Epigenetics

When the association of Histones with DNA is weak, the DNA-Histone complex appears loosly packed - less condensed. Therefore, they have easy accessiblity to transcriptional factors and so can produced mRNA. The gene is therefore SWITCHED ON

When the association of Histones with DNA is strong, the DNA-Histone complex appears tightly packed - more condensed. Therefore, there is no access to transcriptional factors so mRNA cannot be produced. Therefore the gene is SWITCHED OFF.

-Donating Acetyl from Acetylcoenzyme A from the Link Reaction.

Positive charges are increased on histones so this attracts the phosphate groups on the DNA. 

Association between DNA-Histone complex appears stronger and therefore cannot have access to transcriptional factors and the gene switches off.

- CH3 is added to the cytosine bases on DNA. Prevents transcription of genes in two ways: 

1. This prevents instant binding of transcriptional factors

OR

2. Attracts proteins that condense the association of DNA-Histone complex so therefore transcriptional factors turn off and the gene also turns off so that there is no production of mRNA

Translation of mRNA is inhibited by breaking down mRNA into smaller pieces known as small interferance RNA (siRNA)

1. Enzyme cuts the double stranded RNA into small interferance RNA (siRNA)

2. The siRNA combines with an enzyme

3. This then guides the enzyme to bind on a strand of mRNA which has complementary base pairing.

4. The enzyme then cuts the RNA

5. This means that polypeptide production can no longer occur. Cannot code for polypeptides and therefore the gene is NOT expressed. IT IS BLOCKED.