Basics of Gene Expression Control - Online Tutor, Practice Problems & Exam Prep

Cell differentiation is the process by which cells become specialized for specific functions, driven by gene expression. Although all cells in a multicellular organism share the same genetic material, they express different genes, leading to diverse cell types. This regulation occurs at various stages, including transcriptional control, RNA processing, and translational control. Housekeeping genes are essential for all cells, while other genes are selectively expressed based on the cell's role. External factors, such as hormones and environmental signals, also influence gene expression, highlighting the complexity of cellular specialization.

Gene Expression Control Basics

Hello, everyone. In this lesson, we are going to be learning about cell differentiation and gene expression. Specifically, we are going to learn about how cells can have different fates and different jobs depending on the genes that they express.

Okay, everyone. So, gene expression is the process through which cells selectively choose to express some genes and not others. This is important because every cell in a multicellular organism is a clone. Every cell in your body has the exact same genetic material as every other cell in your body. They are all clones, but they don't all do the same thing. The way that you are able to have specialized cells, tissues, organs, and body structures is because of this gene expression, and it is going to be unique to each type of cell. The different appearances and functions depend on selectively expressing certain genes and not others. There are genes in your cells that are purposely built for liver function, and only liver cells are going to express those genes. Your eyeball cells are not going to express your liver genes. Only the liver cells are going to do that. While you have the genetic information to do everything, certain cells are only going to express the correct genes so that they do the correct job. Thus, gene expression controls the expression of proteins and RNAs, basically controlling the gene products. Cell differentiation is the process by which a cell becomes specialized for a particular function. Cell differentiation is very important because it allows us to have all of our unique cell types. Differentiation is entirely directed by gene expression control. Gene expression control can come in many different forms, but it is basically regulating the products that are made in a particular cell from the DNA. Which genes are transcribed, which ones are translated, how the mRNA is created, what exons are chosen to be put in that mRNA, how the proteins are made, how the products of those genes function, and if they are made at all. This allows for multicellular organisms to have a wide array of unique and diverse cell types.

An example of this can be seen here, and differentiation of a stem cell into many blood cell types. You can see here we have a multipotent hematopoietic stem cell. It's multipotent, meaning that this particular cell can make any type of blood cell that it needs to. These are generally found in the bones. You can see it makes all of these different types of cells which have a wide range of characteristics, sizes, and jobs, basically. Over here, we have our lymphocytes, and these are going to be your immune cells. You have your natural killer cells, your small lymphocytes, your T cells, your B cells, your plasma cells. These are going to be very important to maintain your health, your innate immune system, and your adaptive immune system. They all come from these multipotent stem cells. This multipotent stem cell for blood is also going to make all these other cell types. This particular giant cell here is going to make platelets. You can see here, we have the red blood cells which are also called erythrocytes. You have these mast cells which are going to be important for making histamines and then all of these down here are going to be your white blood cells. All of these different cell types of blood cells, because there are many, many different types of blood cells, all came from this cell right here. And all of these cells have the exact same genetic material, but they don't do the exact same job. They don't even look the same. And that is because they choose to express certain genes that are unique to their function. This is cell differentiation and cell specialization based on which genes they express, which genes they transcribe and translate.

Okay, everyone? So let's go down and let's continue our talk on this lesson. Okay, so gene expression can be controlled at various steps in the RNA to RNA protein pathways. There are many steps and many actions that our cells can take to control gene expression. Most people think it only happens at transcription, but gene expression is not only controlled there. You can have control of gene expression at transcription, where an mRNA is made or not. You can have it happen at RNA splicing or RNA times. Gene expression can be controlled by external signals as well. These external signals can be things like hormones. You can have certain hormones that are going to tell certain cells to do certain things, like sex hormones will tell egg cells to be created or sperm cells to be created. You can also have external signals like cold temperatures and less sunlight. In some animals, this is going to trigger some of their cells to begin secreting hormones or begin differentiating into sex cells because it may be mating season. So you can have environmental factors, hormonal factors, external factors that can control this differential expression, not just these genetic factors.

Alright, but there is an exception to this. There are some genes that are always used in all of the cell types and these are going to be called housekeeping genes. The reason that a cell doesn't get to choose whether it expresses these genes or not is because it can't live without these genes. These are going to be genes that are used in every cell because they are critical for life. So, these cells don't get to decide; they don't get to differentiate whether they want to express these genes or not. They have to. And these are going to be things like ribosomal genes, RNA polymerase genes, DNA repair genes, histone genes, oncogenes, transcription factor genes, cell cycle regulation genes, all of these have to be expressed in every cell or the cell either won't create proteins, it won't make mRNA, and it won't divide correctly. So there are a set of genes that every cell is going to express, but a lot of the genes in our cells are going to be unique to particular jobs that a cell might have.

Alright. So let's scroll down a little bit, and this is going to be just a representation of kind of what we've already talked about. This is going to be the many ways and many locations that gene expression can be controlled because there are many steps in this pathway to the creation of a gene product in which it can be controlled. Obviously, we have our gene up here, and it is either going to be expressed or it's not going to be expressed. The first level of expression control is going to be transcriptional control. You can allow transcription to happen, or you might not allow transcription to happen. This can be controlled by histone modifications. This can be controlled by methylation of the DNA, which is going to cause it to not allow replication machinery or transcription machinery polymerase can't even get into that gene to express it. These would be things called transcriptional control. You can also have repressors. Okay, guys? So basically, transcriptional control is, is this gene able to be transcribed? Yes, then it's probably going to be transcribed. No, then we're not expressing that gene. And there are many ways you can control that expression.

Now, you can also have RNA. You can have things like differential splicing where, say, there are 3 exons in a gene, but you only choose to put 2 of them in the mRNA. Then you're going to make a very unique protein. There are different ways that you can change the mRNA to make a unique protein for a unique function, which is going to change the gene expression. And then you can have things like RNA transport. Where are these RNAs going to be transported in the cell so that they can be translated? Because where they are translated is going to depend on where their job is going to be, where they're going to be transported. Remember, they can be translated all over the cytoplasm, but they can also be translated in the rough endoplasmic reticulum, and then they can be transported in or out of the cell. So where it is translated is very important as well. And then of course, you're going to have translational control. Is the mRNA even allowed to be translated? It might not be. It might be degraded before it's even translated, and then technically that gene wasn't expressed because the protein product was not made. So, as you can see, there are tons of different ways that you can control expression. The transcriptional control is going to be the most common, and this is because it requires the least amount of energy, but all of these other processes do happen. You can control gene expression in transcription, RNA processing, RNA transport, and in every step of the expression of a gene. But just remember, guys, that gene expression is going to determine the type of gene and the type of cell, and the function that that cell has. We call this cell differentiation when it chooses to express certain genes that are important for its way of life and its job in the multicellular organism. Okay, everyone. I hope that was helpful. Let's go on to our next topic.

Gene expression is defined as which of the following?

• A. Genes are expressed because each cell contains a different set of genes
• B. Choosing which genes are expression by regulating only transcription
• C. Choosing which genes are expression because on only internal signals
• D. Cells choosing to express some genes and not others at many steps in the DNA to protein pathway

Every gene is regulated differently in each cell type.

• A. True
• B. False

Here’s what students ask on this topic:

What is gene expression and why is it important for cell differentiation?

Gene expression is the process by which cells selectively express certain genes while repressing others. This selective expression is crucial for cell differentiation, which is the process by which cells become specialized for specific functions. Although all cells in a multicellular organism contain the same genetic material, they express different sets of genes, leading to diverse cell types. For example, liver cells express genes necessary for liver function, while muscle cells express genes required for muscle function. This regulation allows for the formation of specialized tissues and organs, enabling the organism to perform complex tasks. Without gene expression control, cells would not be able to differentiate and perform their unique roles.

How do external factors influence gene expression?

External factors such as hormones, environmental signals, and temperature can significantly influence gene expression. Hormones like sex hormones can trigger the expression of genes involved in the development of reproductive cells. Environmental factors, such as cold temperatures or changes in sunlight, can also affect gene expression. For instance, in some animals, these changes can trigger the secretion of hormones that lead to the differentiation of cells for mating season. These external signals interact with the cellular machinery to either promote or inhibit the transcription and translation of specific genes, thereby affecting the cell's function and behavior.

What are housekeeping genes and why are they essential?

Housekeeping genes are a set of genes that are essential for the basic functions and survival of all cells. These genes are expressed in all cell types because they are involved in fundamental processes such as protein synthesis, DNA replication, and cell cycle regulation. Examples include genes encoding ribosomal proteins, RNA polymerases, and DNA repair enzymes. Unlike other genes that are selectively expressed based on the cell's specific function, housekeeping genes are always active to ensure that the cell can perform its essential activities. Without these genes, cells would not be able to survive or maintain their basic functions.

What are the different levels at which gene expression can be controlled?

Gene expression can be controlled at multiple levels, including transcriptional control, RNA processing, RNA transport, and translational control. Transcriptional control involves regulating whether a gene is transcribed into mRNA, often through mechanisms like histone modification and DNA methylation. RNA processing includes splicing, where different exons are included or excluded to produce unique mRNA variants. RNA transport determines where in the cell the mRNA is transported for translation. Finally, translational control involves regulating whether the mRNA is translated into a protein, which can be influenced by factors such as mRNA stability and the presence of specific translation initiation factors. Each of these levels provides a point of regulation to ensure precise control over gene expression.

How does transcriptional control regulate gene expression?

Transcriptional control regulates gene expression by determining whether a gene is transcribed into mRNA. This can be influenced by several mechanisms, including histone modifications and DNA methylation. Histone modifications can either loosen or tighten the DNA around histones, making it more or less accessible to transcription machinery. DNA methylation typically represses gene expression by preventing transcription factors and RNA polymerase from binding to the DNA. Additionally, transcriptional repressors can bind to specific DNA sequences to block transcription. By controlling the transcription of genes, cells can regulate which proteins are produced, thereby influencing cell function and differentiation.