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I want to know more about positive feedback in gene regulation and to know the similarity and differences between positive and negative feedbacks.

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    $\begingroup$ Do you have any specific genes in mind? The way you ask your question is really broad and hard tro answer. $\endgroup$ – Chris Aug 15 '14 at 14:35
  • $\begingroup$ I want to know about E.coli bacteria $\endgroup$ – Sahar Aug 15 '14 at 14:48
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    $\begingroup$ Can you then edit your question and add the relevant information? $\endgroup$ – Chris Aug 15 '14 at 15:18
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Positive and negative feedback are essential motifs that control transcriptional activity of genes. They may involve other genes, mRNA and/or proteins.

A classical pattern of negative feedback is where transcription of a gene results in mRNA and translation of this mRNA in a protein that represses the expression of the gene again. Feedback loops may involve much larger structures, e.g. if a gene codes for a hormone that stimulates the release of another hormone in a remote organ and if that other hormone represses the expression of the first gene again.

In general, negative feedback results in stable levels of certain products. Positive feedback leads to infinite growth (if not limited by a surrounding negative feedback loop or simply by lack of substrate).

It is easy to identify positive and negative feedback by counting the positive and negative correlations within the loop. Feedback loops are defined as positive or regenerative loops if there is at least one positive correlator and the number of negative correlators is even. If the number of negative correlators is odd than the loop is negative or degenerative.

See http://biomedicalcybernetics.wikia.com/wiki/Feedback for a more detailed explanation.

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    $\begingroup$ It is easy to identify positive and negative feedback by counting the positive and negative correlations within the loop.... Not that easy.. Correlations are not to be trusted blindly $\endgroup$ – WYSIWYG Aug 16 '14 at 6:42
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A feedback loop is a network structure in which the gene is able to regulate itself via a single path.

$A\rightarrow B\rightarrow C\rightarrow A$

Interactions can be called positive or negative depending on their effect on the downstream gene. If a gene A represses another gene B, then it is a negative interaction; when it activates then the interaction is positive.

If you multiply the signs of all the interactions in the path from gene-A back to itself you will get the sign of the loop.

So a positive FBL will result (in the above example) if :

  1. A activates (+) B; B activates (+) C; C activates (+) A
  2. A represses (-) B; B activates (+) C; C represses (-) A
  3. A activates (+) B; B represses (-) C; C represses (-) A
  4. A represses (-) B; B represses (-) C; C activates (+) A

Any other combination of interaction will result in a negative feedback loop.

Differences are based on their properties. I'll mention two main unique properties of each.

PFBL

  • Switching circuits
  • Hysteresis

NFBL

  • Expression control (feedback control)
  • Oscillations

Similarity: well, nothing more that they both are called feedback loops and the topology is similar (just the nature of interaction is different).

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