strand breaks. Justify your prediction. [2]
(c) Point mutations alter the DNA sequence at a single nucleotide. Describe how point mutations affect the genetic makeup of the population. [2]
(d) Explain the effect of a STOP codon mutation being introduced to the middle of gene sequence. [2]
The Correct Answer and Explanation is :
(a) Effect of BaP Alone, UVA Alone, and Combined Treatment of BaP and UVA on DNA:
The data from the treatments reveal that exposure to Benzo[a]pyrene (BaP) alone causes a slight increase in DNA double-strand breaks, with a mean percent of 9.7, similar to the untreated control (2.4). When only UVA radiation is applied, the percent of DNA with double-strand breaks increases to 9.8. However, the combined exposure to both BaP and UVA significantly amplifies the damage, increasing the mean percent of DNA with double-strand breaks to 35.1. This indicates that BaP and UVA together cause a much higher degree of DNA damage compared to either mutagen alone. The combined effect suggests an interaction between these two mutagens, leading to more severe DNA damage than would be expected from their individual effects.
(b) Effect on Cell Division for Cells Containing DNA Double-Strand Breaks:
Double-strand breaks (DSBs) in DNA are highly detrimental to cell division. If a cell’s DNA is damaged by DSBs, it can result in errors during the process of cell division, such as the failure of proper chromosome segregation or cell cycle arrest. Cells often activate repair mechanisms to fix the breaks, but if the damage is too severe or the repair mechanisms fail, the cell may undergo apoptosis (programmed cell death) or enter senescence. Thus, cells with DSBs have a high likelihood of experiencing disruptions in cell division, and this can lead to genetic instability and potential cell death.
(c) Effect of Point Mutations on the Genetic Makeup of the Population:
Point mutations are changes in a single nucleotide base in the DNA sequence. Depending on the type of mutation (e.g., silent, missense, or nonsense), point mutations can lead to the production of altered proteins or no protein at all. When point mutations occur in germ cells (sperm or egg), they can be passed on to offspring, thereby introducing genetic variation in the population. If the mutation provides a selective advantage, it can increase in frequency over generations, contributing to evolutionary changes. Conversely, deleterious mutations may decrease in frequency due to natural selection, but they may still persist in a population.
(d) Effect of a STOP Codon Mutation in the Middle of a Gene Sequence:
A STOP codon mutation introduced to the middle of a gene sequence would result in the premature termination of protein synthesis. Normally, STOP codons signal the end of translation, but when a mutation introduces a STOP codon too early in the gene, the ribosome halts translation before the protein is fully synthesized. This would lead to a truncated protein that is likely nonfunctional or dysfunctional. The presence of a truncated protein can disrupt cellular processes and may cause diseases or disorders related to the loss of that protein’s function. Such mutations can have severe consequences if the gene product is essential for normal cell or organism function.