What are two modifications that affect the gene transcripition and?

Question

Describe how they work

Answer 1

EPIGENTIC CONTROL OF DNA TRANSCRIPTION - METHYLATION OF DNA

The fertilized egg is a totipotent cell. That is, through a series of divisions, its progeny cells can eventually become any of about 200 histologically different cell types. With subsequent cell division in the developing embryo, cells find themselves in different topological environments and have different cell-cell contact. Through signal transduction through the cell membrane, these cells start to become different, or differentiate, into other cells types. They do so by activating and inhibiting the expression of a different set of genes to form a different set of proteins in the cells. Most cells become terminally differentiated and eventually (after maybe a hundred cell divisions) lose the ability to divide and hence begin to die. However, a few types of cells, called stem cells, retain the ability to differentiate into other cells types in a regenerative process. These cells are pluripotent in that they can differentiate into other cell types.

How do dividing cells know what types of genes to actively transcribe? How can they have "memory" of the cells type they were before division? This appears to happen without alteration of the nucleotide sequence of the DNA in these cells. The main mechanism appears to be a inheritable but modifiable pattern of chemical modifications to the DNA (not unlike co- or post-translational modification of proteins) involving methylation/demethylation (by a methylase and demethylase) of cytosine in CpG dinucleotide repeats in the DNA. Also proteins can bind to DNA and methylated DNA to modify the course of gene expression in daughter cells. Such chemical modifications to the DNA which modify gene transcription are examples of epigenetic mechanisms controlling gene expression.

Answer 2

Mutation at the promoter site on DNA so that RNA Polymerase doesn't bind as frequently.

But perhaps you are looking for repressible and inducible genes...repressors slow or stop transcription and inducers increase gene transcription.

Answer 3

Sorry...I feel like I might be able to answer this if I had more details, but you probably need to be more specific