Question

13. DNA methyltransferase:

A) Recognizes hemimethylated 5’-CpG-3’ sites on double stranded DNA

B) Adds a methyl group to select cytosine bases in the DNA

C) Is inhibited by decitabine

D) All of the above

E)   None of the above

13. Angelman Syndrome and Prader Willi Syndrome revealed that a similar chromosomal deletion could manifest as two very different (phenotypically distinct) syndromes. The genetic basis for the difference:
    1. Arises from changes in transcription due to promoter swapping at the deletion site.
    2. Depended on whether the deletion occurred on the maternally or the paternally inherited chromosome.
    3. Involves select, differentiated haploid cells inheriting the chromosome with the deletion after reductional cell division.
    4. Involved the generation of miRNA at the deletion site that silenced the wild type transcript on the other chromosome.
    5. Depended on the nurturing environment of the children, well-nurtured children developed Angelman Syndrome, while poorly-nurtured children developed Prader Willi Syndrome.

13. What epigenetic changes would lead to increased transcription?

1. Demethylation of the DNA
2. Acetylation of the histone N-terminal tails
3. Removal of histones from the DNA by remodeling factors
4. All of the above
5. None of the above

Answer 1

1) DNA methyltransferase adds a methyl group to cytosine residues in the DNA, fully methylated DNA is replicated, then the older strand in the daughter DNA molecule is methylated and newly synthesized DNA is not methylated, so the DNA is called hemimethylated, DNA methyltransferase methylates the newly synthesized strand of hemimethylated DNA to fully methylated.

decitabine inhibits DNA methylation.

so the answer s d) all of the above.

2) Angelman Syndrome and Prader Willi Syndrome revealed that a similar chromosomal deletion, butgene for the Angelman syndrome is paternally imprinted, that is allele from the father is not expressed, so if the same region is depeted in the maternal chromosome it results in Angelman syndrome.

Prade Willi syndrome is maternally imprinted, so if the allele from the mother is not expressed, so if the allele from father has deletion in that region (15q11-13)  it results in Prader willi syndrome.

so the answer is B) Depended on whether the deletion occurred on the maternally or the paternally inherited chromosome.

3) DNA methylation decreases transcription, so DNA demethylation increases the transcription of the genes, acetylation of histone N terminal tails makes the chromatin to a loosely packed state so RNA polymerase can transcribe the regions, so acetylation of histone N terminal tails increase gene expression.

during transcription the nucleosomes are removed by chromatin remodelling complexes, so that RNA polymerase can acess the DNA, Removal of histones from the DNA by remodeling factors can increase the expression of genes.

so the answer is D) all of the above.