Epigenetics

Question 1

Methylation

Answer 1

Methylation is the addition of a methyl group to either cytosine or adenine - most common is 5methylcytosine. This prevents transcription as the methyl group prevents the TF from binding.

5-methylcytosine can spontaneously revert to thymine, causing a base pair mismatch, and a mutation.

Question 2

X-inactivation

Answer 2

During early development, one copy of the X chromosome is sometimes left inactive (also called a Barr Body), meaning all descendant cells contain the inactive X chromosome. This creates a mosaic in females as they have two copies of the X chromosome, so one is expressed and one isn’t. However, males only have one copy of the X chromosome (XY) so they either have the inactivation or they don’t - so they don’t exhibit a mosaic. This can be seen in Calico cats.

An exception to this is when the tips of an inactive X chromosome recombine with the tips of the Y chromosome, resulting in pseudoautosomal regions.

Question 3

Histone modification

Answer 3

Histones are important in transcription and gene silencing, and wrap around DNA to keep it compact. Histones have an amino acid tail, which when modified, can attract proteins to the DNA. This results in methylation, acylation, phosphorylation, and citrullination. This alters chromosome structure which therefore alters gene expression

Question 4

Imprinting

Answer 4

Causes genes to be expressed in a ‘parent-of-origin’ manner, meaning the gene expressed in the offspring is a result in the expression in either mum or dad.

In mice, the IGF2 gene controls growth. The mother carries the gene for the IGF2 receptor, while the father carries the individual IGF2 gene. This means that if the mothers gene for the receptor is not expressed, the offspring will be larger than normal. If the fathers gene is not expressed, the offspring will be smaller than normal.