Question

Discuss the relationship between 5 methyl-cytosine and 5 hydroxymethyl-cytosine, mention enzymes responsible for their placement and how each modification affects gene expression.Define, compare and contrast heterochromatin and euchromatin and how they relate to gene expression

1. What does the field of epigenetics aim to study?

2. What would happen to a developing cell if its nucleus were switched for a nucleus of a differentiated cell?

3. Histone deacetylation enhances chromosome ______, thus the chromosomal segment cannot be ______. (options are Condensation; decondensation for the first part and translated or transcribed for the second part).

4. Explain the mechanism of DNA methylation, including what DNA residue is modified, by what enzymes, and the impact on gene expression and chromatin state. Draw it!​

Answer 1

***Hello, I'm answering your first question. Kindly post each question separately.

Methylation of DNA means addition of methyl groups to DNA. Methyl groups are added to DNA in the form of 5-methyl cytosine and 5-hydroxy methyl cytosine.

There are many restriction enzymes which are methylation sensitive. These restriction enzymes do not cut DNA at the sites of methylation. As a result, gene expression is altered. 5-methyl cytosine can also lead to mutation after deamination. Deamination of this base results in formation of thymine. While cytosine pairs with guanine, thymine pairs with adenine. This causes transition mutation in DNA; this finally alters the protein formed due to change in gene expression.

Enzyme responsible for placement of 5-methyl cytosine is DNA methyltransferase.

Presence of 5-hydroxy methyl cytosine causes a gene to switch on or off. 5-hydroxy methyl cytosine causes DNA demethylation. Such altered (de-methylated) DNA will be more labile to cutting via restriction endonucleases. This will further alter gene expression.

Note that 5-hydroxy methyl cytosine is the oxidized form of cytosine. Oxidases are responsible for doing this reaction.