(3) Below is a diagram of a human gene X whose expression is regulated by the en
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Question
(3) Below is a diagram of a human gene X whose expression is regulated by the enhancer shown; this enhancer has binding sites for transcription factors A and B. The table on the right shows the expression of mRNA from the genes that encode these two transcription factors and gene X in four different tissues Tissue A mRNA BmRNA XmRNA ? ??? Heart yes yes yes yes no yes no yes no no yes enhancer gene X Pancreas un (a) Explain the results in the table (i) What type of transcription factor is A most likely to be?: (ii) What type of transcription factor is B most likely to be?: (ii) Explain why X is expressed in the heart (iv) Explain why X is not expressed in the liver: (v) Explain why X is not expressed in the pancreas: (vi) Explain why X is expressed in the lung (b) If the region containing just the coding region of gene X was inverted (as shown below) in relation to the enhancer, what effect would this have on its expression in these tissues? B ABA enhancer (c) Expression of these genes and the genomic region containing gene X were investigated in pigs and it was shown that the sequence of this enhancer was quite strongly conserved with all of the binding sites for A and B still present. However, curiously, gene X was now expressed in the liver. Provide a possible explanation.Explanation / Answer
Transcription is the process of formation of mRNA from DNA strand. The expression of genes are regulated by various types of transcription factors, which helps in ONN and OFF of the gene regulation. Enhancers ,are short stretches of nucleotide present either upstream or downstream of promoter region, which provide binding sites for these factors. Activity of enhancer is tissue specific.
(a) (i) A is likely to be a Generalised type of transcription factor, as its binding and expression is necessary for expression of gene X.
(ii) B is likely to be a specialised (REPRESSOR or ACTIVATOR ) type of transcription factor, depending upon the tissue.
(iii) In heart tissue, B mRNA is not expressing, so the A mRNA activates the promoter site to initiate transcription, thus expressing the gene X.
(iv) In liver, expression of B mRNA represses the activation of gene X.
(v) In pancreas, failue of B mRNA expression leads to loss of gene X expression.
(vi) In Lung, both the transcription factors act as Co-activators, leading to expression of gene X.
(b) Direction of enhancer has no effect on its binding with transcription factors (act in orientation-independent manner). So if the coding region of gene X is inverted, its regulation due to enhancer will not be affected.
(c) Although the sequences for enhancer are conserved, a single mutation in transcription factor B can lead to its inactivation, thus activating the expression of gene X.
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