Cellular Overview

You and I, we’re made up of cells – all sorts of them. Cells are the structural and functional units of all living organisms. The size of one of our cells can be a range between 5 to 100 microns in diameter, or across its width. (As examples of these sizes, consider this: 500 microns is about the size of a granule of table salt and 100 microns is the thickness of a human hair.)

Though very different in their specialized functions, the cells from which we are made are all derived from one cell (the fertilized oocyte) and are all more similar in their basic operations than they are dissimilar. Think about that. Think of how different your eye is from your skin and how different those cells must be compared to the cells in your brain, it’s really amazing to know that they all came from one cell.

The workings of these cells are intricate, intriguing, and still not completely understood. Every day, more discoveries are made about how our bodies do what they do through their cellular components – and how improperly functioning cells can lead to diseases. I have spent years studying cell biology because it is simply fascinating. 

My goal with this blog is to share the complex workings of the cell in such a way that a casual reader will enjoy it. Not everyone enjoys reading textbook-like material, but everyone should be able to marvel at the little cellular wonders working away 24/7 in our own bodies.

For my first post, I feel like I must touch on a few core subjects of mammalian cell biology: 1) Why cells have organelles. 2) The flow of information, from DNA into protein. 3) The purpose of proteins within cells.

Why cells have organelles.

Cells are enclosed by a membrane, and also contain smaller, membrane-enclosed parts called organelles. You can think of an organelle as a mini-environment where certain functions can take place more easily than in other parts of the cell. 

If a cell were a little city, the organelles could be thought of as different public works within that city. For instance, not everyone can collect and purify their own water – they are busy with other aspects of their life, so instead cities have specific places where water is purified and sent to where it is needed. And city buildings often serve as little mini-environments for certain actions. Think of town halls, jails, and broadcasting stations, for instance. And the citizens of the city go to organelle-like mini-environments for many of their needs, like going to a hardware store for tools, grocery stores for food, and amusement parks for some summer fun. Just as we make these environments to make life easier on ourselves, the cell has organelles for the same reason. Instead of going over what these organelles are and what they do, I'm going to move on to another core aspect of cell biology, the flow of the encoded information from DNA into the formation of proteins.

The flow of information, from DNA into protein.

One process over all others is key to cellular function: the creation of protein from DNA. Well, really, the regulation of this process is key to cellular function – and boy, do the cells regulate it! At just about every step of the process, there are multiple ways to start, stop, slow, or accelerate the creation of protein from DNA. This regulation can have a huge effect on the cell, so let’s go over the basic process now.

Picture a computer file containing the script to a new stage production. This computer file represents DNA. This file can be kept relatively safe, and copies of the file can be made to give to actors and stagehands, etc. In this example, these copies represent something called mRNA, or messenger RNA. The process of making mRNA from DNA is called “transcription,” because mRNA is a slightly different way of writing the same script found in DNA.

Importantly, you can make as many copies as you want from the one master file. The number of copies you need depends on your environment – maybe you only need enough for the actors right now, or maybe you need extra for the understudies. You can make more copies as needed, or you can stop making them as well. Or, perhaps you only want to copy and hand out the first act right now, and then the rest later.

The cell does this too. The amount of mRNA made from DNA depends on the environment of the cell and, based on its needs, the cell is able to choose which parts of the DNA it copies and at what time. This makes sense to us. However, in a more odd way of controlling the flow of information, the cell can also make copies from the master file (that is, make mRNA from DNA), and then destroy the mRNA copies before they are used. Very strange, but useful. So useful, that the cell has several ways to destroy copies in this fashion. But for now, let’s get back to how DNA gets turned into protein.

What happens with the copies of the script? They go to the actors, and in their hands the script gets changed from words on a page to vocal sounds and bodily movement. In other words, the content of the script is “translated” by the actor into an emotional performance to affect the audience. In the same way, the mRNA copies are used as templates to create proteins in a process called “translation.”

The DNA master file is transcribed into mRNA copies which become translated into protein.

Okay, so that’s one way of describing the process of going from DNA to mRNA to protein. Let’s briefly go over the process one more time in a different way. The DNA master file is “transcribed” into mRNA copies. This term, “transcription,” is used because both DNA and mRNA use building blocks called nucleic acids. The change from DNA to mRNA is a re-telling of the information using essentially the same language. Then the mRNA copies are “translated” into the building blocks of proteins, called amino acids. The term “translation” is used because conveying the information found in mRNA into protein is like moving from one language into another language.

If DNA was a book, mRNA would be a retelling of that book in the same language (nucleic acids). However, to make protein, the language of mRNA and DNA has to be translated  into a different language (amino acids).

And guess what? The transciptionists and translators working to move the encoded information on DNA to mRNA and then to protein, are proteins themselves. So while protein is often just thought of as part of a healthy diet in the mind of an average person, to a cell biologist, proteins are the engines that drive the actions of a cell. Cells have specialized functions because they make specific sets of proteins from their DNA. And, when it comes to the particular job of the cell, generally, proteins do much of the heavy lifting.

The purpose of proteins in cells.

After water, the next biggest fraction of what is in a cell is protein. Proteins come in many sizes, shapes, and have many different functions in the cell. So in truth, its a bit over-zealous of me to state the purpose of all proteins within a cell - they all have a slightly different function. I originally started listing the various functions of proteins here, but it was just too much. Instead, let me say that for every need the cell has, there is a protein involved in providing for that need. Cells have specialized functions because they make specific sets of proteins from their DNA. And, when it comes to the particular job of the cell, generally, proteins do much of the heavy lifting.

Importantly, proteins are able to be controlled by the cell in many different ways. In response to the environment, the activity of certain proteins can be turned on or off, they can be produced more abundantly or degraded, and they can be moved to different parts of the cell. All these ways of changing the protein (and more besides) can help the cell respond appropriately to certain signals. When proteins are unable to be regulated in this way, diseases like cancer can begin or become more hard to treat. So in summary, the purpose of proteins is essentially to run the cell smoothly, and to allow the cell to react to its environment.

Now, everyone has an idea of what DNA looks like: it’s a beautiful double helix, simple and striking. But its a bit harder to recall what a protein looks like. In general, most proteins are neither simple nor striking, and each looks a bit different than another. The particular shape of a protein is very interesting to cell biologists though, because often a protein's shape can reveal clues about their function.

Proteins get their shape by being the 3D result of a linear code. Let’s picture this in everyday terms. Say you have a big ball of yarn and you want to make pair of baby booties from it. Using a crochet pattern for booties, you can follow along and place different types of stitches at different times. The yarn you are using is linear, but because of the type of stitches used and the position at which the stitches are employed, you can make a functional 3D item from the linear code.

Linear yarn can be used to make different 3D shapes depending on which stitches are placed where. Shown are (poor) representatives of  this process.
If you're curious. what I tried to depict were: 4 single crochets, turn, then four double crochets (lower left), 4 single crochets, then 2 double crochets in each one of the singles (bottom right), and finally, 2 single crochets, followed by 3 double crochets, followed by two single crochets again (upper right). But the point is, different stitches at different times = different 3D structures.

In the cell, DNA and mRNA are the crochet pattern. During translation, the linear code specifies different amino acids be added to create a protein. These amino acids are added in a linear string, like the yarn, but because each amino acid has specific properties, their placement along a chain of other amino acids can result in the formation of a 3D structure. It’s a bit like different stitches along a chain of yarn, except in the case of the protein, the amino acids react with each other and fold up into different patterns. The different folding patterns are commonly called “domains.” These domains have specific roles in how the protein functions: what it interacts with, where it is located in the cell, as well as when and how the protein can be turned on or off.

In sum, proteins exist in a variety of shapes and sizes. This occurs because certain amino acids along the protein create certain 3D shapes which give the protein different attributes. The shape and make-up of the protein allow it to act and to be acted on by other proteins or other things in the cellular environment. Much of cell biology is the study of how different proteins act and react with each other. Well, and also cell biologists are interested in the when, the where, and the why of these protein-mediated reactions. 

And so concludes my broad summary of some of the basic processes of cell biology. Perhaps later, we can dive more deeply into the processes mentioned so briefly here.