Question

In a eukaryotic cell, the hydrophilic ligand CUE binds the enzyme-linked receptor CUE receptor. Describe a hypothetical but realistic signal transduction cascade, that eventually leads to the transcription of the GRAD gene. The GRAD protein is secreted by the cell. Describe the critical steps in gene transcription, RNA processing and in translation, and which structures and organelles are involved.

A detailed description of transcription and translation, and all proteins involved, is NOT expected, but a description of the main steps is.

Include:

RNA polymerase

a signal transduction cascade

transcription factors

amplification

how the signal reaches the nucleus

transcription factors

ribosomes

organelles involved

secretion from the cell

Answer 1

•Signal transduction cascade is refers to a series of sequential events- like-

A.protein phosphorylations,

B.binding of ligand by a transmembrane receptor,

C.transfer a signal through a series of intermediate molecules until final regulatory molecules, such as transcription factors, are modified in response to the signal.

•Cell signaling can be divided into 3 stages.

1.Reception: These type of receptors transmit information from the extracellular environment to the inside of the cell .Membrane receptors function by binding the signal molecule (ligand) and produced a secondary messenger that then causes a intracellular response and start the gene expression [transcription and translation].

2.Transduction: When the signaling molecule binds the receptor it changes the receptor protein and cells often use a multi-step pathway that transmits the signal quickly, while amplifying the signal to numerous molecules at each step. Transduction is a chain reaction where one step is dependent on another. Example;- the protein is activated by removal of phosphate group with the enzyme phosphate kinase, this phosphorylation cascade, causing a chain reaction. where one enzyme phosphorylates another, which then phosphorylates another protein.

3.Response: Finally, the signal triggers cellular response that is gene expression.

•Let us focus the events according to the steps involved in GRAD gene expression like – transcription, m-RNA processing ,translation and formation of GRAD PROTEIN.

1.The GRAD signal receptor protein received signals . This is a tyrosine kinase receptor which undergoes autophosphorylation.

2.It releases a secondary messenger like cAMP

3.Which received by intracellular receptor and form a complex.

4.This complex entered into the nucleus and triggers the GRAD gene for expression and formation of GRAD protein.

•A Grad gene is a DNA segment that has three parts – promoter gene, structural gene and termination gene.

•The gene expression process involved the following steps.

1.Transcription :- formation of m- RNA molecule from the DNA template. This process is catalyzed by a DNA dependent RNA enzyme called RNA polymerase . In the eukaryotic cell RNA polymerase II enzyme is responsible for this transcription process.- during this event – firstly the sigma factor of RNA polymerase is searched for the promoter sequence [tata box] and attached the RNA polymerase to the operator region. After that he sigma factor leaves RNA polymerase and the transcription process is started. According to the structural GRAD gene sequence of the template strand the polymerase made a m- RNA molecule .

2.M- RNA processing:- this prematured m- RNA is processed by G- capping at 5’ end , splicing of exon and intron, joining of all exons and making a m- RNA , and finally addition of poly Adenyl chain on the 3’ end of m- RNA. This matured and processed m- RNA is ready of translation.

3.This m- RNA is now exis from the nucleus and atthached to the RER / Ribosome.

4.Translation :- this event is sub divided into 3 steps – initiation, elongation and termination. This event is catalysed by different factors like- initiating factors, elongation factors and termination factors.

•Initiation :- the smaller sub unit of ribosome search for KOZAK sequence [consensus sequence ] on m- RNA and attached to it, after that large sub unit , activated and charged t- RNA is also attached to m- RNA.

•Elongation :- the first intitiating charged t-RNA [for methionine amino acid] is attached to the p- site of ribosome , after that the second t- RNA [with amino acid] attached to the A site of ribosome . First peptide peptide bond is made between these two amino acid. The process is going on, according to codon sequence on m- RNA , a charged t- RNA is coming a a polypeptide chain is elongated.

•Termination:- when , codon UAA/UAG /UGA came on the m- RNA molecule , the process is stopped and nascent protein molecule is released from ribosome. The transcription complex is broken down and ribosome, released .

a)After this even , the nascent protein [GRAD protein] is entered into the lumen of endoplasmic reticulate , in that the protein processed the events like folding , oligomerization and ready to export by enclosing into export vesicle

b)This vesicle cis portion of Golgi apparatus. In side the golgi bodies the protein glycosylation is performed and now the the protein nis fuctional and entered into transporting clathering coated vesicle.

c)This clathering coated protein containing vescilcle transported towards the cell membrane .

d) here the GRADE protein is secreted from the the cell.