Gene Expression and Regulation

 


Gene Expression:

Gene expression is the mechanism at the molecular level by which a gene is able to express itself  in the phenotype of an organism. The mechanism of gene expression involves biochemical genetics. It consists of synthesis of specific RNAs, polypeptides, structural , prtoein aceous biochemical  or enzymes  which control the structure of functioning of specific traits.



Gene Regulation:

Regulation of gene expression refers to a very broad term that may occur at various levels. Considering that gene expression results in the formation of a polypeptide, it can be regulated at several levels. In Eukaryotes , the regulation could be exerted at:

(i) Transcriptional level ( formation of primary transcript,),

(ii) Processing level ( regulation  of splicing, ), 

(iii) Transport of mRNA from nucleus  to the cytoplasm


(iv) Translational level

In prokaryotes,  control of the rate of transcription  initiation is the predominant site for control of gene expression. In a transcription unit, the activity of RNA polymerase at a given promoter is in turn regulated by interaction with accessory proteins, which affect its ability to recognise start sites. These regulatory proteins can act both positively ( activators) and negatively ( repressors). The accessibility of promoter regions of prokarytic DNA  is in many cases regulated by the interaction of  proteins with sequences termed operators.


        All the mature cells of a living  being possess the same gene complement or genetic material. Even then  they differ in their structure  and functioning. It is possible only if it organism has a mechanism of regulating gene activity, allowing some to function and restraining others through a system of switching on and switching off. From a number of studies on the metabolism of bacterium Escherichia coli, two French scientists,  Jacob and Monod ( 1961) found that the genetic material possesses regulated gene units called operons.

In their  study on lactose utilization with the help of mutants of bacterium  Escherichia coli, Jacob and  Monod found that two enzymes are required __ß_ galactosidase and lactose permease. Permease is required for allowing entry of lactose. A very small quantity of permease is always present in the bacterium otherwise lactose cannot enter the cells. Enzyme ß_ galactosidase breaks lactose into glucose and galactose.  Jacob and Monod (1961) found  that genes  for three enzymes  ( ß__ galactosidase, lactose permease and transacetylase) become active  or inactive simultaneously.  Inducer for activity of these genes is normally lactose. Because of this fact, Jacob and Monod proposed that a repressor produced by a regulator  gene is normally present  in Escherichia coli.  It binds to a site of DNA  called operator gene that governs the working of enzyme producing  or structural  genes. The formation  of mRNA by structural genes is also governed by the availability of RNA polymerase. Later workers proposed that another gene, called promoter  gene ( not described by Jacob and Monod), is also involved in the gene regulation. Operon concept of Jacob and Monod is applicable for prokaryotes only.

An operon is a part of genetic material ( or DNA) which acts as a single regulated unit having one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor and an inducer or corepressor ( from outside). Operons are of two types, inducible and repressible. Some important examples of operon are lac operon, trp operon, ara operon, his operon, val operon etc. 

What are the characteristics of inducible operon system?

Inducible Operon System:

It is an operon, which is switched on in response to the presence of a chemical . It consists  of the following  parts:

Structural Genes: They synthesize mRNAs. An mRNA control metabolic activity  of cytoplasm through the formation  of protein of enzyme. An operon has one or more structural  genes. The lactose or lacoperon of Escherichia coli contains  three structural  genes ( z, y, a). They transcrib a polycistronic mRNA molecule that help in the synthesis of three enzymes __ ß _ galactosidase,  for hydrolysing lactose or gelactoside, lactose or galactoside permease for allowing entry of lactose from outside and galactoside acetylase or transacetylase. Out of these 'z' gene codes for beta_ galactosidase ( ß_ gal), 'y' gene codes for permease and 'a' gene codes for transacetylase. Structural gene is moderately long to large depending upon the polypeptide to be synthesized. 

Operator Gene: It is a small gene which directly control the synthesis of mRNAs over the structural genes.  It is present upstream to the structural gene, i.e., towards 32 end of template strand. It is switched off by the presence of a repressor.An inducer can take away the repressor and switch on the gene. The gene then directs the structural genes to transcribe. Operator gene of lac operon is made of only 27 base pairs. 


Promoter Gene: It is present upstream  to structural gene, i.e., towards 32  end of template strand. It acts as an initiation signal which functions as recognition centre for RNA _ polymerase provided the operator gene is switched on. RNA polymerase is bound to the promoter gene.  When the operator gene is functional, the polymerase moves over it and reaches the structural  genes to  perform  transcription. This gene is also a small gene.

Regulator Gene: ( i Gene). The gene produces a repressor that binds to operator gene and stops the working of the latter. The term 'I' does not refer to inducer, rather it is derived from word_ Inhibitor.  Regulator gene is present somewhere else on the chromosome and not in contiguity  with other genes of operon. 

Repressor: It is proteinaceous substance synthesised by the regulator gene. Repressor is meant for blocking the operator gene so that the structural genes are unable to form mRNAs.  It has two allosteric sites, one for attaching to  operator gene and second for binding to inducer. After coming in contact with inducer, the repressor is unable to combine with operator. The repressor of lac_ operon is a protein made up of four subunits,each having molecular weight of 40,000. Regulator gene is commonly a large gene.




Inducer: It is a chemical  ( substrate, hormone or some other metabolite), which after coming in contact with the repressor, changes the latter into non _ DNA binding state so as to free the operator gene.  The inducer for lac_ operon of Escherichia coli  is lactose ( actually   allolactose, a metabolic of lactose).

cAMP: It exerts a positive control in lac_ operon because in its absence RNA polymerase is unable to recognize  promoter gene. cAMP itself requires catabolite activator protein or CAP for its functioning.The gene  coding for CAP is located away from the operon but the receptor CAP site occurs near the lac promoter. CAP activates lac genes  only when glucose  is absent. 

   RNA polymerase is recognised by promoter  gene. It passes over the freed operator gene and then catalyses transcription of mRNAs  over the structural genes.  The mRNAs pass into the cytoplasm and form particular proteins or enzymes. Out of the three enzymes produced by lac_ operon, lactose_ permease is meant for bringing lactose inside the cell. ß__ galactosidase ( = lactase) breaks lactose into two components, glucose and galactose. The enzyme like lactase or ß__ galactosidase which is formed in response to the presence of its substrate is often called inducible enzyme. Regulation of lac operon by repressor is referred to as negative regulation. Lac operon is under control of positive regulation as well, but it is beyond the scope of discussion at this level.

Inducible operon system generally  occur in catabolic pathways. 


Repressible Operon  System Notes:

Repressible Operon System:

It is commonly found in anabolic pathways.  The operon is active and the enzymes produced by its structural genes are normally present  in the cell. The functioning of the operon is stopped when the concentration of an end product crosses a threshold value.An example of repressible system is tryptophan or trp operon of Escherichia coli. It consists  of the following:

 Structural Genes: The genes are connected to transcription of mRNAs.  Tryptophan operon has five structural genes E, D, C, B, A. They form enzymes  for five steps of tryptophan synthesis. 

Operator Gene: It controls  the functioning of structural  genes. Normally  it is  kept switched on because the aporepressor produced by regulator gene is unable to completely block operator gene. The operator gene is switched off when a corepressor is available along with aporepressor.

Promoter Gene: It is the site for initial binding  of RNA_ polymerase.  The latter  travels from promoter gene to structural genes provided operator gene is switched on.


Regulator Gene: It forms aporepressor for blocking the activity of operator gene. 


Aporepressor:It is a proteinaceous substance  synthesised by regulator gene.  Aporepressor forms a constituent of repressor for blocking the operator gene. For this it requires a corepressor.  When the latter is not available in proper strength,  the operator gene is kept switched on because  by itself, aporepressor is unable to block the operator gene. 


Corepressor: It is a  nonproteinaceous component of repressor, which is also an end product of reactions catalysed by enzymes produced by structural genes.  The end product  is often utilized in some other reaction so that it rarely accumulates and hence does not function as corepressor.  However, whenever it accumulates or becomes available from outside source,  the end product becomes corepressor, combines with aporepressor,  forms repressor and blocks the operator gene. The structural genes now stop  transcription. The phenomenon is known as feed __ back repression. In tryptophan operon, tryptophan ( an aminoacid) functions as corepressor. 



Thus, we have seen that gene encoding enzymes involved in related functions often are located next to each other in the form of a cluster of genes comprises a single transcription unit. The full of genes is transcribed to produce single a mRNA    molecule.  Ribosomes initiate translation at the beginning of each of the genes  in this mRNA producing different polypeptides. Such an mRNA which encodes several polypeptides is said to be polycistronic. In contrast to polycistronic mRNAs, which are common in prokaryotes, most  eukaryotic units produce mRNAs  that encode only one protein. 


       READ AND DISEST
Translation of a bacterial mRNA proceeds sequentially through  its cistrons. Usually  ribosomes terminates translation at the end of the first  cistron ( and dissociate into subunits), and a new ribosome assembles independently at the start of the next coding region. When the mRNA is polycistronic,  each coding region starts with a ribosome binding site. In some bacterial mRNAs, translation between adjacent cistrons is directly linked, because ribosomes gain access to the initiation codon of the second cistron as they complete translation of the  first cistron. This effect requires the space between the two coding regions to be small.


What is gene expression and its regulation?

              OR 

What are the Five levels of  regulation of  gene expression?

Gene expression refers to the process by which the information encoded in a gene is used to synthesize a functional gene product, such as a protein or a non-coding RNA molecule. It involves transcription, where the   DNA sequence of a gene is copied into an RNA molecule, and  translation, where the RNA molecule is used as a template to produce a protein.

Gene expression regulation refers to the mechanisms that control the level and timing of gene expression. It ensures that genes are expressed at the right time, in the right cells, and in the right amount. There are various levels of gene expression regulation, including:

1): Transcriptional  regulation:This involves controlling the initiation and rate of   transcription of a gene. It is achieved by regulatory DNA sequences and proteins (transcription factors) that bind to these sequences and either enhance or repress transcription.

2):Post _ transcriptional regulation: This occurs after RNA molecules are transcribed from DNA. It involves processes such as RNA splicing, where non-coding regions (introns) are removed, and the remaining coding regions (exons) are joined together.



 3): RNA stability and degradation:The stability of RNA molecules can be regulated, determining how long they persist in the cell before being degraded. Certain factors or RNA-binding proteins can influence the stability and degradation rates.

4): Translation regulation:It involves controlling the rate at which mRNA molecules are translated into proteins. Various factors, such as specific RNA sequences or regulatory proteins, can influence translation efficiency.

5): Epigenetic Regulation:This involves modifications to the DNA or associated proteins that can impact gene expression. Examples include DNA methylation or histone modifications, which can alter the structure of chromatin and influence gene accessibility.

Gene expression regulation is crucial for normal cellular function and development, as well as responding to environmental signals and maintaining cellular homeostasis. Dysregulation of gene expression can lead to various diseases, including cancer and genetic disorders.

Gene regulation in Eukaryotes:

Eukaryotes have both inducible and repressible gene regulation. It is more complex then prokaryotes because:

1): Different structural genes connected to a metabolic pathway do not often lie adjacent to one another. They are generally found well spaced, on the same or  different  chromosomes. 

2): Each structural gene has a promoter gene.


3): Eukaryotes possess senser genes which pick up information  of any change in the intracellular environment  and presence or absence of hormones, vitamins, metal ions, chemicals, pathogens, etc.


4): Integrator genes are present for coordinating functioning of structural genes present in different parts of genetic material. 

5): Enhancer  genes and silencer genes enhance or slow down the expression of certain genes. 


6): Eukaryotic structural genes have non essential regions called introns or spacer DNA or intervening sequences ( IVS) and essential  parts called exons. Exons take part in transcription and introns do not. These introns are removed  by nucleases. Exonic regions of r_ RNA are joined together to produce  a single chain RNA required for functioning as translational template. Phenomenon of removal of non_ coding regions and fusion of coding parts of RNA is called splicing. 


7): The freshly formed m_ RNA undergoes several  changes at 5' and 3' ends. It receives a cap at the 5' end and poly A tail at 3' end. The tailored m_ RNA is then transported across the nuclear envelope  for translation through nbosomes and t_ RNAs.

Importance of gene Regulation:

1): Gene action are constitutive  and regulated. Constitutive gene actions occur in those cells which operate all the times and the cell does not survive without them e.g.glycolysis. It is not repressed so regulator and operator genes are not associated  with it.

2): In regulation  gene action all the genes  required  for a multistep reaction can be switched on or off simultaneously. 


3):The genes  are switched  on or off in response to particular chemicals wether required  for  metabolism  or are formed  at the end of a metabolic  pathway. 


4): Gene regulation is required  for growth, division  and differentiation of cells. It bring about morphogensis.


QNo1 Molecular basis of organ differentiation depends on the modulation in  transcription by 

(a) Ribosome

(b) Transcription  factor

(c) Anticodon 

(d) RNA polymerase 

Ans Transcription factor

What is not true for genetic code? 

(a) It is degenerate 

(b) It is unambiguous 

(c) A codon in mRNA is read in a non_ contiguous fashion

(d) It is nearly universal 

Ans (c)

Point mutation involves 

(a) Duplication 

(b) Deletion 

(c) Insertion 

(d) Change in single base pair

Ans (d) 

The Lac operon " Inducer Lactose " serves as an enzyme substrate for

(a) Transacetylase 

(b) Endonuclease 

(c) Permease

(d) Beta__ galactosidase 

Ans (d)





























































 

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