Please help me with the following questions:

1. What does the term "selective gene expression" refer to?

2. Describe how undifferentiated cells specialize to carry out a particular function?

3. what are the 2 differetn ways cells can control gene expression?

4. Explain why most cancers take a long time to develop?
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2 Answers

Each cell contains the entire genome but will only use what is necessary for its functionality.  Selective gene expression simply means that the cell can turn genes on or off depending upon its current needs.

While the cell has the ability to repress or activate genes throughout its life cycle, different cell types make larger and more permanent changes as they differentiate.  This allows for different cell types while preserving the entire genome.  Heart cells, for example may function very differently than liver cells but both contain the entire genome.

 This is important because it allows cells of different types to function in specific ways much more efficiently and greatly reduces the possibility of errors.

Differentiation of stem cells or undifferentiated cells during development is accomplished in a number of ways. Specific cell types often tightly pack DNA which will be less frequently used through what is referred to as Histone Modification.  Tightly packed DNA is difficult to access and prevents genes in that region of the genome from being expressed. DNA methylation is another mechanism that is frequently used to suppress gene expression.  Methyl groups are added to the DNA strand in certain locations to prevent or block the RNA polymerase and other transcription machinery from accessing those genes. 

DNA binding proteins are also often used to block or inhibit transcription.  These proteins, often referred to as transcription factors, bind to an area of the DNA to prevent access.   Many of these changes are permanent and specific to a particular cell type. They often cannot be reversed.  As a result, many tissues cannot repair themselves.  Stem cell research of undifferentiated cells may provide a way in which damaged tissues can be regenerated through the use of undifferentiated stem cells.

Once a cell is differentiated, there are still many functions which require additional regulation.  This regulation is constantly happening in order to meet the cells needs. 

Regulation of gene expression can be achieved through cell signaling and the release of transcription factors.  In this process, signaling molecules bind to receptors on the outside of the cell.  This causes the release of a DNA binding protein (transcription factor) inside the cell.  The transcription factor binds to the DNA and can either repress or activate a particular gene or set of genes.  When activated, an mRNA transcript is produced and translated into protein product.

Promoter and enhancer sequences regulate how many RNA transcripts are produced.

Expression of a gene can also be controlled at the mRNA level.  RNA is single stranded and much less stable than DNA.   More complex RNA strands can be modified to include a poly-a tail and 5 prime cap.  These additions control expression by slowing the degradation of the strand by rnace enzymes.  A longer poly-a tail and addition of a 5 prime cap allow for RNA strands to be translated more times before degradation occurs.

Often, gene regulation of gene expression is also tied to an operon such as the LAC operon as found in E. Coli.  In this case a series of genes are regulated by one promoter.  Another gene encodes for the protein CAP protein which blocks transcription by binding to the promoter region of the operon.  When lactose is present, it binds to the CAP protein and changes its shape so that it can no longer bind to the DNA strand.  View the following animation:

Lac Operon Animation

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Thank you soooooooooo much, anonymous!

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