14.) Epigenetic Inheritance & Imprinting

Question 0

Define epigenome.

Answer 0

A range of modifications that are imposed on the genome and ensure the stable transmission of gene expression patterns without changes to the DNA sequence.

Question 1

Define epigenetics.

Answer 1

Information that is passed down from one generation to the next but is not encoded in DNA. (ie silence of certain to genes).

Question 2

Define cis and trans epigenetic signals.

Answer 2

• Cis epigenetic signals:* Molecular signatures that are physically associated with the DNA an are inherited via chromosome segregation during the cell cycle. (eg methylation)
• Trans epigenetic signals:* This is when a gene expresses a product that has a positive feedback on the gene itself and is transmitted by partition of cytosol with a feedback loop. The proteins are still present during cell division and will be present in both daughter cells.

Question 3

Define dosage compensation.

Answer 3

This is the genetic expression “compensation” mechanism found in females. Since males have the Y chromosome which does not carry any genetic information (for our purposes), females have a mechanism to match this level of transcriptional expression. This is known as X-inactivation. Females will inactivate one of their X-chromosomes, at random, to all of their somatic cells.

Question 4

Describe the general mechanism of X-inactivation.

Answer 4

The X-inactivation center (XIC) will synthesizes XIST RNA. XIST RNA will bind to the X chromosome until the entire genetic information is silenced.

Question 5

What are maintenance methyl transferases?

Answer 5

These are methylating enzymes that will methylate daughter strands after replication. (but only complementary to the original strand).

Question 6

Describe how epigenetic information is inherited “faithfully.”

Answer 6

Methylation of the DNA in vertebrates is specific to cytosine residues of CG sequences. When DNA is replicated, there are maintenance methyltransferases that will recognize the cytosines of these sequences on the daughter strand (provided that the original strand is methylated at this site as well). This will trigger methylation.

Question 7

What are 4 different methods for epigenetic inheritance?

Answer 7

1. ) Positive feedback loop activated (trans epigenetics)
2. ) Histone modification (Chromatins are activated/inactivated)
3. ) DNA methylation (cis epigenetics)
4. ) Conformation change to aggregated state (prions)

Question 8

What is the most abundant methyltransferase in the mammals?

Answer 8

DNA methyltransferase 1 (DNMT-1)

Question 9

How does methylation affect gene expression?

Answer 9

Believed to contribute to gene repression (silencing of a gene).

Question 10

True or False: Genes that don’t have transcriptional factors, aka “turned off,” will still express a gene.

Answer 10

True if it is not methylated: Only to a small extent. This is known as leaky expression. This is prevented by DNA methylation.

Question 11

What were the other two examples of dosage compensation?

Answer 11

Drosophila: Males have two-fold expresssion of single genes on X chromosome

Nematodes: Genes on the two X chromosomes found on the hermaphrodites are each expressed at half the level found on males (single X chromosome).