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Genes Expression Regulation - Module 3 Reading / Watching

సైటు: Justwrite
కోర్సు: An introduction to Gene Regulation and Gene Expression
పుస్తకం: Genes Expression Regulation - Module 3 Reading / Watching
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తేదీ: బుధవారం, 4 జూన్ 2025, 10:39 AM

విషయసూచిక

1. Gene expression Regulation

In this book you will see an overview of the topics of Module Three and reference links.

2. Learning Objectives

By the end of this chapter, readers will be able to

  • Discuss why every cell does not express all of its genes.
  • Describe the importance of gene expression  regulation
  • Explain the stages at which gene expression can be regulated


2.1. Why Gene expression should be regulated?

A human cell, a eukaryotic cell, contains some 21,000 genes. Some of these are expressed in all cells all the time. These so-called They are responsible for the routine metabolic functions (e.g. respiration) which are common to all cells. Some are expressed as a cell enters a particular pathway of differentiation. Some are expressed all the time in only those cells that have differentiated in a particular way.  For example, a plasma cell expresses continuously the genes for the antibody it synthesizes. Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.
 Cellular functions  are determined by the thousands of genes that are expressed in that cell. By altering the quantity and kind of proteins produced, the cell can self-regulate its operations by regulating informational flow  from DNA to RNA to protein.
Gene regulation is the process of controlling which genes in a cell’s DNA are expressed i.e., Which gene is used to make a functional product such as a protein. Different cells in a multicellular organism may express different sets of genes in spite of containing the same DNA. The set of genes expressed in a cell determines the set of proteins and functional RNAs it contains and this determines its unique properties.

In eukaryotes like humans, gene expression involves many steps. Gene regulation can occur at any of these steps. However basically many genes are regulated at the level of transcription.The concentration of a protein in a cell is determined by the the equilibrium between its metabolic pathways for synthesis and degradation.

Protein formation  begins with transcription and continues with translation . The types and concentrations of mRNA molecules present in a cell determine the cell’s function.  Every cell creates thousands of transcripts per second. The  gene expression  is mainly regulated during the transcriptional initiation. One mRNA molecule can produce several proteins, making RNA transcription an effective control point.

Eukaryotes have an additional degree of regulation provided by  post transcriptional processing in the nucleus before being exported to the cytoplasm for translation. While in prokaryotes due to the close proximity of the ribosomes to the nascent mRNA molecules, translation  starts even  before the  transcription is completed .

The gene expression regulation may occur even  after transcription . For instance, following fertilization during embryonic  developmental stages  gene expression is regulated at the level of translation.  Hence, many maternally derived mRNA transcripts are present in eggs as a ready supply for translation following fertilization.

In eukaryotic cells the level of  protein degradation is connected to cellular activities.  This can be well illustrated by the role of cyclins. are regulatory proteins that regulate the various phases of cell cycle.  Each phase produce a characteristic type of cyclins and a cell must breakdown the cyclin that is specific to that phase of the cell cycle before it can move on to the following phase. A cyclin’s inability to be degraded prevents the cycle from continuing the cell cycle. This example thus helps us to understand how appropriate gene expression and protein levels determine the functions of a cell.



2.2. Gene regulation makes cells different

The different patterns of gene expression cause our various cell types to have different sets of proteins, making each cell type uniquely specialized to do its job.  For example, the liver functions to remove toxic substances like alcohol from the bloodstream. By expressing genes encoding the enzyme called alcohol dehydrogenase. This breaks alcohol down into a non-toxic molecule.
Likewise the neurons in brain do not  function to remove toxins from the body, so they keep these genes unexpressed, or “turned off.” Similarly, the cells of the liver do not send signals using neurotransmitters, so they keep neurotransmitter genes turned off.
It is interesting to know that humans and chimpanzees are about 98.8%identical at the DNA level. But the protein-coding sequences of some genes are different between humans and chimpanzees.  This  contributes to the differences between the species explaining the importance of gene regulation and differential expression of the genes.

2.3. Gene expression regulation enables the human body to respond to changes in nutrient concentration

Hormonal and nutrient concentrations affect several regulatory domains of genes, which encode for enzymes involved in anabolic and catabolic pathways. Insulin and glucose concentrations increase mRNA levels and transcription rates of the glycolytic enzymes, and decrease those of the gluconeogenic enzymes. On the contrary glucagon  has the opposite effect of insulin.

The required protein must be generated at the appropriate time and rate for a cell to function effectively. To prevent the buildup of intermediates, particularly the hazardous ones, in the biosynthetic pathway, the activity of the pathway enzymes must be balanced which could be achieved by regulating the expression of genes encoding these enzymes.


2.4. Gene expression regulation helps to conserve Energy and Space

Energy and space are conserved through gene expression control. Just activating the genes when necessary will use less energy. Also, because DNA must be unwound from its tightly coiled shape in order to be translated and transcribed, just expressing a selection of genes in each cell conserves space. If every protein were constantly expressed in every cell, cells would need to be extremely large.

2.5. Mechanisms/ stages of gene regulation

Eukaryotic gene expression can be regulated at many stages
  • Chromatin accessibility. The structure of chromatin can be regulated. More open or “relaxed” chromatin makes a gene more available for transcription.
  • Transcription. Transcription is a key regulatory point for many genes. With the help of transcription factor proteins bind to specific DNA sequences in or near a gene and promote or repress its transcription into an RNA.
  • RNA processing. Processing events like Splicing, capping, alternative splicing and addition of a poly-A tail to an RNA molecule can be regulated in turn regulating gene expression.
  • RNA stability. The lifetime of an mRNA molecule in the cytosol determines the number of proteins can be made from it. Small regulatory RNAs called miRNAs can bind to target mRNAs and cause them to be chopped up.
  • Translation. Translation of an mRNA may be increased or inhibited by regulators. For instance, miRNAs sometimes block translation of their target mRNAs (rather than causing them to be chopped up).
  • Protein activity. Proteins can undergo a variety of modifications, such as being chopped up or tagged with chemical groups. These modifications can be regulated and this in turn affect the activity or behavior of the protein.