Name: how cells switch genes on and off
Uploaded: 2020-11-23T11:47:37.000Z
Duration: 1 min 58 s
Channel: JoVE

Chaque cellule du corps d'un organisme multicellulaire possède le même ADN. Cependant, différents types de cellules présentent des différences structurelles et fonctionnelles notables qui peuvent être attribuées à l'expression différentielle des gènes dans la cellule. En outre, des cellules spécifiques expriment différents gènes au fil du temps en raison des changements dans la cellule ou l'organisme.La variété d'ARN et de protéines produites permet aux cellules d'exercer leurs fonctions uniques aux moments appropriés. Les cellules hépatiques, appelées hépatocytes, expriment un ensemble différent de gènes comparé aux neurones. Par exemple, les cellules hépatiques produisent de l'alcool déshydrogénase, une enzyme qui décompose les alcools toxiques en acétaldéhyde, qui peut ensuite être métabolisé pour produire du dioxyde de carbone et de l'eau.En revanche, les neurones produisent des neurexines, un groupe de protéines dans le cerveau qui aide à transmettre l'information d'un neurone à un neurone adjacent. Les cellules ont des mécanismes pour contrôler l'expression des gènes à plusieurs étapes régulatrices avant, pendant et après la transcription et la traduction. Cependant, les régulateurs transcriptionnels sont communs parce qu'ils empêchent la synthèse des transcripts d'ARNm.Au cours du développement du foie chez l'homme, les facteurs de transcription C/EBP-alpha, C/EBP-bêta, et le facteur nucléaire hépatocyte-1 contribuent à l'expression spécifique dans le foie des gènes de l'alcool déshydrogénase. L'expression des gènes est également régulée en réponse à l'environnement extracellulaire. Lorsque le taux de glucose dans le sang diminue, le pancréas sécrète l'hormone glucagon.Lorsqu'elles sont exposées au glucagon, les cellules hépatiques expriment la phosphoénolpyruvate carboxykinase, une protéine nécessaire à la production de glucose à partir de précurseurs non glucidiques.

<ol>
	<li>Alberts et al., 6e édition ; pages 369-374.</li>
	<li>Lodish et al., 8e édition ; pages 363-364</li>
	<li>Park, Edwards A. Austin L. Gurney, Steven E. Nizielski, Parvin Hakimi, Zhodan Caot, Antoon MoormanY et Richard W. Hanson. « Rôles relatifs de la protéine de liaison CCAAT/Enhancer-protein β et de la protéine de liaison aux éléments régulateurs de l’AMPc dans le contrôle de la transcription du gène de la phosphoénolpyruvate carboxykinase (GTP) » Journal of Biological Chemistry 268, n° 1, (1993) : 613-619</li>
</ol>

expression génétique spécifique à une cellule

Cell Specific Gene Expression

Les organismes multicellulaires contiennent une variété de types de cellules structurellement et fonctionnellement distincts, mais l'ADN de toutes les cellules provient des mêmes cellules parentales. Les différences entre les cellules peuvent être attribuées à l’expression différentielle des gènes. Les cellules hépatiques, dont les fonctions comprennent la détoxification du sang, la production de bile pour métaboliser les graisses et la synthèse de protéines essentielles au métabolisme, doivent exprimer un ensemble spécifique de gènes pour remplir leurs fonctions. L'expression des gènes varie également selon les stades de développement. Avant leur différenciation en cellules hépatiques, les cellules expriment des gènes impliqués dans le cycle cellulaire, la réplication de l'ADN et la prolifération. Plus tard dans le développement, les gènes impliqués dans la différenciation épithéliale et la coagulation sanguine sont fortement exprimés. Une fois que les cellules se différencient en hépatocytes, l'expression de gènes impliqués dans des fonctions spécifiques du foie augmente, comme ceux impliqués dans le métabolisme des lipides et la régulation du cholestérol.

L'expression des gènes peut être régulée à de nombreux points, notamment la transcription, la traduction, le traitement et le transport de l'ARN, ainsi que les modifications post-traductionnelles. Les méthodes courantes de régulation de l'expression sont des facteurs qui se lient directement à l'ADN pour réguler la transcription d'un gène particulier. L’expression des gènes dans le foie peut être régulée par les facteurs de transcription C/EBPα, C/EBPβ et Hepatocyte Nuclear Factor-1, entre autres. La régulation peut avoir lieu avant la transcription en modifiant les histones contenues dans la chromatine. Ces modifications entraînent soit un relâchement, soit un resserrement de la structure de l'ADN, empêchant ou permettant ainsi respectivement aux régulateurs transcriptionnels d'accéder à l'ADN. Différents types de cellules ont différentes modifications covalentes et variantes d'histone, ce qui entraîne une variation de l'accessibilité des gènes.

Les cellules sont soumises à des changements environnementaux et expriment différents gènes en réponse à ces stimuli extracellulaires. Le glucose est une source d’énergie importante et, à mesure que sa concentration dans le sang fluctue, un organisme doit réagir par des changements appropriés dans l’expression des gènes et des protéines. Lorsque la glycémie diminue, le pancréas sécrète l'hormone glucagon. Cette hormone signale au foie d'initier la production de phosphoénolpyruvate carboxykinase (PEPCK), une protéine nécessaire à la production de glucose à partir de précurseurs non glucidiques. Le glucagon induit la transcription de ce gène en stimulant indirectement la liaison des facteurs de transcription C/EBPα et C/EBPβ au promoteur PEPCK. Lorsque la glycémie est élevée, le pancréas sécrète l’hormone insuline ; le gène PEPCK possède une séquence sensible à l'insuline qui inhibe sa transcription.

Expression génétique spécifique à une cellule

Les organismes multicellulaires contiennent une variété de types de cellules distincts sur le plan structurel et fonctionnel, mais l’ADN de toutes les cellules provient du même ...

Education

Enseignement

JoVE Core

Molecular Biology

Biologie moléculaire

Unit 1

Unité 1

Gene Expression

Expression génique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

how cells switch genes on and off

Cells switch genes on and off to make different proteins for different jobs. In multicellular organisms, many cell types are built from the same DNA, but they do not use the same genes in the same way. This difference in gene expression helps explain why cells can look and act so differently.

Liver cells are a clear example. They help detoxify blood, make bile to help digest fats, and produce proteins needed for metabolism. To do these jobs, liver cells must express a specific set of genes. Gene expression also changes during development. Before cells differentiate into liver cells, they express genes for the cell cycle, DNA replication, and cell growth. Later, genes for epithelial differentiation and blood coagulation become highly active. After the cells become hepatocytes, genes linked to liver functions, such as lipid metabolism and cholesterol control, increase in expression.

Cells can regulate gene expression at several steps, including transcription, translation, RNA processing and transport, and post-translational modifications. One common control point is transcription, where proteins bind DNA and affect whether a gene is copied into RNA. In the liver, transcription factors such as C/EBPα, C/EBPβ, and Hepatocyte Nuclear Factor-1 help regulate gene activity.

Control can also happen before transcription begins. Histones are proteins around which DNA is wrapped in chromatin. Chemical changes to histones can loosen or tighten the DNA structure. Looser chromatin gives transcription regulators access to the DNA, while tighter chromatin can block access. Different cell types have different covalent modifications and histone variants, which changes which genes are available to be expressed.

Environmental signals can also change gene expression. Blood glucose levels are one example. When glucose levels drop, the pancreas releases glucagon. This hormone signals the liver to make phosphoenolpyruvate carboxykinase, or PEPCK, a protein needed to produce glucose from non-carbohydrate sources. Glucagon increases transcription of the PEPCK gene by indirectly helping C/EBPα and C/EBPβ bind to its promoter. When blood glucose is high, the pancreas releases insulin, and an insulin-responsive sequence inhibits PEPCK transcription.

How Cells Switch Genes On and Off

cell specific gene expression

Cells switch genes on and off to make different proteins for different jobs. In multicellular organisms, many cell types are built from the same DNA, but they do not use the same genes in the same way. This difference in gene expression helps explain why cells can look and act so differently. Liver cells are a clear example. They help detoxify blood, make bile to help digest fats, and produce proteins needed for metabolism. To do these jobs, liver cells must express a specific set of genes.

regulation of expression occurs at multiple steps

regulation-of-expression-occurs-at-multiple-steps

How Cells Control Gene Expression

Cells control gene expression at several points between DNA and protein. The most common control point is transcription. At this stage, proteins bind to short regulatory DNA sequences and can either increase or reduce transcription of the linked gene. After transcription, a precursor mRNA is made. This pre-mRNA contains exons, which code for protein, and introns, which do not. It must be processed before translation, and splicing removes the introns and joins the exons. Cells can also regulate...

cis-regulatory sequences

How Cis-Regulatory DNA Controls Transcription

Cis-regulatory sequences are short, non-coding DNA segments that control gene transcription. They sit on the same chromosome as the genes they regulate and act as binding sites for transcriptional regulators. In eukaryotes, these proteins help control gene expression patterns in different cell types. These DNA sequences can be close to a gene or thousands of bases away. When they are far from the gene in the DNA sequence, they are often brought close together in space because chromosomes are...

cooperative binding of transcription regulators

cooperative-binding-of-transcription-regulators

How Transcription Factors Bind DNA

Transcriptional regulators bind to cis-regulatory DNA sequences to control gene transcription. These DNA sequences are very short, usually fewer than ten nucleotide pairs long. Because they are so short, the same sequence can appear by chance in many places across the genome. Some regulators can also bind to groups of similar sequences. That raises the chance of random binding even more. To lower this risk, transcription regulators often form dimers, which are pairs of proteins that bind a DNA...

prokaryotic transcriptional activators and repressors

prokaryotic-transcriptional-activators-and-repressors

Operon Control by Activators and Repressors

Operon control in prokaryotes depends on activators and repressors. Prokaryotic genes are often arranged in groups, not one by one like in eukaryotes. Genes that help carry out the same biochemical process are placed together with their regulatory elements. This group is called an operon, and it is transcribed into a single polycistronic mRNA strand. Activators and repressors are DNA-binding proteins that control transcription in an operon. Activators bind near the promoter, which is the site...

operons

Operons in Bacterial Gene Control

Operons help bacteria control groups of genes at the same time. An operon is a set of genes that are regulated together and can be turned on or off as a unit. This shared control helps bacteria respond quickly to changes in their environment. In many operons, a promoter gives RNA polymerase a place to bind and start transcription. The operon also includes a regulatory region that helps control whether the genes are expressed. When conditions change, the cell can use this setup to manage gene...

the eukaryotic promoter region

Eukaryotic Promoter Motifs and Gene Control

The eukaryotic promoter region is a stretch of DNA located upstream of a gene. It includes an RNA polymerase binding site, a transcription start site, and several cis-regulatory sequences. These DNA signals help control when transcription begins. The promoter can be divided into proximal and distal regions. The proximal promoter lies close to the gene and includes cis-regulatory sequences and the core promoter. The core promoter is the RNA polymerase binding site, and it is usually found...

co-activators and co-repressors

Chromatin Control in Gene Transcription

Chromatin control plays a major role in gene transcription in eukaryotes. Gene expression is regulated by several proteins that work together at a gene regulatory site. Some of these proteins bind directly to specific DNA sequences, while co-regulators attach to those regulators but do not bind DNA on their own. Co-regulators can act as co-activators or co-repressors, depending on the complex they join. One co-regulator may even switch roles in different settings. For example, the...

eukaryotic transcription activators

How Transcription Activators Turn Genes On

Transcription activators are proteins that help turn genes on by promoting transcription from DNA to RNA. They often have two parts. One part binds to DNA, and the other part activates transcription. In some proteins, such as the glucocorticoid receptor and MyoD, one domain can do both jobs. The DNA-binding part recognizes specific regulatory sequences on the DNA. These binding domains are grouped into families based on their structure. Common examples include the leucine zipper, zinc finger,...

eukaryotic transcription inhibitors

How Eukaryotic Gene Repressors Work

Eukaryotic gene repressors help turn specific genes off when the cell needs to control growth, development, or other biochemical processes. These DNA-binding proteins regulate gene expression in eukaryotes. Their presence at the right time and place can be triggered by hormones and signals from other cells. Most transcription inhibitors have two main parts. One part binds DNA, and the other is a repressor domain, which helps shut down transcription. Repressor domains can connect with basal...

combinatorial gene control

Gene Expression by Transcription Factor Combinations

Gene expression can depend on combinations of transcription factors working together. In combinatorial gene control, several transcription factors act in a coordinated way to regulate a single gene. If one or more of these factors are missing, the level of gene expression or repression can change a lot. This kind of control helps explain how a relatively small number of transcription factors can regulate a very large number of genes. More than 30,000 genes are controlled by about 2,000 to...

induced pluripotent stem cells

Reprogramming Cells into Stem Cells

Induced pluripotent stem cells are created by reprogramming mature cells back into a stem-cell state. This process shows that specialized cells can be changed into cells with broader developmental potential. The topic is important in biology because it links cell identity, gene control, and stem cell research. These cells begin as ordinary body cells and are pushed to de-differentiate, or lose their specialized features. After reprogramming, they behave like pluripotent stem cells, which means...

master transcription regulators

MEF2C and Gene Control Cascades

MEF2C is a master transcription regulator that controls gene expression in coordinated cascades. These regulators are proteins that manage the activity of multiple genes that often work together to produce a complex outcome. When a master transcription regulator is activated, it can start a chain of transcriptional activation that supports that outcome. Master transcription regulators can act in two main ways. They may bind directly to the regulatory sequences of several genes involved in a...

epigenetic regulation

DNA Methylation and Histone Changes

Epigenetic regulation controls gene activity without changing the DNA sequence. It helps determine when genes are turned on or off in a cell. Two major epigenetic mechanisms are DNA methylation and histone modification. DNA methylation adds chemical groups to DNA, while histone changes affect how tightly DNA is packed around histone proteins. These changes can make genes easier or harder to use. In high school biology, epigenetic regulation is important for understanding how cells with the...

genomic imprinting and inheritance

Genomic Imprinting in Human Disease

Genomic imprinting affects how some inherited genes are turned on or off in human cells. Diploid organisms inherit chromosomes from both parents, and each gene usually has two alleles. In most cases, both alleles are expressed. If one allele is missing or mutated, the other can often compensate, but some genes do not work that way. In genomic imprinting, a gene or its regulatory sequence carries chemical tags, such as methyl groups, on the copy inherited from only one parent. These tags are...

key terms and concepts

Learn how cells control gene expression through development, chromatin changes, transcription factors, and hormone signals like glucagon and insulin.

How Cells Switch Genes On and Off | JoVE K12

molecular biology

JoVE Core Biologie Moléculaire illustre les processus moléculaires au sein de la cellule grâce à des leçons vidéos claires et concises. Les applications de ces concepts sont ensuite démontrées grâce à des expériences concrètes en laboratoire.

How Cells Switch Genes On and Off

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