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

Cada célula en el cuerpo de un organismo multicelular posee el mismo ADN; sin embargo, los diferentes tipos de células tienen diferencias estructurales y funcionales notables que pueden atribuirse a la expresión diferencial de genes en la célula. 
Además, células específicas expresan diferentes genes a lo largo del tiempo debido a cambios en la célula u organismo. La variedad de ARN y proteínas producidas permite a las células llevar a cabo sus funciones únicas en los momentos adecuados.
Las células hepáticas, llamadas hepatocitos, expresan un conjunto de genes diferente en comparación con las neuronas.
Por ejemplo, las células hepáticas producen alcohol deshidrogenasa, una enzima que descompone los alcoholes tóxicos en acetaldehído, que luego puede metabolizarse para producir dióxido de carbono y agua. 
En contraste, las neuronas producen neurexinas, un grupo de proteínas en el cerebro que ayuda a transmitir información de una neurona a una adyacente. 
Las células tienen mecanismos para controlar la expresión génica en múltiples pasos reguladores antes, durante y después de la transcripción y traducción. Sin embargo, los reguladores transcripcionales son comunes porque impiden la síntesis de transcripciones de ARNm. 
Durante el desarrollo del hígado en humanos, los factores de transcripción C/EBP α, C/EBP β y el Factor Nuclear de Hepatocitos-1 contribuyen a la expresión hepática específica de los genes de la alcohol deshidrogenasa. 
La expresión génica también se regula en respuesta al entorno extracelular. 
Cuando los niveles de glucosa en sangre disminuyen, el páncreas segrega la hormona glucagón. Cuando se exponen al glucagón, las células hepáticas expresan fosfoenolpiruvato carboxiquinasa, una proteína necesaria para la producción de glucosa a partir de precursores no carbohidratados.

<ol>
	<li>Alberts et al., 6ª edición; páginas 369-374.</li>
	<li>Lodish et al., 8ª edición; páginas 363-364</li>
	<li>Park, Edwards A. Austin L. Gurney, Steven E. Nizielski, Parvin Hakimi, Zhodan Caot, Antoon MoormanY y Richard W. Hanson. "Roles relativos de la proteína de unión a CCAAT / potenciador β y la proteína de unión a elementos reguladores de cAMP en el control de la transcripción del gen para la fosfoenolpiruvato carboxiquinasa (GTP)" Revista de Química Biológica 268, No. 1, (1993): 613-619</li>
</ol>

expresión genética específica de células

Cell Specific Gene Expression

Los organismos multicelulares contienen una variedad de tipos de células estructural y funcionalmente distintos, pero el ADN de todas las células se originó a partir de las mismas células madre. Las diferencias en las células pueden atribuirse a la expresión genética diferencial. Las células del hígado, cuyas funciones incluyen la desintoxicación de la sangre, la producción de bilis para metabolizar las grasas y la síntesis de proteínas esenciales para el metabolismo, deben expresar un conjunto específico de genes para realizar sus funciones. La expresión genética también varía según las etapas de desarrollo. Antes de diferenciarse en células hepáticas, las células expresan genes implicados en el ciclo celular, la replicación del ADN y la proliferación. Más adelante en el desarrollo, los genes implicados en la diferenciación epitelial y la coagulación sanguínea se expresan intensamente. Una vez que las células se diferencian en hepatocitos, aumenta la expresión de genes implicados en funciones específicas del hígado, como los implicados en el metabolismo de los lípidos y la regulación del colesterol.

La expresión genética se puede regular en muchos puntos, incluida la transcripción, la traducción, el procesamiento y transporte del ARN y las modificaciones postraduccionales. Los métodos comunes para regular la expresión son factores que se unen directamente al ADN para regular la transcripción de un gen en particular. La expresión génica en el hígado puede estar regulada por los factores de transcripción C/EBPα, C/EBPβ y el factor nuclear de hepatocitos-1, entre otros. La regulación puede ocurrir antes de la transcripción alterando las histonas contenidas en la cromatina. Estas modificaciones dan como resultado un aflojamiento o un endurecimiento de la estructura del ADN, impidiendo o permitiendo respectivamente que los reguladores transcripcionales accedan al ADN. Los diferentes tipos de células tienen diferentes modificaciones covalentes y variantes de histonas, lo que da como resultado una variación en la accesibilidad de los genes.

Las células están sujetas a cambios ambientales y expresan diferentes genes en respuesta a estos estímulos extracelulares. La glucosa es una fuente importante de energía y, a medida que su concentración en el torrente sanguíneo fluctúa, un organismo debe responder con cambios apropiados en la expresión de genes y proteínas. Cuando los niveles de glucosa en sangre disminuyen, el páncreas secreta la hormona glucagón. Esta hormona le indica al hígado que inicie la producción de fosfoenolpiruvato carboxiquinasa (PEPCK), una proteína necesaria para producir glucosa a partir de precursores distintos de los carbohidratos. El glucagón induce la transcripción de este gen estimulando indirectamente los factores de transcripción C/EBPα y C/EBPβ para que se unan al promotor PEPCK. Cuando los niveles de glucosa en sangre son altos, el páncreas secreta la hormona insulina; el gen PEPCK tiene una secuencia sensible a la insulina que inhibe su transcripción.

Expresión genética específica de células

Los organismos multicelulares contienen una variedad de tipos de células estructural y funcionalmente distintos, pero el ADN de todas las células se originó a partir de las mismas células.

Education

Educación

JoVE Core

Molecular Biology

Biología Molecular

Unit 1

Unidad 1

Gene Expression

Expresión génica

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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 Biología Molecular presenta los procesos moleculares de la célula a través de lecciones claras y concisas de animaciones audiovisuales. Además, en los vídeos de científicos en acción, se muestran aplicaciones de estos conceptos mediante experimentos reales en laboratorios especializados.

How Cells Switch Genes On and Off

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