Methylation state of cytosines at the QRT gene promoter is tested in one pair of identical twins, Ana and Becca, at 1 year of age, again when they are 25 years old. At 1 year old, the twins’ DNA is the same, and this part of the genome is not methylated. At age 25, Ana’s DNA remains unmethylated, but Becca’s cells show methylation at this site in skin cells.a. What most likely caused this change in Becca? i. A mutation event ii. An environmental stimulusb. When the twins were 1 year old, QRC mRNA was expressed at similar levels in both Ana and Becca. At age 25, which twin likely expresses more of the QRC mRNA? i. Ana, whose QRC promoter DNA is unmethylated ii. Becca, whose QRC promoter DNA is methylated iii. Both are the samec. At age 25, which twin has more tightly packed chromatin in the region of the QRC gene? Explain briefly. i. Ana, whose QRC promoter DNA is unmethylated ii. Becca, whose QRC promoter DNA is methylated iii. Both are the samed. Some of Becca’s skin cells, in which the QRC promoter is methylated, often go through mitosis to replace skin tissue. The daughter cells still have the same methylation pattern as the original cell. What could prevent maintenance of the methylation state through mitosis? Choose one or more. i. A drug that prevents DNA polymerase from incorporating 5-methyl-cytosine ii. A drug that prevents DNA methyltransferase Dnmt1 from methylating cytosines iii. A mutation that substitutes another base for the G beside a methylated cytosine iv. A mutation that substitutes another base for the A beside a methylated cytosine v. None of these

The sequences in the DNA determine the phenotype of an organism. But not all sequences in a DNA are expressed. In higher organisms the gene expression is a complicated mechanism influenced by several factors, both inherited/genetic and environmental. Methylation and acetylation are two chemical processes through which gene expression controlled without changing the sequence of the DNA.a. ii) An environment stimulus is more likely to happen than mutation. b. i) Methylation leads to silencing of the gene expression. Therefore, 25 year old Becca's mRNA would be less since the gene is methylated. But Ana's is unmethylated. so her mRNA quantity of the gene would be higher than Beccas. c. ii) Becca's gene is methylated. Methylation would add a positive methyl group to the DNA region. Therefore, the DNA would bind more tightly with the histone proteins surrounding them. So, Becca's chromatin in the gene region would be more dense than unmethylated gene of Ana's d.ii) DNA methyltransferases is the enzyme involved in the process of DNA methylation in humans. Any drug that blocks the activity of this enzyme would affect DNA methylation.